Abstract 4101: Control of orthotopic melanoma and breast tumor growth and metastases is achieved by alphav beta3 CAR T cells and is augmented via PD-1 blockade

Purpose: The purpose of this study was to determine the efficacy of αvβ3 CAR T cells deployed against melanoma and triple-negative breast cancer tumors, two malignancies recognized for harnessing the αvβ3 pathway for angiogenic and invasion purposes.

Procedures: CAR T cells expressing an anti-αvβ3 scFv containing either a CD28 or 4-1BB co-stimulatory domain and CD3zeta were generated by retroviral transduction. In vitro cytotoxicity of αvβ3 CAR T cells was assessed by co-culture with melanoma or breast tumor cells and evaluated by impedance-based real-time cell analysis and effector cytokine production by ELISA. Xenograft studies, including melanoma and orthotopic breast tumors, were carried out in NSG mice to evaluate in vivo efficacy of systemically administered αvβ3 CAR T cells. Immunohistochemistry was performed to evaluate T cell infiltration of tumors and changes in the microenvironment. NSG mice implanted with orthotopic tumors were monitored for disease progression and development of metastases using bioluminescent imaging.

Results: αvβ3 CAR T cells exert rapid cytotoxicity and prominent cytokine production against all melanoma and triple-negative breast tumor lines tested. Systemic administration of αvβ3 CAR T cells potently inhibited growth of SK-MEL-28 melanoma xenografts as demonstrated by significant differences in tumor volume relative to control CARs. In orthotopic MD-AMB-231 breast tumors, αvβ3 CAR T cells were able to control tumor growth, prevent the formation of spontaneous lung metastases from the primary tumor, and resulted in improved survival. αvβ3 CAR T cells also mediated control and clearance of established lung metastases established via intravenous injection of breast tumor cells. Histological analysis of tumors at endpoint revealed striking infiltration of residual tumors by T cells in mice administered αvβ3.28z CARs, but to a lesser extent in αvβ3.BBz CAR-treated xenografts. Infiltration was accompanied by tumors up-regulating PD-L1 expression in response to treatment. Coadministration of αvβ3 CAR T cells with anti-PD-1 treatment led to augmentation of the therapeutic response against melanoma tumors.

Conclusions: These studies highlight a renewed potential for targeting integrins, specifically αvβ3, for the treatment of solid tumors. These data suggest that αvβ3 CAR T cell therapy for melanoma can be further improved by combination therapy with checkpoint inhibition. Together with our prior work demonstrating robust efficacy of αvβ3 CAR T cells to treat orthotopic glioblastoma and DIPG, these results highlight the broad applicability of utilizing CAR T cells targeting αvβ3 for treatment of multiple cancer types with reduced risk of on-target, off-tumor toxicity due to the restricted expression of αvβ3 in normal tissues. Results of these studies warrant further development of αvβ3 CAR T cells for clinical use.

Citation Format: Dustin A. Cobb, Philip Mollica, Lixia Liu, Barbara Dziegielewska, Daniel W. Lee. Control of orthotopic melanoma and breast tumor growth and metastases is achieved by alphav beta3 CAR T cells and is augmented via PD-1 blockade. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4101.

Abstract 577: Control of solid tumors by alphav beta3 CAR T cells is accompanied by profound tumor penetration and prevention of metastasis in pre-clinical models

Purpose: Using in vitro and xenograft models we aimed to determine the efficacy of αvβ3 CAR T cells deployed against melanoma and breast cancer tumors, two malignancies previously recognized for harnessing the αvβ3 pathway for angiogenic and invasion purposes.

Procedures: CAR T cells expressing an anti-αvβ3 scFv containing either a CD28 or 4-1BB co-stimulatory domain and CD3zeta were generated by retroviral transduction. In vitro cytotoxicity of αvβ3 CAR T cells was assessed by co-culture with melanoma or breast tumor cells assayed with bioluminescent- and impedance-based methods and effector cytokine production by ELISA. Xenograft studies, including melanoma (SK-MEL-28) and orthotopic breast tumors (MD-AMB-231), were carried out in NSG mice to evaluate in vivo efficacy of systemically administered αvβ3 CAR T cells. Immunohistochemistry was performed to evaluate T cell infiltration of melanoma tumors and changes within the tumor microenvironment. NSG mice harboring orthotopic breast tumors were monitored for disease progression, development of metastases using bioluminescent imaging and flow cytometry analysis of lung tissue, and overall survival.

