TATDN2 resolution of R-loops is required for survival of BRCA1-mutant cancer cells

BRCA1-deficient cells have increased IRE1 RNase, which degrades multiple microRNAs. Reconstituting expression of one of these, miR-4638-5p, resulted in synthetic lethality in BRCA1-deficient cancer cells. We found that miR-4638-5p represses expression of TATDN2, a poorly characterized member of the TATD nuclease family. We discovered that human TATDN2 has RNA 3' exonuclease and endonuclease activity on double-stranded hairpin RNA structures. Given the cleavage of hairpin RNA by TATDN2, and that BRCA1-deficient cells have difficulty resolving R-loops, we tested whether TATDN2 could resolve R-loops. Using in vitro biochemical reconstitution assays, we found TATDN2 bound to R-loops and degraded the RNA strand but not DNA of multiple forms of R-loops in vitro in a Mg2+-dependent manner. Mutations in amino acids E593 and E705 predicted by Alphafold-2 to chelate an essential Mg2+ cation completely abrogated this R-loop resolution activity. Depleting TATDN2 increased cellular R-loops, DNA damage and chromosomal instability. Loss of TATDN2 resulted in poor replication fork progression in the presence of increased R-loops. Significantly, we found that TATDN2 is essential for survival of BRCA1-deficient cancer cells, but much less so for cognate BRCA1-repleted cancer cells. Thus, we propose that TATDN2 is a novel target for therapy of BRCA1-deficient cancers.

Cellular Responses to Widespread DNA Replication Stress

Replicative DNA polymerases are blocked by nearly all types of DNA damage. The resulting DNA replication stress threatens genome stability. DNA replication stress is also caused by depletion of nucleotide pools, DNA polymerase inhibitors, and DNA sequences or structures that are difficult to replicate. Replication stress triggers complex cellular responses that include cell cycle arrest, replication fork collapse to one-ended DNA double-strand breaks, induction of DNA repair, and programmed cell death after excessive damage. Replication stress caused by specific structures (e.g., G-rich sequences that form G-quadruplexes) is localized but occurs during the S phase of every cell division. This review focuses on cellular responses to widespread stress such as that caused by random DNA damage, DNA polymerase inhibition/nucleotide pool depletion, and R-loops. Another form of global replication stress is seen in cancer cells and is termed oncogenic stress, reflecting dysregulated replication origin firing and/or replication fork progression. Replication stress responses are often dysregulated in cancer cells, and this too contributes to ongoing genome instability that can drive cancer progression. Nucleases play critical roles in replication stress responses, including MUS81, EEPD1, Metnase, CtIP, MRE11, EXO1, DNA2-BLM, SLX1-SLX4, XPF-ERCC1-SLX4, Artemis, XPG, FEN1, and TATDN2. Several of these nucleases cleave branched DNA structures at stressed replication forks to promote repair and restart of these forks. We recently defined roles for EEPD1 in restarting stressed replication forks after oxidative DNA damage, and for TATDN2 in mitigating replication stress caused by R-loop accumulation in BRCA1-defective cells. We also discuss how insights into biological responses to genome-wide replication stress can inform novel cancer treatment strategies that exploit synthetic lethal relationships among replication stress response factors.

Effect of surgical intervention on serum insulin-like growth factor 1 in patients with obstructive sleep apnoea

OBJECTIVE

To evaluate the effect of surgical intervention on serum insulin-like growth factor 1 levels in patients with obstructive sleep apnoea.

METHODS

A prospective study was conducted in a tertiary care hospital of adult patients with obstructive sleep apnoea for whom continuous positive airway pressure therapy failed or was refused. All patients underwent polysomnography and serum insulin-like growth factor 1 evaluation pre-operatively and at three months post-operatively. The site of surgery was determined using Müller's manoeuvre and ApneaGraph AG 200.

RESULTS

Fifteen patients were included with a mean age of 38 years: 11 males and 4 females. The mean pre-operative Apnoea-Hypopnoea Index using polysomnography was 53.7 events per hour, and the mean post-operative Apnoea-Hypopnoea Index at three months was 15.3 events per hour (p = 0.0001). The mean pre-operative serum insulin-like growth factor 1 was 160.2 μg/l, while the mean post-operative value was 236.98 μg/l (p = 0.005).

CONCLUSION

In adult patients with obstructive sleep apnoea for whom continuous positive airway pressure therapy fails, site-specific surgical intervention to treat the obstruction leads to an increase in serum insulin-like growth factor 1 levels.

EEPD1 promotes repair of oxidatively-stressed replication forks

Unrepaired oxidatively-stressed replication forks can lead to chromosomal instability and neoplastic transformation or cell death. To meet these challenges cells have evolved a robust mechanism to repair oxidative genomic DNA damage through the base excision repair (BER) pathway, but less is known about repair of oxidative damage at replication forks. We found that depletion or genetic deletion of EEPD1 decreases clonogenic cell survival after oxidative DNA damage. We demonstrate that EEPD1 is recruited to replication forks stressed by oxidative damage induced by H2O2 and that EEPD1 promotes replication fork repair and restart and decreases chromosomal abnormalities after such damage. EEPD1 binds to abasic DNA structures and promotes resolution of genomic abasic sites after oxidative stress. We further observed that restoration of expression of EEPD1 via expression vector transfection restores cell survival and suppresses chromosomal abnormalities induced by oxidative stress in EEPD1-depleted cells. Consistent with this, we found that EEPD1 preserves replication fork integrity by preventing oxidatively-stressed unrepaired fork fusion, thereby decreasing chromosome instability and mitotic abnormalities. Our results indicate a novel role for EEPD1 in replication fork preservation and maintenance of chromosomal stability during oxidative stress.

In Vitro Reconstitutive Base Excision Repair (BER) Assay

The mammalian cell genome is continuously exposed to endogenous and exogenous insults that modify its DNA. These modifications can be single-base lesions, bulky DNA adducts, base dimers, base alkylation, cytosine deamination, nitrosation, or other types of base alteration which interfere with DNA replication. Mammalian cells have evolved with a robust defense mechanism to repair these base modifications (damages) to preserve genomic stability. Base excision repair (BER) is the major defense mechanism for cells to remove these oxidative or alkylated single-base modifications. The base excision repair process involves replacement of a single-nucleotide residue by two sub-pathways, the single-nucleotide (SN) and the multi-nucleotide or long-patch (LP) base excision repair pathways. These reactions have been reproduced in vitro using cell free extracts or purified recombinant proteins involved in the base excision repair pathway. In the present chapter, we describe the detailed methodology to reconstitute base excision repair assay systems. These reconstitutive BER assay systems use artificially synthesized and modified DNA. These reconstitutive assay system will be a true representation of biologically occurring damages and their repair.

Roles of homologous recombination in response to ionizing radiation-induced DNA damage

PURPOSE

Ionizing radiation induces a vast array of DNA lesions including base damage, and single- and double-strand breaks (SSB, DSB). DSBs are among the most cytotoxic lesions, and mis-repair causes small- and large-scale genome alterations that can contribute to carcinogenesis. Indeed, ionizing radiation is a 'complete' carcinogen. DSBs arise immediately after irradiation, termed 'frank DSBs,' as well as several hours later in a replication-dependent manner, termed 'secondary' or 'replication-dependent DSBs. DSBs resulting from replication fork collapse are single-ended and thus pose a distinct problem from two-ended, frank DSBs. DSBs are repaired by error-prone nonhomologous end-joining (NHEJ), or generally error-free homologous recombination (HR), each with sub-pathways. Clarifying how these pathways operate in normal and tumor cells is critical to increasing tumor control and minimizing side effects during radiotherapy.

CONCLUSIONS

The choice between NHEJ and HR is regulated during the cell cycle and by other factors. DSB repair pathways are major contributors to cell survival after ionizing radiation, including tumor-resistance to radiotherapy. Several nucleases are important for HR-mediated repair of replication-dependent DSBs and thus replication fork restart. These include three structure-specific nucleases, the 3' MUS81 nuclease, and two 5' nucleases, EEPD1 and Metnase, as well as three end-resection nucleases, MRE11, EXO1, and DNA2. The three structure-specific nucleases evolved at very different times, suggesting incremental acceleration of replication fork restart to limit toxic HR intermediates and genome instability as genomes increased in size during evolution, including the gain of large numbers of HR-prone repetitive elements. Ionizing radiation also induces delayed effects, observed days to weeks after exposure, including delayed cell death and delayed HR. In this review we highlight the roles of HR in cellular responses to ionizing radiation, and discuss the importance of HR as an exploitable target for cancer radiotherapy.

The splicing component ISY1 regulates APE1 in base excision repair

The integrity of cellular genome is continuously challenged by endogenous and exogenous DNA damaging agents. If DNA damage is not removed in a timely fashion the replisome may stall at DNA lesions, causing fork collapse and genetic instability. Base excision DNA repair (BER) is the most important pathway for the removal of oxidized or mono-alkylated DNA. While the main components of the BER pathway are well defined, its regulatory mechanism is not yet understood. We report here that the splicing factor ISY1 enhances apurinic/apyrimidinic endonuclease 1 (APE1) activity, the multifunctional enzyme in BER, by promoting its 5'-3' endonuclease activity. ISY1 expression is induced by oxidative damage, which would provide an immediate up-regulation of APE1 activity in vivo and enhance BER of oxidized bases. We further found that APE1 and ISY1 interact, and ISY1 enhances the ability of APE1 to recognize abasic sites in DNA. Using purified recombinant proteins, we reconstituted BER and demonstrated that ISY1 markedly promoted APE1 activity in both the short- and long-patch BER pathways. Our study identified ISY1 as a regulator of the BER pathway, which would be of physiological relevance where suboptimal levels of APE1 are present. The interaction of ISY1 and APE1 also establishes a connection between DNA damage repair and pre-mRNA splicing.

