The role of myc oncogenes in cell growth and differentiation

Stabilization of c-Myc by the atypical cell cycle regulator, Spy1, decreases efficacy of breast cancer treatments

PURPOSE

c-Myc is frequently upregulated in breast cancers, however, targeting c-Myc has proven to be a challenge. Targeting of downstream mediators of c-Myc, such as the 'cyclin-like' cell cycle regulator Spy1, may be a viable therapeutic option in a subset of breast cancer subtypes.

METHODS

Mouse mammary tumor cells isolated from MMTV-Myc mice and human breast cancer cell lines were used to manipulate Spy1 levels followed by tamoxifen or chemotherapeutic treatment with a variety of endpoints. Patient samples from TNBC patients were obtained and constructed into a TMA and stained for c-Myc and Spy1 protein levels.

RESULTS

Over time, MMTV-Myc cells show a decreased response to tamoxifen treatment with increasing levels of Spy1 in the tamoxifen-resistant cells. shRNA against Spy1 re-establishes tamoxifen sensitivity. Spy1 was found to be highly elevated in human TNBC cell and patient samples, correlating to c-Myc protein levels. c-Myc was found to be stabilized by Spy1 and knocking down Spy1 in TNBC cells shows a significant increase in response to chemotherapy treatments.

CONCLUSION

Understanding the interplay between protein expression level and response to treatment is a critical factor in developing novel treatment options for breast cancer patients. These data have shown a connection between Spy1 and c-Myc protein levels in more aggressive breast cancer cells and patient samples. Furthermore, targeting c-Myc has proven difficult, these data suggest targeting Spy1 even when c-Myc is elevated can confer an advantage to current chemotherapies.

Emerging Roles of lncRNAs in the Formation and Progression of Colorectal Cancer

Colorectal cancer (CRC) is the primary cause of cancer-related death worldwide; however, specific and sensitive tools for the early diagnosis and targeted therapy of CRC are currently lacking. High-throughput sequencing technology revealed that gene expression of long-chain non-coding RNAs (lncRNAs) in a number of cancers directly or indirectly interferes with various biological processes. Emerging evidence suggests that lncRNAs regulate target genes and play an important role in the biological processes of malignancies, including CRC. Many carcinostatic/oncogenic lncRNAs have been identified as biomarkers for metastasis and prognosis in CRC; hence, they serve as therapeutic tools. In this article, we systematically review the literature on the disordered lncRNAs in CRC from four aspects: DNA transcription, RNA level regulation, post-translational level, and the translation of lncRNAs into polypeptides. Subsequently, we analyze the mechanism through which lncRNAs participate in the biological process of CRC. Finally, we discuss the application and prospects of these lncRNAs in CRC.

The c-MYC protooncogene expression in cholesteatoma

Cholesteatoma is an epidermoid cyst, which is most frequently found in the middle ear. The matrix of cholesteatoma is histologically similar to the matrix of the epidermoid cyst of the skin (atheroma); their epithelium is characterized by hyperproliferation. The c-MYC protooncogene located on chromosome 8q24 encodes a transcription factor involved in the regulation of cell proliferation and differentiation. Previous studies have found aneuploidy of chromosome 8, copy number variation of c-MYC gene, and the presence of elevated level c-MYC protein in cholesteatoma. In this study we have compared the expression of c-MYC gene in samples taken from the matrix of 26 acquired cholesteatomas (15 children and 11 adults), 15 epidermoid cysts of the skin (atheromas; head and neck region) and 5 normal skin samples (retroauricular region) using RT-qPCR, providing the first precise measurement of the expression of c-MYC gene in cholesteatoma. We have found significantly elevated c-MYC gene expression in cholesteatoma compared to atheroma and to normal skin samples. There was no significant difference, however, in c-MYC gene expression between cholesteatoma samples of children and adults. The significant difference in c-MYC gene expression level in cholesteatoma compared to that of atheroma implies a more prominent hyperproliferative phenotype which may explain the clinical behavior typical of cholesteatoma.

Myc: the beauty and the beast

The iconic history of the Myc oncoprotein encompasses 3 decades of intense scientific discovery. There is no question that Myc has been a pioneer, advancing insight into the molecular basis of cancer as well as functioning as a critical control center for several diverse biological processes and regulatory mechanisms. This narrative chronicles the journey and milestones that have defined the understanding of Myc, and it provides an opportunity to consider future directions in this challenging yet rewarding field.