Results: αvβ3 CAR T cells exhibited robust cytotoxicity and cytokine production against several melanoma and triple-negative breast tumor cell lines. Systemic administration of αvβ3 CAR T cells potently inhibited growth of SK-MEL-28 melanoma xenografts as demonstrated by significant differences in tumor volume relative to CD19 CAR treatment. Immunohistochemical analysis of tumors at the experimental endpoint revealed striking infiltration of residual tumors by human T cells in mice administered αvβ3.28z CARs, but to a lesser extent in αvβ3.BBz CAR-treated xenografts. This T cell infiltration was accompanied by marked expression of PD-L1 that was mostly absent in tumors of CD19 CAR-treated controls, suggesting treatment-induced up-regulation of PD-L1. In orthotopic xenografts of MD-AMB-231 breast tumors, αvβ3 CAR T cells were able to control tumor growth, prevent the formation of lung metastases, and resulted in enhanced overall survival.

Conclusions: These pre-clinical studies highlight a renewed potential for targeting integrins, specifically αvβ3, for the treatment of solid tumors. Data from this study suggests that αvβ3 CAR T cell therapy for melanoma may be further improved by combination therapy with checkpoint inhibition. Together with our prior work demonstrating robust efficacy of αvβ3 CAR T cells to treat glioblastoma and DIPG xenografts, these results highlight the broad applicability of utilizing CAR T cells targeting αvβ3 for treatment of multiple cancer types with reduced risk of on-target, off-tumor toxicity due to the restricted expression of αvβ3 in normal tissues. Results of these studies warrant further development of αvβ3 CAR T cells for clinical use.

Citation Format: Dustin A. Cobb, Lixia Liu, Barbara Dziegielewska, Philip Mollica, Maria Lee, Daniel W. Lee. Control of solid tumors by alphav beta3 CAR T cells is accompanied by profound tumor penetration and prevention of metastasis in pre-clinical models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 577.

Abstract P3-07-03: Targeting low voltage-activated calcium channels in HER2-positive breast cancer

Under physiological conditions low voltage-activated calcium channels (LVA, also called T-type channels) occur mostly in the brain, peripheral nervous system, smooth muscles, and pacemaker of the heart. However, LVA are often aberrantly expressed in various cancers and their overexpression is inversely correlated with outcomes. Active LVA contribute to proliferation, progression and resistance to therapy in several human cancers, including breast cancer (BC). The molecular mechanism of this activity is still not fully understood, and there is a lack of pre-clinical studies that target LVA in animal models of BC.Data from the Cancer Genome Atlas (TCGA) show a positive correlation between HER2 gene amplification and LVA channel overexpression, particularly of Cav3.1 (CACNA1G) subunit. The goal of this study is to identify functional links between HER2 receptor signaling and LVA channels, and validate LVA Ca2+ channels as a potential target for BC therapy. Inhibition of LVA, by small molecule antagonist mibefradil (MIB) or shRNA, significantly decreased the activity of PI3K/mTOR/AKT pathway in HER2-positive BC cell lines, blocked cell cycle progression, and at higher concentrations induced apoptotic cell death. Furthermore, LVA antagonist, mibefradil, sensitized cells to trastuzumab, lapatinib and paclitaxel, commonly used standard treatments for HER2-positive BC. To test the effects of LVA inhibition in vivo, the MMTV-PyMT mouse model of BC was used. In this model several features of HER2-positive BC are recapitulated, such as activation of PI3K/AKT/mTOR pathway, overexpression of HER2, BIRC5 and cyclin D1, and low expression of ERα, PGR and FOXA1. In PyMT animals palpable tumors can be detected at the age of 10-12 weeks, and 100% of animals will develop high tumor burden, including lung metastases, at the age of 15-20 weeks. Expression of CACNA1G subunit was detected in breast epithelial cells of 7 week old mice, and throughout the adult life, in both wild-type and PyMT animals, but CACNA1G was not expressed in lung or skeletal muscles. In in vitro experiments, cell lines derived from PyMT tumors were sensitive to MIB. In in vivo experiment, 5 to 7 weeks-old PyMT mice were fed either control or MIB-containing diet ad lib. Analysis of mouse serum revealed stable MIB concentration of ˜1500 ng/mL achieved within one week of treatment. In MIB-fed animals tumor appearance was delayed by 2 weeks, and the tumors were smaller as compared to control, thus suggesting a supporting role for LVA channels in breast tumor development and progression. Together, our observations provide new insights into the role of Ca2+ channels in HER2 driven BC and posit a future use of LVA channel antagonists for the treatment HER2-positive breast tumors.