MiR223-3p promotes synthetic lethality in BRCA1-deficient cancers

Defects in DNA repair give rise to genomic instability, leading to neoplasia. Cancer cells defective in one DNA repair pathway can become reliant on remaining repair pathways for survival and proliferation. This attribute of cancer cells can be exploited therapeutically, by inhibiting the remaining repair pathway, a process termed synthetic lethality. This process underlies the mechanism of the Poly-ADP ribose polymerase-1 (PARP1) inhibitors in clinical use, which target BRCA1 deficient cancers, which is indispensable for homologous recombination (HR) DNA repair. HR is the major repair pathway for stressed replication forks, but when BRCA1 is deficient, stressed forks are repaired by back-up pathways such as alternative nonhomologous end-joining (aNHEJ). Unlike HR, aNHEJ is nonconservative, and can mediate chromosomal translocations. In this study we have found that miR223-3p decreases expression of PARP1, CtIP, and Pso4, each of which are aNHEJ components. In most cells, high levels of microRNA (miR) 223-3p repress aNHEJ, decreasing the risk of chromosomal translocations. Deletion of the miR223 locus in mice increases PARP1 levels in hematopoietic cells and enhances their risk of unprovoked chromosomal translocations. We also discovered that cancer cells deficient in BRCA1 or its obligate partner BRCA1-Associated Protein-1 (BAP1) routinely repress miR223-3p to permit repair of stressed replication forks via aNHEJ. Reconstituting the expression of miR223-3p in BRCA1- and BAP1-deficient cancer cells results in reduced repair of stressed replication forks and synthetic lethality. Thus, miR223-3p is a negative regulator of the aNHEJ DNA repair and represents a therapeutic pathway for BRCA1- or BAP1-deficient cancers.

ASR352, A potent anticancer agent: Synthesis, preliminary SAR, and biological activities against colorectal cancer bulk, 5-fluorouracil/oxaliplatin resistant and stem cells

Despite new agent development and short-term benefits in patients with colorectal cancer (CRC), metastatic CRC cure rates have not improved due to high rates of 5-fluorouracil (5-FU)/leucovorin/oxaliplatin (FOLFOX)-resistance and a clinical therapeutic plateau. At the same time, this treatment regime leads to significant toxicity, cost, and patient inconvenience. Drug-resistance is linked to CRC stem cells, which are associated with the epidermal-to-mesenchymal transition (EMT) pathway. Thus, to optimally treat CRC, a therapy that can target the cell survival and EMT pathways in both CRC bulk and stem cell populations is critical. We recently identified a novel small molecule NSC30049 (7a) that is effective alone, and in combination potentiates 5-FU-mediated growth inhibition of CRC bulk, FOLFOX-resistant, and CRC stem cells both in vitro and in vivo models. In the present study, we report the synthesis and anti-CRC evaluation of several stable and effective 7a analogs. ASR352 (7b) was identified as one of the equipotent 7a analogs that inhibited the growth of CRC bulk cells, sensitized FOLFOX-resistant cells, and reduced the sphere formation capacity of CRC stem cells. It appears that the complex mechanism of cytotoxicity for 7b includes abrogation of 5-FU-induced the S phase, reduction of the phosphorylation of Chk1 at S317P, S345P and S296P, increased γH2AX staining, activation of caspase 3/PARP1 cleavage, and enhancement of Bax/Bcl2 ratio. Further 7b-mediated reduced phosphorylation of Chk1 was an indirect effect, since it did not inhibit Chk1 activity in an in vitro kinase assay. Our findings suggest that 7b as a single agent, or in combination with 5-FU can be developed as a therapeutic agent in CRC bulk, FOLFOX-resistant, and CRC stem cell populations for unmanageable metastatic CRC conditions.

NSC30049 inhibits Chk1 pathway in 5-FU-resistant CRC bulk and stem cell populations

The 5-fluorouracil (5-FU) treatment induces DNA damage and stalling of DNA replication forks. These stalled replication forks then collapse to form one sided double-strand breaks, leading to apoptosis. However, colorectal cancer (CRC) stem cells rapidly repair the stalled/collapsed replication forks and overcome treatment effects. Recent evidence suggests a critical role of checkpoint kinase 1 (Chk1) in preventing the replicative stress. Therefore, Chk1 kinase has been a target for developing mono or combination therapeutic agents. In the present study, we have identified a novel orphan molecule NSC30049 (NSC49L) that is effective alone, and in combination potentiates 5-FU-mediated growth inhibition of CRC heterogeneous bulk and FOLFOX-resistant cell lines in culture with minimal effect on normal colonic epithelial cells. It also inhibits the sphere forming activity of CRC stem cells, and decreases the expression levels of mRNAs of CRC stem cell marker genes. Results showed that NSC49L induces 5-FU-mediated S-phase cell cycle arrest due to increased load of DNA damage and increased γ-H2AX staining as a mechanism of cytotoxicity. The pharmacokinetic analysis showed a higher bioavailability of this compound, however, with a short plasma half-life. The drug is highly tolerated by animals with no pathological aberrations. Furthermore, NSC49L showed very potent activity in a HDTX model of CRC stem cell tumors either alone or in combination with 5-FU. Thus, NSC49L as a single agent or combined with 5-FU can be developed as a therapeutic agent by targeting the Chk1 pathway in 5-FU-resistant CRC heterogeneous bulk and CRC stem cell populations.

Synthetic lethality in malignant pleural mesothelioma with PARP1 inhibition

Malignant pleural mesotheliomas (MPM) are most often surgically unresectable, and they respond poorly to current chemotherapy and radiation therapy. Between 23 and 64% of malignant pleural mesothelioma have somatic inactivating mutations in the BAP1 gene. BAP1 is a homologous recombination (HR) DNA repair component found in the BRCA1/BARD1 complex. Similar to BRCA1/2 deficient cancers, mutation in the BAP1 gene leads to a deficient HR pathway and increases the reliance on other DNA repair pathways. We hypothesized that BAP1-mutant MPM would require PARP1 for survival, similar to the BRCA1/2 mutant breast and ovarian cancers. Therefore, we used the clinical PARP1 inhibitors niraparib and olaparib to assess whether they could induce synthetic lethality in MPM. Surprisingly, we found that all MPM cell lines examined, regardless of BAP1 status, were addicted to PARP1-mediated DNA repair for survival. We found that niraparib and olaparib exposure markedly decreased clonal survival in multiple MPM cell lines, with and without BAP1 mutations. This clonal cell death may be due to the extensive replication fork collapse and genomic instability that PARP1 inhibition induces in MPM cells. The requirement of MPM cells for PARP1 suggests that they may generally arise from defects in HR DNA repair. More importantly, these data demonstrate that the PARP1 inhibitors could be effective in the treatment of MPM, for which little effective therapy exists.

Abstract 1420: The endonuclease Metnase promotes base excision repair of Clustered abasic DNA lesions

Metnase, a human SET-transposase fusion protein contains two functional domains: a SET domain and transposase domain. Transposase domain exhibit strand transfer and end joining activity while set domain is responsible for histone lysine methyltransferase activity (KMT). Metnase also increases the efficiency of double strand break repair by non-homologous end joining (NHEJ); however, its role in the excision of clustered DNA damage remains to be investigated. We hypothesize that 5’-3’ endonuclease activity of metnase could possibly substitute for APE1 in BER and serve as an alternative initiator of BER in tumor cells lacking APE1. In the present study, we examined in vitro endonuclease activity of Metnase using abasic site containing artificial DNA, and describe its activity in BER. Its unique 5’ endonuclease activity was selective only for abasic lesions, but not other base modifications such as 8-oxoguanine, uracil, hypoxanthine or xanthenes. Metnase specifically initiates removal of reduced abasic lesions from DNA, and allows completion of short-patch or long-patch BER. APE1 has difficulty cleaving 5’ of clustered abasic lesions. This endonuclease that promotes BER in clustered oxidative DNA damage is not known. Metnase also cleaves multiple abasic lesions (clustered DNA damage) and facilitates the repair of DNA if the multiple DNA damages are 3 or more nucleotides apart on the opposing strands. However, repair of multiple DNA damages remains inefficient if these are in close proximity of each other (< 3nt apart on the opposing strands) and results in DNA double strand break (DSB). These results suggest that Metnase can promote BER of oxidative nucleotides, where APE1 is unable to function.

Citation Format: Aruna S. Jaiswal, Elizabeth A. Williamson, Bhavita Patel, Gayathri Srinivasan, Satya Narayan, Robert A. Hromas. The endonuclease Metnase promotes base excision repair of Clustered abasic DNA lesions [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1420. doi:10.1158/1538-7445.AM2017-1420

Endonuclease EEPD1 Is a Gatekeeper for Repair of Stressed Replication Forks

Replication is not as continuous as once thought, with DNA damage frequently stalling replication forks. Aberrant repair of stressed replication forks can result in cell death or genome instability and resulting transformation to malignancy. Stressed replication forks are most commonly repaired via homologous recombination (HR), which begins with 5′ end resection, mediated by exonuclease complexes, one of which contains Exo1. However, Exo1 requires free 5′-DNA ends upon which to act, and these are not commonly present in non-reversed stalled replication forks. To generate a free 5′ end, stalled replication forks must therefore be cleaved. Although several candidate endonucleases have been implicated in cleavage of stalled replication forks to permit end resection, the identity of such an endonuclease remains elusive. Here we show that the 5′-endonuclease EEPD1 cleaves replication forks at the junction between the lagging parental strand and the unreplicated DNA parental double strands. This cleavage creates the structure that Exo1 requires for 5′ end resection and HR initiation. We observed that EEPD1 and Exo1 interact constitutively, and Exo1 repairs stalled replication forks poorly without EEPD1. Thus, EEPD1 performs a gatekeeper function for replication fork repair by mediating the fork cleavage that permits initiation of HR-mediated repair and restart of stressed forks.