Bone morphogenetic protein-7 is a MYC target with prosurvival functions in childhood medulloblastoma

Medulloblastoma (MB) is the most common malignant brain tumor in children. It is known that overexpression and/or amplification of the MYC oncogene is associated with poor clinical outcome, but the molecular mechanisms and the MYC downstream effectors in MB remain still elusive. Besides contributing to elucidate how progression of MB takes place, most importantly, the identification of novel MYC-target genes will suggest novel candidates for targeted therapy in MB. A group of 209 MYC-responsive genes was obtained from a complementary DNA microarray analysis of a MB-derived cell line, following MYC overexpression and silencing. Among the MYC-responsive genes, we identified the members of the bone morphogenetic protein (BMP) signaling pathway, which have a crucial role during the development of the cerebellum. In particular, the gene BMP7 was identified as a direct target of MYC. A positive correlation between MYC and BMP7 expression was documented by analyzing two distinct sets of primary MB samples. Functional studies in vitro using a small-molecule inhibitor of the BMP/SMAD signaling pathway reproduced the effect of the small interfering RNA-mediated silencing of BMP7. Both approaches led to a block of proliferation in a panel of MB cells and to inhibition of SMAD phosphorylation. Altogether, our findings indicate that high MYC levels drive BMP7 overexpression, promoting cell survival in MB cells. This observation suggests the potential relevance of targeting the BMP/SMAD pathway as a novel therapeutic approach for the treatment of childhood MB.

Reflecting on 25 years with MYC

Just over 25 years ago, MYC, the human homologue of a retroviral oncogene, was identified. Since that time, MYC research has been intense and the advances impressive. On reflection, it is astonishing how each incremental insight into MYC regulation and function has also had an impact on numerous biological disciplines, including our understanding of molecular oncogenesis in general. Here we chronicle the major advances in our understanding of MYC biology, and peer into the future of MYC research.

Cdk5-mediated phosphorylation of c-Myc on Ser-62 is essential in transcriptional activation of cyclin B1 by cyclin G1

It has been reported previously that cyclin G1 enables cells to overcome radiation-induced G(2) arrest and increased cell death and that these effects are mediated by transcriptional activation of cyclin B1. In this study, we further investigated the mechanism by which cyclin G1 transcriptionally activates cyclin B1. Deletion or point mutations within the cyclin B1 promoter region revealed that the c-Myc binding site (E-box) is necessary for cyclin G1-mediated transcriptional activation of cyclin B1 to occur. In addition, the kinase activity of Cdk5 was increased by cyclin G1 overexpression, and Cdk5 directly phosphorylated c-Myc on Ser-62. Furthermore, cyclin G1 mediated increased radiosensitivity, and radiation-induced M phase arrest was attenuated when RNA interference of Cdk5 was treated. Taken together, the results of this study indicate that Cdk5 activation in cells that overexpress cyclin G1 leads to c-Myc phosphorylation on Ser-62, which is responsible for cyclin G1-mediated transcriptional activation of cyclin B1.

Expressions of CCAAT-enhancer-binding proteins and c-Myc in the parotid gland of the rat: in vivo effects of isoprenaline, bethanechol, vasoactive intestinal peptide and food intake

Parotid glands of adult female rats were exposed to agonists mimicking sympathetic (isoprenaline, 1mg/kg, I.P.) or parasympathetic activity (bethanechol, 10 microg/kg/min i.v. for 30 min, and vasoactive intestinal peptide, VIP, 0.2 microg/kg/min, i.v. for 30 min) or they were reflexly activated by a meal demanding chewing. The stimulated glands were removed at varying times (15(30)-360 min) following the onset of the agonist administration or 75-300 min after the start of a 1h long feeding period, and a number of transcription factors was studied using Western blot. The protein bands were semi-quantitatively measured by densitometry. In response to isoprenaline, C/EBPalpha of 42, 38 and 30 kDa increased by 45-50% above control value, C/EBPbeta LAP (38/35 kDa) by 80% and C/EBPdelta (35 kDa) by 230%, while C/EBPbeta LIP (20 kDa) decreased by 45%. In response to VIP, C/EBPalpha of 42 kDa increased by 75% and C/EBPalpha of 30 kDa by 10%, C/EBPbeta LAP by 65% and C/EBPdelta by 410%, while C/EBPalpha of 38 kDa as well as C/EBPbeta LIP were not changed. In response to bethanechol, C/EBPalpha of 42 kDa increased by 105%, C/EBPbeta LAP by 40% and C/EBPdelta by 170%, while C/EBPalpha of 30 kDa decreased by 30% and C/EBPalpha of 38 kDa and C/EBPbeta LIP remained unchanged. c-Myc increased in response to isoprenaline and VIP by 40-55%, but not to bethanechol. In rats offered a pelleted diet, the parotid glands displayed increases in C/EBPalpha of 42 kDa by 105%, of 30 kDa by 40% and of 38 kDa, by 10%, in C/EBPbeta LAP by 65% and in C/EBPdelta by 215%, whereas C/EBPbeta LIP decreased by 25%. Thus, in parotid glands transcription factors of importance for growth and metabolism were shown to be influenced by autonomimetics as well as by nervous activity.