Citation Format: Dziegielewska B, Dunlap-Brown ME, Warnock WT, Dillon PM, Bouton AH, Dziegielewski J. Targeting low voltage-activated calcium channels in HER2-positive breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P3-07-03.

T-Type Ca2+ Channel Inhibition Sensitizes Ovarian Cancer to Carboplatin

Ovarian cancer is the deadliest gynecologic cancer, due in large part to the diagnosis of advanced stage disease, the development of platinum resistance, and inadequate treatment alternatives. Recent studies by our group and others have shown that T-type calcium (Ca(2+)) channels play a reinforcing role in cancer cell proliferation, cell-cycle progression, and apoptosis evasion. Therefore, we investigated whether T-type Ca(2+) channels affect ovarian tumor growth and response to platinum agents. Inhibition of T-type Ca(2+) channels with mibefradil or by silencing expression resulted in growth suppression in ovarian cancer cells with a simultaneous increase in apoptosis, which was accompanied by decreased expression of the antiapoptotic gene survivin (BIRC5). Analysis of intracellular signaling revealed mibefradil reduced AKT phosphorylation, increased the levels and nuclear retention of FOXO transcription factors that repress BIRC5 expression, and decreased the expression of FOXM1, which promotes BIRC5 expression. Combining carboplatin with mibefradil synergistically increased apoptosis in vitro. Importantly, mibefradil rendered platinum-resistant ovarian tumors sensitive to carboplatin in a mouse model of peritoneal metastasis. Together, the data provide rationale for future use of T-type channel antagonists together with platinum agents for the treatment of ovarian cancer.

Abstract 750: Overcoming resistance to anti-EGFR therapy in non-small cell lung cancer with a T-type Ca2+ channels inhibitor

This study's aim was to define a novel molecular target and to develop an effective agent capable of overcoming intrinsic and acquired resistance to anti-EGFR therapy. Despite introduction of new therapies, lung cancer is a major cause of cancer death. One of its characteristics is the expression and activation of epidermal growth factor receptor (EGFR). Activating mutations in EGFR are present in approximately 20% of patients with non-small cell lung cancer (NSCLC), mostly in never-smokers. While most patients initially respond to treatment with anti-EGFR therapy, essentially all patients develop acquired drug resistance. T-type calcium channels, are present in lung cancer and support cell proliferation. Importantly, their expression often coincides with abnormal expression or mutations in EGFR. In this study, we investigated the effects of T-type channel inhibition on NSCLC survival and resistance to targeted therapy in vitro. We used a panel of cell lines differing in EGFR mutation status and sensitivity to EGFR tyrosine kinase inhibitors (TKIs). Our results show that T-type channel inhibitors alone arrest cells in the G1/G0 phase of the cell cycle and induce cell death. The treatment reduces activity of both PI3K/AKT/mTOR and JAK2/STAT5 pathways, often upregulated as a mechanism of acquired resistance to TKIs. Importantly, combined treatment with TKI, gefitinib, and T-type channels inhibitor, mibefradil, resulted in synergistic growth inhibition. Our results demonstrate a connection between T-type Ca2+ channels and aberrant EGFR activity in NSCLC, and provide the rationale for future use of two targeted therapies together. Since T-type calcium channel inhibitors are already undergoing clinical trials, our discoveries could be quickly translated into clinical practice.