NSC666715 and Its Analogs Inhibit Strand-Displacement Activity of DNA Polymerase β and Potentiate Temozolomide-Induced DNA Damage, Senescence and Apoptosis in Colorectal Cancer Cells

Recently approved chemotherapeutic agents to treat colorectal cancer (CRC) have made some impact; however, there is an urgent need for newer targeted agents and strategies to circumvent CRC growth and metastasis. CRC frequently exhibits natural resistance to chemotherapy and those who do respond initially later acquire drug resistance. A mechanism to potentially sensitize CRC cells is by blocking the DNA polymerase β (Pol-β) activity. Temozolomide (TMZ), an alkylating agent, and other DNA-interacting agents exert DNA damage primarily repaired by a Pol-β-directed base excision repair (BER) pathway. In previous studies, we used structure-based molecular docking of Pol-β and identified a potent small molecule inhibitor (NSC666715). In the present study, we have determined the mechanism by which NSC666715 and its analogs block Fen1-induced strand-displacement activity of Pol-β-directed LP-BER, cause apurinic/apyrimidinic (AP) site accumulation and induce S-phase cell cycle arrest. Induction of S-phase cell cycle arrest leads to senescence and apoptosis of CRC cells through the p53/p21 pathway. Our initial findings also show a 10-fold reduction of the IC₅₀ of TMZ when combined with NSC666715. These results provide a guide for the development of a target-defined strategy for CRC chemotherapy that will be based on the mechanisms of action of NSC666715 and TMZ. This combination strategy can be used as a framework to further reduce the TMZ dosages and resistance in CRC patients.

Anti-tumor activity of novel biisoquinoline derivatives against breast cancers

Breast cancer is classified into three groups according to its expression of hormone/growth factor receptors: (i) estrogen receptor (ER) and progesterone receptor (PR)-positive; (ii) human epidermal growth factor receptor 2 (HER2)-positive; and (iii) ER, PR, and HER2-negative (triple-negative). A series of methoxy-substituted biisoquinoline compounds have been synthesized as a potential chemotherapeutic agent for the triple-negative breast cancers which are especially challenging to manage. Structure activity relationship study revealed that rigid 6,6'-dimethoxy biisoquinoline imidazolium compound (1c, DH20931) exhibited the significant growth inhibitory effects on both triple-positive and triple-negative human breast cancer cell lines with IC50 in the range of 0.3-3.9 μM. The 1c (DH20931) is more potent than structurally related noscapine for growth inhibition of MCF7 cell line (IC50=1.3 vs 57 μM) and MDA-MB231 cell line (IC50=3.9 vs 64 μM).

Abstract 2721: Novel biisoquinoimidazolium-derivatives for breast cancer therapy

Breast cancer is the most frequently diagnosed cancer in women. This cancer is classified according to its expression of hormone/growth factor receptors. It is divided into three groups: (i) Estrogen receptor (ER) and progesterone receptor (PR)-positive; (ii) Human epidermal growth factor receptor 2 (HER2)-positive; and (iii) ER, PR, and HER2-negative (triple-negative). Most chemotherapeutic anti-cancer drugs used in the clinical practice include an anti-estrogenic agent that interferes with ER and prevents tumor progression; however, it is still a big challenge to manage the triple-negative breast tumors. Mitosis and other distinct pathways of apoptosis are considered as a potential target for the development of novel anti-mitotic drugs. In the present investigation, we designed the biisoquinoimidazoliums class of drugs which is structurally similar to anti-mitotic drugs. We synthesized various derivatives of biisoquinoimidazoliums, tested their potency, and suitability as therapeutic agents in preclinical biological system using clonogenic assay. Results showed a strong growth inhibitory effect of these compounds on both triple-positive and triple-negative breast cancer cell lines in the following order: DHO2-93-1 ≥ DHO2-85-3 ≥ DHO2-75-3 ≥ DHO2-85-1. DH02-75-1 also showed some activity, while DHO2-75-2 and DHO2-107-1 did not show any growth inhibitory effect on these cells. From these results, we concluded that DHO2-93-1 may have therapeutic implication as a novel anti-tumor drug for the intervention of both triple-positive and triple-negative breast cancers.

Citation Format: Satya Narayan, Aruna S. Jaiswal, Sukwon Hong. Novel biisoquinoimidazolium-derivatives for breast cancer therapy. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2721. doi:10.1158/1538-7445.AM2014-2721

Adenomatous polyposis coli-mediated accumulation of abasic DNA lesions lead to cigarette smoke condensate-induced neoplastic transformation of normal breast epithelial cells

Adenomatous polyposis coli (APC) is a multifunctional protein having diverse cellular functions including cell migration, cell-cell adhesion, cell cycle control, chromosomal segregation, and apoptosis. Recently, we found a new role of APC in base excision repair (BER) and showed that it interacts with DNA polymerase β and 5'-flap endonuclease 1 and interferes in BER. Previously, we have also reported that cigarette smoke condensate (CSC) increases expression of APC and enhances the growth of normal human breast epithelial (MCF10A) cells in vitro. In the present study, using APC overexpression and knockdown systems, we have examined the molecular mechanisms by which CSC and its major component, Benzo[α]pyrene, enhances APC-mediated accumulation of abasic DNA lesions, which is cytotoxic and mutagenic in nature, leading to enhanced neoplastic transformation of MCF10A cells in an orthotopic xenograft model.

Abstract 4687: DNA polymerase ≤ inhibitors enhance the chemotherapeutic efficacy of Temozolomide by increasing senescence in colon cancer cells

While much is known about the genetic background of colon cancer development, surgical removal still remains the main strategy for its treatment. One of the critical strategies for cancer chemotherapy is blocking a cancer cell's ability to recognize and repair the damaged DNA. The fine balance between DNA damage and repair determines the final therapeutic consequences of these drugs. One such drug that induces DNA damage is Temozolomide (TMZ). However, tumor resistance to TMZ remains a significant problem in the treatment of colon cancer. In addition, the major limitations of the use of high doses of TMZ to maximize therapeutic efficacy in colon cancer is due to serious side effects, which makes TMZ unsuitable for colon cancer treatment. Thus, there is an urgent need for the development of new strategies to increase the therapeutic efficacy of TMZ with minimum or no side-effects. The type of DNA alkylation damage induced by TMZ is primarily repaired by DNA polymerase ≤ (Pol-α)-directed base excision repair (BER) pathway. Thus, Pol-α activity blockade can be a potential mechanism to sensitize colon cancer cells. We used structure-based molecular docking of Pol-α and identified potent small molecular inhibitors (SMI) (NSC-666715; NSC-666719) that specifically interacts with Pol-α. Our initial findings showed that the interaction of NSC-666715 with Pol-α blocks Pol-α-directed BER and enhances the TMZ-induced killing of colon cancer cells both in vitro and in vivo at 10-fold lower concentrations. We also found that the disruption of BER negates its contribution to drug-resistance and bypasses other resistance factors such as mismatch repair (MMR) defects. In the present study, we propose to determine the molecular mechanisms by which NSC-666715 and its potent derivative (NSC-666719) enhance the chemotherapeutic efficacy of TMZ against colon cancer growth. Our central hypothesis is that the blockade of Pol-α activity by NSC-666715 or NSC-666719 will significantly enhance the cytotoxic effects of TMZ-induced DNA damage that is repaired by BER pathway. Results show that NSC-666715 or NSC-666719 in combination with TMZ blocks BER pathway in vitro and increases accumulation of apurinic/apyrimidinic (AP)-site lesions in cells. Results also show that the AP-lesion accumulation is linked with p53/p21 signaling pathway. Results further show that increased cellular senescence in a p53/p21-dependent pathway might be one of the mechanisms by which NSC-666715 or NSC-666719 increases the therapeutic efficacy of TMZ in colon cancer cells. More importantly, the combination of SMI's reduces TMZ concentration, but keeps the biological outcome same, which is shown by the reduced IC50 of TMZ in combination experiments. The proposed mechanistic studies will allow us to establish the “proof-of-principle” by which Pol-α-targeted compounds enhance the efficacy and reduce the dose of TMZ.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4687. doi:1538-7445.AM2012-4687

Abstract 4201: Role of adenomatous poliposis coli (APC) in cigarette smoke condensate-induced accumulation of apurinic/apyrimidinic lesions and enhanced transformation of normal breast epithelial cells

Active and passive smoking are major risk factors for cancer and have been implicated in the etiology of breast cancer. The mechanisms through which smoking promotes breast cancer are poorly understood. Our previous studies have shown that cigarette smoke condensate (CSC), a surrogate for cigarette smoke, induces transformation of normal human breast epithelial (MCF10A) cells in culture. We have described the involvement of a multifunctional protein, adenomatous polyposis coli (APC), in the CSC-induced transformation of MCF10A cells through the interaction with DNA polymerase β and 5’-flap endonuclease 1 (Fen1), and thus, the blockade of the BER pathway. In the present study, we have examined the underlying mechanisms involved in CSC and Benzo[α]pyrene (B[α]P)-induced transformation of MCF10A cells in which APC was overexpressed or knocked down. Our results suggest a positive correlation between the increased levels of APC expression with the decreased BER activity and concomitant accumulation of apurinic/apyrimidinic (AP)-site lesions. If not repaired on time, the cytotoxic and mutagenic AP-site lesions ultimately lead to enhanced transformation of MCF10A cells. These studies suggest a possible mechanism by which active and/or passive cigarette smoking might be linked with breast carcinogenesis. This work was supported by Flight attendant Medical Research Institute, Miami, FL to SN.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 4201. doi:10.1158/1538-7445.AM2011-4201

Abstract 3524: Structure-based molecular docking of DNA polymerase β as a novel target for chemotherapeutic intervention of colorectal cancer

Colorectal cancer is the third most common cancer and the second most common cause of cancer related deaths worldwide. In the year 2010, an estimated 102,900 new colorectal cases will be diagnosed and 51,370 deaths will occur in the United States only. Chemoprevention presents a major strategy for the medical management of colorectal cancer risk. Most drugs used for colorectal cancer therapy induce DNA alkylation damage, which are primarily repaired by the base excision repair (BER) pathway. Thus, blockade of BER pathway is an attractive option to inhibit the spread of colorectal cancer. Here we used structure-based molecular docking of DNA polymerase β (Pol-β) and identified a potent anti-Pol-β compound NSC-124854. Our aim was to examine whether NSC-124854 could enhance the therapeutic efficacy of DNA-alkylating agent, Temozolomide (TMZ), by blocking BER. First, we determined the specificity of NSC-124854 for Pol-β by using in vitro activities of APE1, Fen1, DNA ligase I, and Pol-β-directed single nucleotide (SN)- and long-patch (LP)-BER. Second, we determined the effect of NSC-124854 on the cytotoxicity of TMZ to mismatch repair (MMR)-deficient and MMR-proficient colon cancer cells by using in vitro clonogenic assays. Third, we used female homozygous SCID mice as a pre-clinical xenograft model to determine the effect of NSC-124854 on the in vivo tumor growth inhibition of MMR-deficient and MMR-proficient colonic tumors in mice. Our data showed that NSC-124854 elicit specificity to Pol-β and blocked Pol-β-directed SN- and LP-BER activities in vitro reconstituted system. Furthermore, NSC-124854 effectively induced the sensitivity of TMZ to MMR-deficient and MMR-proficient colon cancer cells both in vitro cell culture system and in vivo mouse model. Our findings suggest a potential novel strategy for the development of highly specific, and thus safer structure-based inhibitor(s) for the prevention of colonic tumor progression. This work has been supported by NCI/NIH grants (R01 CA-097031 and CA-100247) to SN.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3524. doi:10.1158/1538-7445.AM2011-3524

Assembly of the base excision repair complex on abasic DNA and role of adenomatous polyposis coli on its functional activity

The assembly and stability of base excision repair (BER) proteins in vivo with abasic DNA and the role of adenomatous polyposis coli (APC) protein in this process are currently unclear. We have studied the assembly of a multiprotein BER complex onto abasic DNA (F-DNA) and characterized the physical and functional activity of the associated proteins. We found that the BER complex contained all the essential components of the long-patch BER system, such as APE1, Pol-β, Fen1, and DNA ligase I. Interestingly, wild-type APC was also present in the BER complex. Kinetics of the assembly of BER proteins onto the F-DNA were rapid and appeared in sequential order depending upon their requirement in the repair process. The presence of wild-type APC in the BER complex caused a decrease in the level of assembly of BER proteins and negatively affected long-patch BER. These results suggest that major BER proteins in the complex are assembled onto F-DNA and are competent in performing DNA repair. Wild-type APC in the BER complex reduces the repair activity, probably because of interaction with multiple components of the system.

DNA polymerase β as a novel target for chemotherapeutic intervention of colorectal cancer

Chemoprevention presents a major strategy for the medical management of colorectal cancer. Most drugs used for colorectal cancer therapy induce DNA-alkylation damage, which is primarily repaired by the base excision repair (BER) pathway. Thus, blockade of BER pathway is an attractive option to inhibit the spread of colorectal cancer. Using an in silico approach, we performed a structure-based screen by docking small-molecules onto DNA polymerase β (Pol-β) and identified a potent anti-Pol-β compound, NSC-124854. Our goal was to examine whether NSC-124854 could enhance the therapeutic efficacy of DNA-alkylating agent, Temozolomide (TMZ), by blocking BER. First, we determined the specificity of NSC-124854 for Pol-β by examining in vitro activities of APE1, Fen1, DNA ligase I, and Pol-β-directed single nucleotide (SN)- and long-patch (LP)-BER. Second, we investigated the effect of NSC-124854 on the efficacy of TMZ to inhibit the growth of mismatch repair (MMR)-deficient and MMR-proficient colon cancer cell lines using in vitro clonogenic assays. Third, we explored the effect of NSC-124854 on TMZ-induced in vivo tumor growth inhibition of MMR-deficient and MMR-proficient colonic xenografts implanted in female homozygous SCID mice. Our data showed that NSC-124854 has high specificity to Pol-β and blocked Pol-β-directed SN- and LP-BER activities in in vitro reconstituted system. Furthermore, NSC-124854 effectively induced the sensitivity of TMZ to MMR-deficient and MMR-proficient colon cancer cells both in vitro cell culture and in vivo xenograft models. Our findings suggest a potential novel strategy for the development of highly specific structure-based inhibitor for the prevention of colonic tumor progression.

Abstract 692: Antitumor activity of novel structure-based chemotherapeutic agent for the intervention of colorectal cancer

Colorectal cancer is the second most common cause of cancer-related death in both men and women in the Western hemisphere. The success of treatment of colon cancer patients depends on matching the most effective therapeutic regimen with the characteristics of the individual patient. The primary challenge in achieving this goal is the heterogeneity of the disease. In the past 10 years, the overall survival of colon cancer patients has significantly improved with adjuvant trials of drugs; the recurrence rate over 5 years is still high. Thus, there is clearly an urgent need for the development of new chemotherapeutic drugs and strategies. The recent emerging concept to sensitize cancer cells to DNA-alkylating drugs is by inhibiting various proteins in the base excision repair (BER) pathway. We used structure-based molecular docking of DNA polymerase β (Pol-β) and identified a potent small molecular weight inhibitor (SMI), NSC-666715. We determined the specificity of this SMI for Pol-β by using in vitro activities of APE1, Fen1, DNA ligase I, and Pol-β-directed single nucleotide (SN)- and long-patch (LP)-BER. The binding specificity of NSC-666715 with Pol-β was also determined by using fluorescence anisotropy. The effect of NSC-666715 on the cytotoxicity of the DNA-alkylating drug, Temozolomide (TMZ), to colon cancer cells was determined by in vitro clonogenic and in vivo xenograft assays. The reduction in tumor growth was higher in the combination treatment relative to untreated or monotherapy treatment. NSC-666715 showed a high specificity for blocking Pol-β activity. It blocked Pol-β-directed SN- and LP-BER without affecting the activity of APE1, Fen1 and DNA ligase I. Fluorescence anisotropy data suggested that NSC-666715 directly and specifically interacts with Pol-β and interferes with binding to damaged DNA. NSC-666715 drastically induces the sensitivity of TMZ to colon cancer cells both in vitro and in vivo assays. The results further suggest that the disruption of BER by NSC-666715 negates its contribution to drug-resistance and bypasses other resistance factors, such as mismatch repair defects. Our findings provide the “proof-of-concept” for the development of highly specific and thus safer structure-based inhibitors for the prevention of tumor progression and/or treatment of colorectal cancer. Funded by NCI-NIH grants CA-097031 and CA-100247 to S.N.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 692.

A novel inhibitor of DNA polymerase beta enhances the ability of temozolomide to impair the growth of colon cancer cells

The recent emerging concept to sensitize cancer cells to DNA-alkylating drugs is by inhibiting various proteins in the base excision repair (BER) pathway. In the present study, we used structure-based molecular docking of DNA polymerase beta (Pol-beta) and identified a potent small molecular weight inhibitor, NSC-666715. We determined the specificity of this small molecular weight inhibitor for Pol-beta by using in vitro activities of APE1, Fen1, DNA ligase I, and Pol-beta-directed single-nucleotide and long-patch BER. The binding specificity of NSC-666715 with Pol-beta was also determined by using fluorescence anisotropy. The effect of NSC-666715 on the cytotoxicity of the DNA-alkylating drug temozolomide (TMZ) to colon cancer cells was determined by in vitro clonogenic and in vivo xenograft assays. The reduction in tumor growth was higher in the combination treatment relative to untreated or monotherapy treatment. NSC-666715 showed a high specificity for blocking Pol-beta activity. It blocked Pol-beta-directed single-nucleotide and long-patch BER without affecting the activity of APE1, Fen1, and DNA ligase I. Fluorescence anisotropy data suggested that NSC-666715 directly and specifically interacts with Pol-beta and interferes with binding to damaged DNA. NSC-666715 drastically induces the sensitivity of TMZ to colon cancer cells both in in vitro and in vivo assays. The results further suggest that the disruption of BER by NSC-666715 negates its contribution to drug resistance and bypasses other resistance factors, such as mismatch repair defects. Our findings provide the "proof-of-concept" for the development of highly specific and thus safer structure-based inhibitors for the prevention of tumor progression and/or treatment of colorectal cancer.