Citation Format: Barbara Dziegielewska, Lloyd S. Gray, Jaroslaw Dziegielewski. Overcoming resistance to anti-EGFR therapy in non-small cell lung cancer with a T-type Ca2+ channels inhibitor. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 750. doi:10.1158/1538-7445.AM2015-750

Abstract 5407: T-type Ca2+ channel inhibitors sensitize ovarian cancer to carboplatin through downregulation of survivin gene expression

Recent studies by our group and others have shown that T-type Ca2+ channels play a reinforcing role in cancer cell proliferation, cell cycle progression and apoptosis evasion. We have demonstrated drug cooperativity between T-type Ca2+ channel inhibitor, mibefradil and carboplatin in vivo in ovarian cancer xenografts. To investigate the molecular mechanism by which T-type Ca2+ channels affect tumor growth, we used a panel of ovarian cancer cell lines together with mibefradil or small interfering RNA (siRNA) to silence the T-channels gene expression. Inhibition of T-type Ca2+ channels with mibefradil or by silencing expression resulted in growth suppression in ovarian cancer with simultaneous increase in apoptosis and decrease in expression of the anti-apoptotic gene survivin (BIRC5). Combining carboplatin with mibefradil resulted in increased apoptosis. Analysis of intracellular signaling revealed T-type Ca2+ channel inhibitors induced changes in the levels of FOXO proteins, which regulate BIRC5 gene expression. Mibefradil treatment caused greater nuclear retention of FoxO1 and FoxO3a, two transcription factors that repress BIRC5 expression, while protein expression of the transcription factor that activates BIRC5 expression, FoxM1, decreased. Chromatin immunoprecipitation of FOXO proteins from mibefradil treated ovarian cancer cells demonstrated binding of FoxM1 and FoxO1 within the BIRC5 (survivin) promoter. Together, the data show that T-type Ca2+ channels support ovarian cancer cell cycle, proliferation and regulate pro-survival pathways through the FOXO-survivin signaling axis, which has been reported to be deregulated in ovarian cancer. Moreover, our results provide the rationale for future use of T-type antagonists as sensitizing agents in combination with standard chemotherapeutics.

Citation Format: Barbara Dziegielewska, Eli V. Casarez, Wesley Z. Yang, Jaroslaw Dziegielewski, Jill K. Slack-Davis. T-type Ca2+ channel inhibitors sensitize ovarian cancer to carboplatin through downregulation of survivin gene expression. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 5407. doi:10.1158/1538-7445.AM2015-5407

Abstract 1808: Enhancing the Stupp regimen in glioblastoma cancer cells with a T-type calcium channels inhibitor

Glioblastoma multiforme (GBM) is the most common and lethal primary brain tumor. Although introduction of the Stupp regimen combining radiation (IR) with concomitant and adjuvant temozolomide (TMZ) led to increased patient survival, the cure rate of GBM is disappointing. We postulated that sensitization of cancer cells to therapy can be achieved by targeting intracellular pathways and processes. Recently, T-type Ca2+ channels have been proposed as a molecular target for GBM sensitization to chemo- and radiotherapy (Keir et al., 2013; Sheehan et al., 2013). In the present studies, we investigated the effects of T-type channel inhibition on GBM cells’ responses to the Stupp regimen. GBM cells differing in their resistance to TMZ and IR were treated with theT-type channel antagonist, mibefradil, either concomitantly with TMZ or sequentially (Interlaced TherapyTM). Cell viability and clonogenic survival were determined, demonstrating significant enhancement of the Stupp regimen's anticancer effects with co-treatment with T-type channels inhibitors. Importantly for DNA-targeted therapies, we observed a decrease in cells’ ability to repair DNA damage in the presence of mibefradil. The molecular mechanism of this decrease was investigated by assessing expression levels, localization and phosphorylation status of ATM, DNA-PK, γH2AX, Chk1/2, Kap1, p53, Rad51. These mechanistic studies demonstrated that inhibiting T-type Ca2+ channels in GBM affects DNA damage signaling and repair through reduction of repair protein activation and changes their cellular localization. The observed effects are specific for T-type channels inhibition, since L-type channels inhibitors had no effect. Our study provide a rational molecular mechanism for sensitization of GBM to the Stupp regimen by T-type Ca2+ channel inhibition, which will facilitate current and future clinical trials and potentially lead to new treatment options for this deadly brain tumor.

Citation Format: Jaroslaw Dziegielewski, Barbara Dziegielewska, Lloyd S. Gray. Enhancing the Stupp regimen in glioblastoma cancer cells with a T-type calcium channels inhibitor. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1808. doi:10.1158/1538-7445.AM2015-1808

T-type calcium channels blockers as new tools in cancer therapies

T-type calcium channels are involved in a multitude of cellular processes, both physiological and pathological, including cancer. T-type channels are also often aberrantly expressed in different human cancers and participate in the regulation of cell cycle progression, proliferation, migration, and survival. Here, we review the recent literature and discuss the controversies, supporting the role of T-type Ca(2+) channels in cancer cells and the proposed use of channels blockers as anticancer agents. A growing number of reports show that pharmacological inhibition or RNAi-mediated downregulation of T-type channels leads to inhibition of cancer cell proliferation and increased cancer cell death. In addition to a single agent activity, experimental results demonstrate that T-type channel blockers enhance the anticancer effects of conventional radio- and chemotherapy. At present, the detailed biological mechanism(s) underlying the anticancer activity of these channel blockers is not fully understood. Recent findings and ideas summarized here identify T-type Ca(2+) channels as a molecular target for anticancer therapy and offer new directions for the design of novel therapeutic strategies employing channels blockers. Physiological relevance: T-type calcium channels are often aberrantly expressed or deregulated in cancer cells, supporting their proliferation, survival, and resistance to treatment; therefore, T-type Ca(2+) channels could be attractive molecular targets for anticancer therapy.

T-type Ca2+ channel inhibition induces p53-dependent cell growth arrest and apoptosis through activation of p38-MAPK in colon cancer cells

Epithelial tumor cells express T-type Ca(2+) channels, which are thought to promote cell proliferation. This study investigated the cellular response to T-type Ca(2+) channel inhibition either by small-molecule antagonists or by RNAi-mediated knockdown. Selective T-type Ca(2+) channel antagonists caused growth inhibition and apoptosis more effectively in HCT116 cells expressing wild-type p53 (p53wt), than in HCT116 mutant p53(-/-) cells. These antagonists increased p53-dependent gene expression and increased genomic occupancy of p53 at specific target sequences. The knockdown of a single T-type Ca(2+) channel subunit (CACNA1G) reduced cell growth and induced caspase-3/7 activation in HCT116 p53wt cells as compared with HCT116 mutant p53(-/-) cells. Moreover, CaCo2 cells that do not express functional p53 were made more sensitive to CACNA1G knockdown when p53wt was stably expressed. Upon T-type Ca(2+) channel inhibition, p38-MAPK promoted phosphorylation at Ser392 of p53wt. Cells treated with the inhibitor SB203580 or specific RNAi targeting p38-MAPKα/β (MAPK14/MAPK11) showed resistance to T-type Ca(2+) channel inhibition. Finally, the decreased sensitivity to channel inhibition was associated with decreased accumulation of p53 and decreased expression of p53 target genes, p21Cip1 (CDKN1A) and BCL2-binding component 3 (BBC3/PUMA).

IMPLICATIONS

A novel pathway involving p53 and p38-MAPK is revealed and provides a rationale for antitumor therapies that target T-type Ca(2+) channels.