Amino acid Asp181 of 5'-flap endonuclease 1 is a useful target for chemotherapeutic development

DNA alkylation-induced damage is one of the most efficacious anticancer therapeutic strategies. Enhanced DNA alkylation and weakened DNA repair capacity in cancer cells are responsible for the effectiveness of DNA-alkylating therapies. 5'-Flap endonuclease 1 (Fen1) is an important enzyme involved in base excision repair (BER), specifically in long-patch BER (LP-BER). Using the site-directed mutagenesis approach, we have identified an important role for amino acid Asp181 of Fen1 in its endonuclease activity. Asp181 is thought to be involved in Mg(2+) binding in the active site. Using structure-based molecular docking of Fen1 targeted to its metal binding pocket M2 (Mg(2+) site), we have identified a potent low-molecular weight inhibitor (LMI, NSC-281680) that efficiently blocks LP-BER. In this study, we have demonstrated that the interaction of this LMI with Fen1 blocked its endonuclease activity, thereby blocking LP-BER and enhancing the cytotoxic effect of DNA-alkylating agent Temozolomide (TMZ) in mismatch repair (MMR)-deficient and MMR-proficient colon cancer cells. The results further suggest that blockade of LP-BER by NSC-281680 may bypass other drug resistance mechanisms such as mismatch repair (MMR) defects. Therefore, our findings provide groundwork for the development of highly specific and safer structure-based small molecular inhibitors targeting the BER pathway, which can be used along with existing chemotherapeutic agents, like TMZ, as combination therapy for the treatment of colorectal cancer.

Genome based cell population heterogeneity promotes tumorigenicity: the evolutionary mechanism of cancer

Cancer progression represents an evolutionary process where overall genome level changes reflect system instability and serve as a driving force for evolving new systems. To illustrate this principle it must be demonstrated that karyotypic heterogeneity (population diversity) directly contributes to tumorigenicity. Five well characterized in vitro tumor progression models representing various types of cancers were selected for such an analysis. The tumorigenicity of each model has been linked to different molecular pathways, and there is no common molecular mechanism shared among them. According to our hypothesis that genome level heterogeneity is a key to cancer evolution, we expect to reveal that the common link of tumorigenicity between these diverse models is elevated genome diversity. Spectral karyotyping (SKY) was used to compare the degree of karyotypic heterogeneity displayed in various sublines of these five models. The cell population diversity was determined by scoring type and frequencies of clonal and non-clonal chromosome aberrations (CCAs and NCCAs). The tumorigenicity of these models has been separately analyzed. As expected, the highest level of NCCAs was detected coupled with the strongest tumorigenicity among all models analyzed. The karyotypic heterogeneity of both benign hyperplastic lesions and premalignant dysplastic tissues were further analyzed to support this conclusion. This common link between elevated NCCAs and increased tumorigenicity suggests an evolutionary causative relationship between system instability, population diversity, and cancer evolution. This study reconciles the difference between evolutionary and molecular mechanisms of cancer and suggests that NCCAs can serve as a biomarker to monitor the probability of cancer progression.

9-bromonoscapine-induced mitotic arrest of cigarette smoke condensate-transformed breast epithelial cells

In the present investigation, we determined the chemotherapeutic efficacy of 9-bromonoscapine (Br-Nos), a more potent noscapine analog, on MCF10A, spontaneously immortalized human normal breast epithelial cells and MCF10A-CSC3, cigarette smoke condensate (CSC)-transformed cells. The results from cytogenetic analysis showed that Br-Nos induced polyploidy and telomeric association in MCF10A-CSC3 cells, while MCF10A cells remained unaffected. Our immunofluorescence data further demonstrated that MCF10A-CSC3 cells were susceptible to mitotic catastrophe on exposure to Br-Nos and failed to recover after drug withdrawal. MCF10A-CSC3 cells exhibited Br-Nos-induced aberrant multipolar spindle formation, which irreversibly impaired the alignment of replicated chromosome to the equatorial plane and finally culminated in cell death. Although MCF10A cells upon Br-Nos treatment showed bipolar spindles with some uncongressed chromosomes, these cells recovered fairly well after drug withdrawal. Our flow-cytometry analysis data reconfirmed that MCF10A-CSC3 cells were more susceptible to cell death compared to MCF10A cells. Furthermore, our results suggest that decreased levels of cdc2/cyclin B1 and cdc2 kinase activity are responsible for Br-Nos-induced mitotic cell arrest leading to cell death in MCF10A-CSC3 cells. This study thus explores the underlying mechanism of Br-Nos-induced mitotic catastrophe in CSC-transformed MCF10A-CSC3 cells and its potential usefulness as a chemotherapeutic agent for prevention of cigarette smoke-induced breast cancer growth.

C/EBPbeta-mediated transcriptional regulation of bcl-xl gene expression in human breast epithelial cells in response to cigarette smoke condensate

In previous studies, we have shown that cigarette smoke condensate (CSC), a surrogate for cigarette smoke, is capable of transforming the spontaneously immortalized human breast epithelial cell line, MCF10A. These transformed cells displayed upregulation of the anti-apoptotic gene, bcl-xl. Upregulation of this gene may impede the apoptotic pathway and allow the accumulation of DNA damage that can lead to cell transformation and carcinogenesis. In the present study, we have determined the mechanism of CSC-mediated transcriptional upregulation of bcl-xl gene expression in MCF10A cells. We cloned the human bcl-xl promoter (pBcl-xLP) and identified putative transcription factor binding sites. Sequential deletion constructs that removed the putative cis-elements were constructed and transfected into MCF10A cells to determine the CSC-responsive cis-element(s) on the pBcl-xLP. Gel-shift, supershift, and chromatin immunoprecipitation (ChIP) analysis confirmed that C/EBPβ specifically bound to a C/EBP-binding site on the pBcl-xLP in vitro and in vivo. Additionally, overexpression of C/EBPβ-LAP2 stimulated pBcl-xLP activity and Bcl-xL protein levels, which mimicked the conditions of CSC treatment. Our results indicate that C/EBPβ regulates bcl-xl gene expression in MCF10A cells in response to CSC treatment, therefore making it a potential target for chemotherapeutic intervention of cigarette smoke-induced breast carcinogenesis.

Structure/function analysis of the interaction of adenomatous polyposis coli with DNA polymerase beta and its implications for base excision repair

Mutations in the adenomatous polyposis coli (APC) gene are associated with an early onset of colorectal carcinogenesis. Previously, we described a novel role for the APC polypeptide in base excision repair (BER). The single-nucleotide (SN) and long-patch (LP) BER pathways act to repair the abasic sites in DNA that are induced by stressors, such as spontaneous oxidation/reduction, alkylation, and hyperthermia. We have shown that APC interacts with DNA polymerase beta (Pol-beta) and flap endonuclease 1 (Fen-1) and blocks Pol-beta-directed strand-displacement synthesis. In this study, we have mapped the APC interaction site in Pol-beta and have found that Thr79, Lys81, and Arg83 of Pol-beta were critical for its interaction with APC. The Pol-beta protein (T79A/K81A/R83A) blocked strand-displacement DNA synthesis in which tetrahydrofuran was used as DNA substrate. We further showed that the APC-mediated blockage of LP-BER was due to inhibition of Fen-1 activity. Analysis of the APC-mediated blockage of SN-BER indicated that the interaction of APC with Pol-beta blocked SN-BER activity by inhibiting Pol-beta-directed deoxyribose phosphate lyase activity. Collectively, our findings indicate that APC blocked both Pol-beta-directed SN- and LP-BER pathways and increased sensitivity of cells to alkylation induced DNA damage.

Cigarette smoke condensate-induced level of adenomatous polyposis coli blocks long-patch base excision repair in breast epithelial cells

Our previous studies have shown that treatment with cigarette smoke condensate (CSC) transforms normal breast epithelial cell line, MCF-10A. In the present study, the mechanism of CSC-induced transformation of breast epithelial cells was examined. We first determined whether benzo[a]pyrene (B[a]P)- and CSC-induced levels of APC are capable of inhibiting long-patch base excision repair (LP-BER) since our earlier studies had shown that an interaction of APC with DNA polymerase beta (pol-beta) blocks strand-displacement synthesis. With the use of a novel in vivo LP-BER assay, it was demonstrated that increased and decreased APC levels in different breast cancer cell lines were associated with a decrease or increase in LP-BER activity, respectively. The effect of APC on LP-BER in malignant and pre-malignant breast epithelial cell lines was produced by either overexpression or knockdown of APC. Furthermore, it was shown that the decreased LP-BER in B[a]P- or CSC-treated pre-malignant breast epithelial cells is associated with an increased level of APC and decreased cell growth. Our results suggest that the decreased growth allows cells to repair the damaged DNA before mitosis, and failure to repair damaged DNA has the potential to transform pre-malignant breast epithelial cells.