Abstract A239: T-type calcium channels as a novel molecular target for tumor therapy

T-type Ca2+ channels are a group of low voltage-gated calcium channels which seem to be crucial for embryonic or stem cell proliferation and differentiation; however, they could be also aberrantly expressed in several human tumors. Since calcium entry is required for smooth transition through the cell cycle, it has been hypothesized that the inhibition of T-type Ca2+ channels blocks cells in G0/G1 phase of cell cycle, while a release from inhibition leads to increased number of cells entering cell cycle and thus increased susceptibility to conventional antitumor therapy. In this study we investigated the effects of T-type channel inactivation, or downregulation, on cell cycle progression, cell death and survival, and resistance to radiotherapy (RT). The experiments were conducted using several cancer cell lines expressing T-type Ca2+ channels (glioblastoma, colon and prostate) and selective and structurally unrelated calcium channels antagonists, including Mibefradil, a drug that is currently undergoing clinical trial for its antitumor properties, and is planned to be tested as radiosensitizing agent in recurrent gliobastoma tumors. The effects of T-type Ca2+ channel inhibition on cell cycle progression, cell death and the expression/activation of cell cycle regulated proteins were assessed. Finally, the combination treatments including T-type channel antagonist and radiotherapy were assayed. We show that T-type Ca2+ channels are important for cell cycle progression and resistance to RT, and that their inhibition leads to enhanced cell death and reduced survival. Our study demonstrates that one of the earliest events evoked by T-type channel inhibition is a deregulation of pro-survival pathway PI3K/Akt/mTOR and activation of pro-apoptotic stress-activated p38MAPK pathway. The results support the idea of T-type Ca2+ channel as a novel molecular target for antitumor therapy and provide a rational and plausible biological mechanism responsible for the effects induced by T-type Ca2+ channel antagonists in cancer cells.

Citation Information: Mol Cancer Ther 2013;12(11 Suppl):A239.

Citation Format: Barbara Dziegielewska, Nicholas C.K. Valerie, Amol S. Hosing, James M. Larner, David L. Brautigan, Lloyd S. Gray, Jaroslaw Dziegielewski. T-type calcium channels as a novel molecular target for tumor therapy. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr A239.

Inhibition of T-type calcium channels disrupts Akt signaling and promotes apoptosis in glioblastoma cells

Glioblastoma multiforme (GBM) are brain tumors that are exceptionally resistant to both radio- and chemotherapy regimens and novel approaches to treatment are needed. T-type calcium channels are one type of low voltage-gated channel (LVCC) involved in embryonic cell proliferation and differentiation; however they are often over-expressed in tumors, including GBM. In this study, we found that inhibition of T-type Ca(2+) channels in GBM cells significantly reduced their survival and resistance to therapy. Moreover, either T-type selective antagonists, such as mibefradil, or siRNA-mediated knockdown of the T-type channel alpha subunits not only reduced cell viability and clonogenic potential, but also induced apoptosis. In response to channel blockade or ablation, we observed reduced phosphorylation of Akt and Rictor, suggesting inhibition of the mTORC2/Akt pathway. This was followed by reduction in phosphorylation of anti-apoptotic Bad and caspases activation. The apoptotic response was specific for T-type Ca(2+) channels, as inhibition of L-type Ca(2+) channels did not induce similar effects. Our results implicate T-type Ca(2+) channels as distinct entities for survival signaling in GBM cells and suggest that they are a novel molecular target for tumor therapy.

Phosphorylation of serine 51 regulates the interaction of human DNA ligase I with replication factor C and its participation in DNA replication and repair