Adenomatous polyposis coli-mediated hypersensitivity of mouse embryonic fibroblast cell lines to methylmethane sulfonate treatment: implication of base excision repair pathways

The role of adenomatous polyposis coli (APC) has been implicated in various cellular functions including cell migration, cell-cell adhesion, cell cycle control, chromosomal segregation and apoptosis. Recently, we discovered a novel role of APC in DNA base excision repair (BER) and showed that APC interacts with DNA polymerase beta (Pol-beta) and flap endonuclease 1 and interferes long-patch base excision repair (LP-BER) by blocking strand displacement synthesis. Many times, the chemotherapeutic drugs induce DNA alkylation damage, which is primarily repaired by the BER pathway. Thus, the efficacy of such drugs can be increased by APC resulting in the blockage of LP-BER. In the present study, we tested this hypothesis by using isogenic wild-type and Pol-beta-knockout mouse embryonic fibroblast (MEF) cell lines in which the Apc gene was knocked down by the small interfering RNA technique and treated with methylmethane sulfonate (MMS). The MEF-Apc(WT)/Polbeta-/- cells were hypersensitive to MMS treatment compared with the MEF-Apc(WT)/Polbeta+/+ cells. However, once the Apc gene was knocked down, these cells became more resistant to MMS treatment, suggesting that the MMS-induced hypersensitivity was associated with Apc. We then determined whether the hypersensitivity of MEF-Apc(WT)/Polbeta-/- and MEF-Apc(WT)/Polbeta+/+ cell lines were due to decreased Pol-beta-independent and Pol-beta-dependent LP-BER pathways, respectively. The results of in vivo and in vitro LP-BER assays supported our findings. Furthermore, Apc-mediated hypersensitivity to MMS treatment was correlated with increased apoptosis of MEF-Apc(WT)/Polbeta-/- and MEF-Apc(WT)/Polbeta+/+ as compared with MEF-Apc(KD)/Polbeta-/- and MEF-Apc(KD)/Polbeta+/+ cells. These results suggest that an increased level of Apc can increase the efficacy of DNA-alkylating drugs used as a curative therapy.

Mechanism of adenomatous polyposis coli (APC)-mediated blockage of long-patch base excision repair

Recently, we found an interaction between adenomatous polyposis coli (APC) and DNA polymerase beta (pol-beta) and showed that APC blocks strand-displacement synthesis of long-patch base excision repair (LP-BER) (Narayan, S., Jaiswal, A. S., and Balusu, R. (2005) J. Biol. Chem. 280, 6942-6949); however, the mechanism is not clear. Using an in vivo LP-BER assay system, we now show that the LP-BER is higher in APC-/- cells than in APC+/+ cells. In addition to pol-beta, the pull-down experiments showed that the full-length APC also interacted with flap endonuclease 1 (Fen-1). To further characterize the interaction of APC with pol-beta and Fen-1, we performed a domain-mapping of APC and found that both pol-beta and Fen-1 interact with a 138-amino acids peptide from the APC at the DRI-domain. Our functional assays showed that APC blocks pol-beta-mediated 1-nucleotide (1-nt) as well as strand-displacement synthesis of reduced abasic, nicked-, or 1-nt gapped-DNA substrates. Further studies demonstrated that APC blocks 5'-flap endonuclease as well as the 5'-3' exonuclease activity of Fen-1 resulting in the blockage of LP-BER. From these results, we concluded that APC can have three different effects on the LP-BER pathway. First, APC can block pol-beta-mediated 1-nt incorporation and strand-displacement synthesis. Second, APC can block LP-BER by blocking the coordinated formation and removal of the strand-displaced flap. Third, APC can block LP-BER by blocking hit-and-run synthesis. These studies will have important implications for APC in DNA damage-induced carcinogenesis and chemoprevention.

7,12-Dimethylbenzanthracene-dependent transcriptional regulation of adenomatous polyposis coli (APC) gene expression in normal breast epithelial cells is mediated by GC-box binding protein Sp3

In the present investigation, we have examined the transcriptional regulation of adenomatous polyposis coli (APC) gene expression in the spontaneously immortalized human normal breast epithelial cell line, MCF10A, in response to carcinogen 7,12-dimethylbenz(a)anthracene (DMBA) treatment. The APC mRNA levels and the APC gene's promoter (pAPCP) activity were increased in MCF10A cells after treatment with DMBA. A sequential deletion analysis and site-directed mutagenesis of the pAPCP promoter revealed that the DMBA response is mediated through a GC-box element. Also, the GC-box binding agent mithramycin A, which prevents binding of proteins to the GC-box region, abolished DMBA-mediated increase of the pAPCP promoter activity. The specificity of the proteins binding to the GC-box region was characterized by gel-shift analysis. An increased binding of the GC-box binding proteins was observed in the gel-shift analysis with nuclear extracts from DMBA-treated MCF10A cells, which corresponded to the increased levels of Sp1 and Sp3 proteins. However, a super-shift of the DNA-protein complexes was observed with only anti-Sp3 antibody. Based on the chromatin-immunoprecipitation assay results, the Sp3 appeared to be a genuine protein binding to the GC-box site of the pAPCP promoter. In RNA interference experiments, in which the Sp3 expression was knocked down, the DMBA response on the pAPCP promoter activity was reduced, suggesting that the binding of Sp3 to the GC-box site is critical for DMBA-induced pAPCP promoter activity. From these results we conclude that the increased pAPCP promoter activity in the MCF10A cell line in response to DMBA treatment is mediated by Sp3.

Zinc stabilizes adenomatous polyposis coli (APC) protein levels and induces cell cycle arrest in colon cancer cells

In the present study, we investigated the mechanisms by which zinc causes growth arrest in colon cancer cells. The results suggest that zinc treatment stabilizes the levels of the wild-type adenomatous polyposis coli (APC) protein at the post-translational level since the APC mRNA levels and the promoter activity of the APC gene were decreased in HCT-116 cells (which express the wild-type APC gene) after treatment with ZnCl2. Increased levels of wild-type but not truncated APC proteins were required for the ZnCl2-mediated G2/M phase arrest in different colon cancer cell lines. We further tested whether serum-stimulation, which induces cell cycle arrest in the S phase, can relieve ZnCl2-induced G2/M phase arrest of HCT-116 cells. Results showed that in the HCT-116 cells pretreated with ZnCl2, the serum-stimulation neither changed the distribution of G2/M phase arrested cells nor the increased levels of APC protein. The G2/M phase arrest correlated with retarded growth of HCT-116 cells. To further establish that wild-type APC protein plays a role in ZnCl2-induced G2/M arrest, we treated SW480 colon cancer cells that express truncated APC protein. We found that ZnCl2 treatment did not induce G2/M phase arrest in SW480 cells; however, the cell growth was retarded due to the loss of E-cadherin and alpha-tubulin levels. These results suggest that ZnCl2 inhibits the proliferation of colon cancer cells (which carry the wild-type APC gene) through stabilization of the APC protein and cell cycle arrest in the G2/M phase. On the other hand, ZnCl2 inhibits the proliferation of colon cancer cells (which carry the mutant APC gene) by disrupting cellular attachment and microtubule stability.

Involvement of adenomatous polyposis coli in colorectal tumorigenesis

Colorectal cancer arises after a series of mutations in various tumor suppressor and proto-oncogenes, each of which is accompanied by specific alterations and pathological conditions. Recent advances have contributed a great deal of understanding of the molecular basis of events that lead to colorectal tumorigenesis. Mutation in the adenomatous polyposis coli (APC) gene is considered to be one of the earliest events in the colon cancer development. The familial adenomatous polyposis (FAP) and hereditary nonpolyposis colorectal cancer (HNPCC) are the most commonly inherited colorectal cancers. FAP and HNPCC develop due to mutations in APC and DNA mismatch repair (MMR) genes, respectively. APC is known to regulate the levels of beta-catenin, an important mediator of cell-cell adhesion and transcriptional regulator. Mutations in APC gene are also linked with chromosomal instability in colon cancer cells. The role of APC is also implicated in cell migration, cell-cell adhesion, cell cycle control, and apoptosis. This article summarizes the structure-function studies and the role of APC mutations in colon cancer development.

Tumor suppressor APC blocks DNA polymerase beta-dependent strand displacement synthesis during long patch but not short patch base excision repair and increases sensitivity to methylmethane sulfonate

In the present investigation, we report a previously unsuspected function of the tumor suppressor protein, APC (adenomatous polyposis coli), in the regulation of base excision repair (BER). We identified a proliferating cell nuclear antigen-interacting protein-like box sequence in APC that binds DNA polymerase beta and blocks DNA polymerase beta-mediated strand-displacement synthesis in long patch BER without affecting short patch BER. We further showed that the colon cancer cell line expressing the wild-type APC gene was more sensitive to a DNA-methylating agent due to decreased DNA repair by long patch BER than the cell line expressing the mutant APC gene lacking the proliferating cell nuclear antigen-interacting protein-like box. Experiments based on RNA interference showed that the wild-type APC gene expression is required for DNA methylation-induced sensitivity of colon cancer cells. Thus, APC may play a critical role in determining utilization of long versus short patch BER pathways and affect the susceptibility of colon cancer cells to carcinogenic and chemotherapeutic agents.

Reduced levels of the adenomatous polyposis coli (APC) protein are associated with ceramide-induced apoptosis of colon cancer cells

PURPOSE

Mutations of the adenomatous polyposis coli (APC) and p53 genes are commonly found in colorectal cancers. We therefore analyzed the relative roles of APC and p53 in the induction of apoptosis of colon cancer cells by comparing the effects of the natural chemopreventive agent, C(2)-ceramide, on different human colon cancer cell lines with and without wild-type p53 and APC genes.

METHODS

We studied the effect of C(2)-ceramide and C(2)-dihydroceramide on proliferation and/or apoptosis of colon cancer cell lines in vitro and determined the role of p53 and APC proteins in these processes. The protein and mRNA levels in colon cancer cell lines with and without treatments were determined by Western and Northern blot analysis, respectively. The cell cycle and apoptosis profiles were determined by FACS analysis and PARP-1 cleavage.

RESULTS

Our findings indicate that C(2)-ceramide can induce apoptosis independently of the p53/p21(Waf-1/Cip-1) pathway. In addition, the C(2)-ceramide induction of apoptosis showed a correlation with a reduction in the levels of the APC protein and mRNA. Moreover, the C(2)-ceramide-induced apoptosis was blocked by pre-treatment with ZnCl(2), which stabilizes APC protein levels.

CONCLUSIONS

These results suggest that C(2)-ceramide treatment reduces the levels of APC protein and that the reduction in the levels of this protein plays a key role in the ability of C(2)-ceramide to induce apoptosis of colon cancer cells.

Cigarette smoke condensate-induced transformation of normal human breast epithelial cells in vitro

In the present study, we showed that a single-dose treatment of normal breast epithelial cell line, MCF10A, for 72 h with cigarette smoke condensate (CSC) resulted in a transformed phenotype. The anchorage-dependent growth of these cells was decreased due to increased cell cycle arrest in S-G2/M phase; however, the surviving cells developed resistance due to an increased Bcl-xL to Bax ratio. Levels of PCNA and gadd45 proteins--involved in DNA repair in response to genomic damage--were increased, suggesting that the cells were responding to CSC-induced genomic damage. The transformation of MCF10A cells was determined by their colony-forming efficiency in soft-agar in an anchorage-independent manner. CSC-treated MCF10A cells efficiently formed colonies in soft-agar. We then re-established cell lines from the soft-agar colonies and further examined the persistence of their transforming characteristics. The re-established cell lines, when plated after 17 passages without CSC treatment, still formed colonies in the soft-agar. An increased staining of neuropilin-1 (NRP-1) further showed a transformation characteristic of MCF10A cells treated with CSC. In summary, our results suggest that CSC is capable of transforming the MCF10A cells in vitro, supporting the role of cigarette smoking and increased risk for breast cancer.

N-methyl-N'-nitro-N-nitrosoguanidine-induced senescence-like growth arrest in colon cancer cells is associated with loss of adenomatous polyposis coli protein, microtubule organization, and telomeric DNA

BACKGROUND

Cellular senescence is a state in which mammalian cells enter into an irreversible growth arrest and altered biological functions. The senescence response in mammalian cells can be elicited by DNA-damaging agents. In the present study we report that the DNA-damaging agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) is able to induce senescence in the HCT-116 colon cancer cell line.

RESULTS

Cells treated with lower concentrations of MNNG (0-25 microM) for 50 h showed a dose-dependent increase in G2/M phase arrest and apoptosis; however, cells treated with higher concentrations of MNNG (50-100 microM) showed a senescence-like G0/G1 phase arrest which was confirmed by increased expression of beta-galactosidase, a senescence induced marker. The G2/M phase arrest and apoptosis were found to be associated with increased levels of p53 protein, but the senescence-like G0/G1 phase arrest was dissociated with p53 protein levels, since the p53 protein levels decreased in senescence-like arrested cells. We further, determined whether the decreased level of p53 was a transcriptional or a translational phenomenon. The results revealed that the decreased level of p53 protein in senescence-like arrested cells was a transcriptional phenomenon since p53 mRNA levels simultaneously decreased after treatment with higher concentrations of MNNG. We also examined the effect of MNNG treatment on other cell cycle-related proteins such as p21, p27, cyclin B1, Cdc2, c-Myc and max. The expression levels of these proteins were increased in cells treated with lower concentrations of MNNG, which supported the G2/M phase arrest. However, cells treated with higher concentrations of MNNG showed decreased levels of these proteins, and hence, may not play a role in cell cycle arrest. We then examined a possible association of the expression of APC protein and telomeric DNA signals with cellular senescence in MNNG-treated cells. We found that protein and mRNA levels of APC were drastically reduced in cells treated with higher concentrations of MNNG. The loss of APC expression might lead to chromosomal instability as well as microtubular disorganization through its dissociation with tubulin. In fact, the protein level of alpha-tubulin was also drastically decreased in senescence-like arrested cells treated with higher concentrations of MNNG. The levels of telomeric DNA also decreased in cells treated with higher concentrations of MNNG.

CONCLUSIONS

These results suggest that in response to DNA alkylation damage the senescence-like arrest of HCT-116 cells was associated with decreased levels of APC protein, microtubular organization, and telomeric DNA.

Beta-catenin-mediated transactivation and cell-cell adhesion pathways are important in curcumin (diferuylmethane)-induced growth arrest and apoptosis in colon cancer cells

The development of nontoxic natural agents with chemopreventive activity against colon cancer is the focus of investigation in many laboratories. Curcumin (feruylmethane), a natural plant product, possesses such chemopreventive activity, but the mechanisms by which it prevents cancer growth are not well understood. In the present study, we examined the mechanisms by which curcumin treatment affects the growth of colon cancer cells in vitro. Results showed that curcumin treatment causes p53- and p21-independent G(2)/M phase arrest and apoptosis in HCT-116(p53(+/+)), HCT-116(p53(-/-)) and HCT-116(p21(-/-)) cell lines. We further investigated the association of the beta-catenin-mediated c-Myc expression and the cell-cell adhesion pathways in curcumin-induced G(2)/M arrest and apoptosis in HCT-116 cells. Results described a caspase-3-mediated cleavage of beta-catenin, decreased transactivation of beta-catenin/Tcf-Lef, decreased promoter DNA binding activity of the beta-catenin/Tcf-Lef complex, and decreased levels of c-Myc protein. These activities were linked with decreased Cdc2/cyclin B1 kinase activity, a function of the G(2)/M phase arrest. The decreased transactivation of beta-catenin in curcumin-treated HCT-116 cells was unpreventable by caspase-3 inhibitor Z-DEVD-fmk, even though the curcumin-induced cleavage of beta-catenin was blocked in Z-DEVD-fmk pretreated cells. The curcumin treatment also induced caspase-3-mediated degradation of cell-cell adhesion proteins beta-catenin, E-cadherin and APC, which were linked with apoptosis, and this degradation was prevented with the caspase-3 inhibitor. Our results suggest that curcumin treatment impairs both Wnt signaling and cell-cell adhesion pathways, resulting in G(2)/M phase arrest and apoptosis in HCT-116 cells.

Long-patch base excision repair of apurinic/apyrimidinic site DNA is decreased in mouse embryonic fibroblast cell lines treated with plumbagin: involvement of cyclin-dependent kinase inhibitor p21Waf-1/Cip-1

Molecular interactions among cell cycle and DNA repair proteins have been described, but the impact of many of these interactions on cell cycle control and DNA repair remains unclear. The cyclin-dependent kinase inhibitor, p21, is known to be involved in DNA damage-induced cell cycle arrest and blocking DNA replication and repair. Participation of p21 has been implicated in nucleotide excision repair. However, the role of p21 in the base excision repair (BER) pathway has not been thoroughly studied. In the present investigation, we treated isogenic mouse embryonic fibroblast (MEF) cell lines containing wild-type (MEF-polbeta) or DNA polymerase beta (polbeta) gene-knockout (MEFpolbetaKO) with oxidative DNA-damaging agent, plumbagin, and examined its effect on p21 levels and BER activity. Plumbagin treatment caused a S-G(2)/M phase arrest and cell death of both MEF cell lines, induced p21 levels, and decreased p21-mediated long-patch (LP) BER by blocking DNA ligase activity in the polbeta-dependent pathway and by blocking both FEN1 and DNA ligase activity in polbeta-independent pathway. These findings suggest that plumbagin induced p21 levels play a regulatory role in cell cycle arrest, apoptosis, and polbeta-dependent and -independent LP-BER pathways in MEF cells.

SN2 DNA-alkylating agent-induced phosphorylation of p53 and activation of p21 gene expression

p53 is an important player in the cellular response to genotoxic stress whose functions are regulated by phosphorylation of a number of serine and threonine residues. Phosphorylation of p53 influences its DNA-binding and gene regulation activities. This study examines p53 phosphorylation in HCT-116 (MMR-deficient) and HCT-116+ch3 (MMR-proficient) human colon cancer cells treated with a S(N)2 DNA-alkylating agent, methylmethane sulfonate (MMS). MMS induces phosphorylation of p53 on Ser15 and Ser392 in a dose- and time-dependent manner. MMS-induced p53 phosphorylation is independent of DNA mismatch repair (MMR) activity. Nuclear extracts from MMS-treated HCT-116 cells had higher p21WAF1/Cip1 (p21) promoter DNA-binding activity in vitro opposed to untreated cells. After MMS treatment, the activation of the cloned p21 promoter in a transient transfection assay and endogenous p21 mRNA levels in HCT-116(p53+/+) versus HCT-116(p53-/-) cells increased, which correlates with an increased levels of phospho-p53(Ser15) and phospho-p53(Ser392). These results suggest that SN2 DNA-alkylating agent-induced phosphorylation of p53 on Ser15 and Ser392 increases its DNA-binding properties to cause an increased expression of p21 that may play a role in cell cycle arrest and/or apoptosis of HCT-116 cells.