Human DNA ligase I (hLigI) joins Okazaki fragments during DNA replication and completes excision repair via interactions with proliferating cell nuclear antigen and replication factor C (RFC). Unlike proliferating cell nuclear antigen, the interaction with RFC is regulated by hLigI phosphorylation. To identity of the site(s) involved in this regulation, we analyzed phosphorylated hLigI purified from insect cells by mass spectrometry. These results suggested that serine 51 phosphorylation negatively regulates the interaction with RFC. Therefore, we constructed versions of hLigI in which serine 51 was replaced with either alanine (hLigI51A) to prevent phosphorylation or aspartic acid (hLigI51D) to mimic phosphorylation. hLigI51D but not hLigI51A was defective in binding to purified RFC and in associating with RFC in cell extracts. Although DNA synthesis and proliferation of hLigI-deficient cells expressing either hLig51A or hLig51 was reduced compared with cells expressing wild-type hLigI, cellular senescence was only observed in the cells expressing hLigI51D. Notably, these cells had increased levels of spontaneous DNA damage and phosphorylated CHK2. In addition, although expression of hLigI51A complemented the sensitivity of hLigI-deficient cells to a poly (ADP-ribose polymerase (PARP) inhibitor, expression of hLig151D did not, presumably because these cells are more dependent upon PARP-dependent repair pathways to repair the damage resulting from the abnormal DNA replication. Finally, neither expression of hLigI51D nor hLigI51A fully complemented the sensitivity of hLigI-deficient cells to DNA alkylation. Thus, phosphorylation of serine 51 on hLigI plays a critical role in regulating the interaction between hLigI and RFC, which is required for efficient DNA replication and repair.

Identification and validation of human DNA ligase inhibitors using computer-aided drug design

Linking together of DNA strands by DNA ligases is essential for DNA replication and repair. Since many therapies used to treat cancer act by causing DNA damage, there is growing interest in the development of DNA repair inhibitors. Accordingly, virtual database screening and experimental evaluation were applied to identify inhibitors of human DNA ligase I (hLigI). When a DNA binding site within the DNA binding domain (DBD) of hLigI was targeted, more than 1 million compounds were screened from which 192 were chosen for experimental evaluation. In DNA joining assays, 10 compounds specifically inhibited hLigI, 5 of which also inhibited the proliferation of cultured human cell lines. Analysis of the 10 active compounds revealed the utility of including multiple protein conformations and chemical clustering in the virtual screening procedure. The identified ligase inhibitors are structurally diverse and have druglike physical and molecular characteristics making them ideal for further drug development studies.

Rational design of human DNA ligase inhibitors that target cellular DNA replication and repair

Based on the crystal structure of human DNA ligase I complexed with nicked DNA, computer-aided drug design was used to identify compounds in a database of 1.5 million commercially available low molecular weight chemicals that were predicted to bind to a DNA-binding pocket within the DNA-binding domain of DNA ligase I, thereby inhibiting DNA joining. Ten of 192 candidates specifically inhibited purified human DNA ligase I. Notably, a subset of these compounds was also active against the other human DNA ligases. Three compounds that differed in their specificity for the three human DNA ligases were analyzed further. L82 inhibited DNA ligase I, L67 inhibited DNA ligases I and III, and L189 inhibited DNA ligases I, III, and IV in DNA joining assays with purified proteins and in cell extract assays of DNA replication, base excision repair, and nonhomologous end-joining. L67 and L189 are simple competitive inhibitors with respect to nicked DNA, whereas L82 is an uncompetitive inhibitor that stabilized complex formation between DNA ligase I and nicked DNA. In cell culture assays, L82 was cytostatic whereas L67 and L189 were cytotoxic. Concordant with their ability to inhibit DNA repair in vitro, subtoxic concentrations of L67 and L189 significantly increased the cytotoxicity of DNA-damaging agents. Interestingly, the ligase inhibitors specifically sensitized cancer cells to DNA damage. Thus, these novel human DNA ligase inhibitors will not only provide insights into the cellular function of these enzymes but also serve as lead compounds for the development of anticancer agents.