DNA alkylation-induced phosphorylation of p53 and activation of kinases in colon cancer cells

In response to DNA damage, p53 protein transiently stabilizes and accumulates in the nucleus, where it performs its role as a transcription factor. Phosphorylation of p53 increases its sequence-specific DNA-binding activity. In the present study, we have examined the effect of methylmethane sulfonate (MMS) to HCT-116 human colon cancer cells on the phosphorylation of p53. Results show that p53 protein becomes phosphorylated at serine 15 (Ser15) and Ser392 residues after treatment with MMS in a time-dependent manner. Increased levels of phospho-p53(Ser15) and phospho-p53(Ser392) were maintained up to 50 h of the MMS treatment. We also examined the involvement of probable kinase(s), which could be responsible for MMS-induced phosphorylation of p53 at Ser15 and Ser392. In vitro phosphorylation assay, carried out with the immunoprecipates of MMS-treated cells, showed an increased phosphorylation of p53 by c-Jun kinase 1 (JNK1) at early time points (2.5 h). However, with cyclin-dependent kinase (Cdk2) and TFIIH complex associated kinase CAK, the phosphorylation of p53 was increased at later time points (25 h). The phosphorylation of p53 by Cdc2 and MAPK (p38) kinases remained unaffected in the MMS-treated versus untreated cells. The MMS-induced phosphorylation of p53 correlates with our previous findings of p53's ability for increased sequence-specific DNA-binding and transcriptional activity in the cells treated with DNA alkylating agents.

DNA damage-induced cell cycle checkpoints involve both p53-dependent and -independent pathways: role of telomere repeat binding factor 2

Treatment of colon cancer cells with MNNG causes DNA damage with reduced telomeric signals in a p53-dependent manner, but increased cell cycle arrest in S-G(2)/M by both p53-dependent and independent mechanisms. Results also indicate that cellular levels of TRF2 may play a critical role in MNNG-induced cell cycle arrest and apoptosis of colon cancer cells.

Upstream stimulating factor-1 (USF1) and USF2 bind to and activate the promoter of the adenomatous polyposis coli (APC) tumor suppressor gene

The adenomatous polyposis coli (APC) gene product is involved in cell cycle arrest and apoptosis, and loss of function is associated with the development of colorectal carcinogenesis. Although it has been demonstrated that the APC gene is inducible, its transcriptional regulation has not been elucidated. Therefore, we characterized the promoter region of the APC gene and transcription factors required for basal expression. The APC gene has a TATA-less promoter and contains consensus binding sites for Octamer, AP2, Sp1, a CAAT-box, and three nucleotide sequences for E-box A, B, and M. The E-boxes are functional in several cancer cell lines and upstream stimulating factor-1 (USF1) and USF2 interact with these sites, with a preferred sequence-specificity for the B site. Analysis of activation of the cloned APC promoter by USF1 and USF2 in transient transfection assays in HCT-116 cells demonstrated that mutation of the E-box B site completely abolished the basal promoter activity. Further, the ectopic USF1 and USF2 expression in HCT-116 cells with deletion mutations of E-box A, B, and M sites showed that these E-boxes contribute to USF1- and USF2-mediated transcriptional activation of the APC promoter, with maximum promoter activity being associated with the E-box B site. Thus, USF1 and USF2 transcription factors are critical for APC gene expression.

p53-dependent transcriptional regulation of the APC promoter in colon cancer cells treated with DNA alkylating agents

The APC (adenomatous polyposis coli) gene product is involved in cell cycle arrest and in apoptosis. The loss of APC function is associated with the development of colorectal carcinogenesis. In previous studies, we have shown that the APC gene is inducible and that the DNA damage-induced level of APC mRNA requires p53. In the present study, we examined the role of p53 in the transcriptional regulation of APC promoter and characterized two p53-binding sites on the cloned APC promoter (pAPCP). Results of electrophoretic mobility shift assay showed specific interactions of p53 protein with p53-binding site oligonucleotides. The DNA-protein complex formed in electrophoretic mobility shift assay was competed with unlabeled excess of p53-binding site oligonucleotide, unaffected with p53-binding site mutant or Sp1-binding site oligonucleotides, and supershifted with anti-p53 antibodies. In a transient transfection assay, the pAPCP promoter activity was lower in HCT-116(p53(+/+)) cells versus HCT-116(p53(-/-)) cells. p53-dependent down-regulation was further confirmed after co-transfection of pAPCP plasmid with pCMV-p53 into HCT-116(p53(-/-)) and SAOS-2 (p53-negative) cells. However, the treatment of cells with DNA alkylating agents methylmethane sulfonate and N-methyl-N'-nitro-N-nitrosoguanidine, which cause phosphorylation of p53 at Ser(15) and Ser(392), induced pAPCP promoter activity in HCT-116(p53(+/+)) cells. Other than p53-binding sites, using deletion mutation constructs, we have shown that N-methyl-N'-nitro-N-nitrosoguanidine-induced transcriptional activation of the pAPCP promoter in HCT-116(p53(+/+)) cells depended upon the Sp1-binding site and the E-box B site. From these results, we conclude that unphosphorylated p53 can down-regulate and phosphorylated p53 can up-regulate the pAPCP promoter activity involving the p53, Sp1, or E-box B elements. These studies are important to understanding the role of p53 and APC in DNA damage-induced cell cycle arrest and/or apoptosis of cancer cells.

Upstream stimulating factor-1 (USF1) and USF2 bind to and activate the promoter of the adenomatous polyposis coli (APC) tumor suppressor gene

The adenomatous polyposis coli (APC) gene product is involved in cell cycle arrest and apoptosis, and loss of function is associated with the development of colorectal carcinogenesis. Although it has been demonstrated that the APC gene is inducible, its transcriptional regulation has not been elucidated. Therefore, we characterized the promoter region of the APC gene and transcription factors required for basal expression. The APC gene has a TATA-less promoter and contains consensus binding sites for Octamer, AP2, Sp1, a CAAT-box, and three nucleotide sequences for E-box A, B, and M. The E-boxes are functional in several cancer cell lines and upstream stimulating factor-1 (USF1) and USF2 interact with these sites, with a preferred sequence-specificity for the B site. Analysis of activation of the cloned APC promoter by USF1 and USF2 in transient transfection assays in HCT-116 cells demonstrated that mutation of the E-box B site completely abolished the basal promoter activity. Further, the ectopic USF1 and USF2 expression in HCT-116 cells with deletion mutations of E-box A, B, and M sites showed that these E-boxes contribute to USF1- and USF2-mediated transcriptional activation of the APC promoter, with maximum promoter activity being associated with the E-box B site. Thus, USF1 and USF2 transcription factors are critical for APC gene expression.

Effect of feeding protein deficient diet on phospholipid turnover and protein kinase C mediated protein phosphorylation in rat brain

Feeding of protein deficient diet is known to alter the transmembrane signalling in brain of rat by reducing total protein kinase C (PKC) activity. Phospholipid metabolism regulates the activation of PKC through generation of second messengers and the extent of PKC activation accordingly influences the magnitude of phosphorylation of its endogenous substrate proteins. Thus it was speculated that ingestion of protein deficient diet may modify the turnover rate of membrane phospholipids and magnitude of phosphorylation of endogenous substrate proteins of PKC. The experiments were conducted on rats fed on three different types of laboratory prepared diets viz. casein (20% casein), deficient (4% protein, rice flour as source of protein) and supplemented (deficient diet supplemented with L-lysine and DL-threonine) for 28 days. The metabolism of phosphoinositides (PIs) and phosphatidyl choline (PC) was studied by equilibrium labeling with [3H] myo inositol and [14C methyl] choline chloride respectively. The phosphorylation of endogenous substrate proteins of PKC was studied by using 32P-gamma-ATP followed by SDS-PAGE and autoradiography. The results suggest that in deficient group, there is an increased incorporation of [3H] myo inositol in PIs and inositol phosphate pool in comparison to the casein group. The phosphatidyl inositol (PI) turnover reduced, although there was a marginal increase in the phosphatidyl inositol monophosphate (PIP) and phosphatidyl inositol bis phosphate (PIP2). Supplementation of diet showed a reversal of the pattern towards control to a considerable extent. In the deficient group, PC metabolism showed an increased incorporation of [14C methyl] choline in choline phospholipids but decreased incorporation in phosphoryl choline in comparison with the casein group. The increase in total PC contents was significant but marginal in residue contents. The turnover rate of PC increased only marginally and that of residue declined. Supplementation of diet reduced the total contents of PC and residue, but the turnover rate of PC and residue remained still higher. Phosphorylation of endogenous proteins showed four different proteins of 78, 46, 33 and 16 kDa to be the substrates of PKC in casein group. In deficient group, phosphorylation of these proteins increased markedly while supplementation of diet had a reversing effect rendering the values to be intermediate between casein and the supplemented group. The changes in phospholipid metabolism and in phosphorylation of endogenous substrate proteins of PKC suggest that dietary protein deficiency causes alterations in transmembrane signalling mechanism in rat brain. These effects are partially reversed by improving the quality of proteins in the diet.

A correlation of APC and c-myc mRNA levels in lung cancer cell lines

One of the functions of adenomatous polyposis coli (APC) in colorectal cancers is regulation of c-myc gene expression. However, the role of APC in lung cancers has not been elucidated. In the present study, the levels of APC and c-myc mRNA were determined in one strain of normal human bronchial epithelial (NHBE) cells, an SV-40-immortalized non-tumorigenic human bronchial epithelial cell line (BEAS-2B), 13 non-small cell lung cancer cell lines, and 4 small cell lung cancer cell lines. To establish a relationship between c-Myc and APC, we determined the ratio of c-myc and APC mRNA levels in different lung cancer cell lines. Out of 19 lung cancer cell lines, we found that 13 exhibited c-myc/APC mRNA ratio of more than two. Among the cell lines CaLu-3, NCI-H82, A427 and SW900 showed a very low level of APC mRNA and a high level of c-myc mRNA. The ratio of c-myc/APC mRNA in these cell lines was 48, 127, 325 and 708, respectively. The results of these analyses revealed an inverse relationship between APC and c-myc mRNA levels, suggesting that APC may regulate c-myc expression in lung cancer cells in a manner analogous to its role in colon cancer.