Inhibition of RecBCD enzyme by antineoplastic DNA alkylating agents

To understand how bulky adducts might perturb DNA helicase function, three distinct DNA-binding agents were used to determine the effects of DNA alkylation on a DNA helicase. Adozelesin, ecteinascidin 743 (Et743) and hedamycin each possess unique structures and sequence selectivity. They bind to double-stranded DNA and alkylate one strand of the duplex in cis, adding adducts that alter the structure of DNA significantly. The results show that Et743 was the most potent inhibitor of DNA unwinding, followed by adozelesin and hedamycin. Et743 significantly inhibited unwinding, enhanced degradation of DNA, and completely eliminated the ability of the translocating RecBCD enzyme to recognize and respond to the recombination hotspot chi. Unwinding of adozelesin-modified DNA was accompanied by the appearance of unwinding intermediates, consistent with enzyme entrapment or stalling. Further, adozelesin also induced "apparent" chi fragment formation. The combination of enzyme sequestering and pseudo-chi modification of RecBCD, results in biphasic time-courses of DNA unwinding. Hedamycin also reduced RecBCD activity, albeit at increased concentrations of drug relative to either adozelesin or Et743. Remarkably, the hedamycin modification resulted in constitutive activation of the bottom-strand nuclease activity of the enzyme, while leaving the ability of the translocating enzyme to recognize and respond to chi largely intact. Finally, the results show that DNA alkylation does not significantly perturb the allosteric interaction that activates the enzyme for ATP hydrolysis, as the efficiency of ATP utilization for DNA unwinding is affected only marginally. These results taken together present a unique response of RecBCD enzyme to bulky DNA adducts. We correlate these effects with the recently determined crystal structure of the RecBCD holoenzyme bound to DNA.

SV40 DNA replication inhibition by the monofunctional DNA alkylator Et743

Ecteinascidin 743 (Et743) is a highly cytotoxic anticancer agent isolated from the squirt Ecteinascidia turbinate, which alkylates DNA in the minor groove at GC-rich sequences resulting in an unusual bending toward the major groove. The ability of Et743 to block DNA replication was studied using the well-established simian virus (SV40) model for mammalian DNA replication in cells and cell-free extracts. Intracellular SV40 DNA isolated from Et743-treated BSC-1 cells was analyzed by native, two-dimensional agarose gel electrophoresis. A low frequency of Et743 adducts detected at 30-100 nM drug concentrations inhibited SV40 origin activity and induced formation of unusual DNA replication intermediates. Under cell-free conditions, only a high Et743 adduct frequency reduced SV40 DNA synthesis. Comparative studies involving related DNA alkylators, tomamycin and saframycin A, revealed inhibition of SV40 DNA replication in cells at concentrations approximately 10 times higher than Et743. Under cell-free conditions tomamycin- or saframycin-A-adducted DNA templates inhibited DNA synthesis similarly to Et743. Et743 appears to be unusual among other alkylators, because its adducts strongly inhibit intracellular SV40 DNA replication but are relatively weak as cis inhibitors as measured under cell-free conditions.

DNA crosslinking and biological activity of a hairpin polyamide-chlorambucil conjugate

A prototype of a novel class of DNA alkylating agents, which combines the DNA crosslinking moiety chlorambucil (Chl) with a sequence-selective hairpin pyrrole-imidazole polyamide ImPy-beta-ImPy-gamma-ImPy-beta-Dp (polyamide 1), was evaluated for its ability to damage DNA and induce biological responses. Polyamide 1-Chl conjugate (1-Chl) alkylates and interstrand crosslinks DNA in cell-free systems. The alkylation occurs predominantly at 5'-AGCTGCA-3' sequence, which represents the polyamide binding site. Conjugate-induced lesions were first detected on DNA treated for 1 h with 0.1 micro M 1-Chl, indicating that the conjugate is at least 100-fold more potent than Chl. Prolonged incubation allowed for DNA damage detection even at 0.01 micro M concentration. Treatment with 1-Chl decreased DNA template activity in simian virus 40 (SV40) in vitro replication assays. 1-Chl inhibited mammalian cell growth, genomic DNA replication and cell cycle progression, and arrested cells in the G2/M phase. Moreover, cellular effects were observed at 1-Chl concentrations similar to those needed for DNA damage in cell-free systems. Neither of the parent compounds, unconjugated Chl or polyamide 1, demonstrated any cellular activity in the same concentration range. The conjugate molecule 1-Chl possesses the sequence-selectivity of a polyamide and the enhanced DNA reactivity of Chl.