Random mutagenesis of PDZ(Omi) domain and selection of mutants that specifically bind the Myc proto-oncogene and induce apoptosis

Omi is a mammalian serine protease that is localized in the mitochondria and released to the cytoplasm in response to apoptotic stimuli. Omi induces cell death in a caspase-dependent manner by interacting with the X-chromosome linked inhibitor of apoptosis protein, as well as in a caspase-independent way that relies on its proteolytic activity. Omi is synthesized as a precursor polypeptide and is processed to an active serine protease with a unique PDZ domain. PDZ domains recognize the extreme carboxyl terminus of target proteins. Internal peptides that are able to fold into a beta-finger are also reported to bind some PDZ domains. Using a modified yeast two-hybrid system, PDZ(Omi) mutants were isolated by their ability to bind the carboxyl terminus of human Myc oncoprotein in yeast as well as in mammalian cells. One such PDZ(m) domain (PDZ-M1), when transfected into mammalian cells, was able to bind to endogenous Myc protein and induce cell death. PDZ-M1-induced apoptosis was entirely dependent on the presence of Myc protein and was not observed when c-myc null fibroblasts were used. Our studies indicate that the PDZ domain of Omi can provide a prototype that could easily be exploited to target specifically and inactivate oncogenes by binding to their unique carboxyl terminus.

Use of oncogene expression as an independent prognostic marker for primary melanoma

For 25 years, the Breslow depth of primary tumors has remained the most accurate prognostic test of survival for primary melanoma. However a number of studies have indicated that outcome for intermediate-thickness melanomas (0.75-2.49 mm) is often at variance with that predicted by the Breslow depth. This study investigated c-myc oncogene expression in 92 primary tumors of intermediate thickness using flow cytometry. Oncoprotein expression was detected in 87 tumors (95%) with a median positivity of 48% (range, 0%-96%). Survival analysis performed using the Kaplan-Meier method revealed a significant association between oncoprotein positivity and clinical outcome (p < 0.01, log-rank test). Multifactorial analysis of survival using Cox's proportional hazards model revealed c-myc oncoprotein to be an independent prognostic marker more accurate than all other clinicopathological parameters including the Breslow depth (chi(2) = 9.68, p< 0.01). Estimation of c-myc oncoprotein is therefore recommended as a powerful prognostic marker for intermediate-thickness primary melanoma.

The c-myc oncogene: use of a biological prognostic marker as a potential target for gene therapy in melanoma

The c-myc oncogene has been shown to be overexpressed in a number of malignancies, and may play an important role in the pathogenesis of malignant melanoma. Previous prognostic studies have demonstrated c-myc overexpression in a range of cutaneous melanomas, and levels of c-myc oncoprotein expression have been shown to correlate with clinical outcome in both primary and secondary disease. The purpose of this study was to investigate the in vitro manipulation of c-myc expression using antisense oligonucleotides. The human melanoma cell lines A375M, Be11 and WM115 were treated with c-myc antisense oligonucleotides, and the cellular growth was compared with controls. Antisense oligonucleotides reduced the growth rate of all three cell lines, and produced a reduction in c-myc gene expression as measured by flow cytometry. The growth inhibitions in the A375M, Be11 and WM115 cell lines at 72 h were 36.6%, 35.8% and 29.3%, respectively. Each of these was significantly different from control cultures (P<0.01). The c-myc antisense produced a mean 75% reduction in c-myc oncoprotein expression when compared with controls in the A375M cells (P<0.001), a 49% reduction in the Be11 cells (P<0.001) and a 28% reduction in the WM115 cells (P=0.005). This study demonstrates the importance of the c-myc oncogene in controlling melanoma growth. It suggests that blocking the expression of this gene, using an antisense approach, reduces melanoma cell growth, and may potentially provide a novel gene-therapy strategy for the treatment of advanced melanoma.

Circadian Transcription. Thinking outside the E-Box

The E-Box is a widely used DNA control element. Despite its brevity and broad distribution the E-Box is a remarkably versatile sequence that affects many different genetic programs, including proliferation, differentiation, tissue-specific responses, and cell death. The circadian clock is one of the latest pathways shown to employ this element. In this context, E-Boxes are likely to play a key role in establishing the robust waves of gene expression characteristic of circadian transcription. The regulatory flexibility of the E-Box hinges on the sequence ambiguity allowed at its core, the strong influence of the surrounding sequences, and the recruitment of spatially and temporally regulated E-Box-binding factors. Therefore, understanding how a particular E-Box can accomplish a specific task entails the identification and systematic analysis of these cis- and trans-acting E-Box modifiers. In the present study we compared the E-Box-containing minimal promoters of vasopressin and cyclin B1, two genes that can respond to the transcriptional oscillators driving the circadian clock and cell cycle, respectively. Results of this comparison will help elucidate the manner in which discreet DNA modules associate to either augment or restrain the activation of potential circadian E-Boxes in response to competing regulatory signals.

The prognostic importance of c-myc oncogene expression in head and neck melanoma

Melanomas of the head and neck have a poorer prognosis than melanomas arising at other cutaneous sites. To study the biology of this disease, the expression of the c-myc oncogene was studied in tumors from 97 patients with head and neck melanoma using the technique of flow cytometry. Survival analysis revealed that stratification of patients according to oncogene expression provided a prognostic marker with shorter overall survival in tumors with high nuclear c-myc oncoprotein positivity (log-rank test, chi2 = 8.77, p < 0.005). Multifactorial analysis using Cox's proportional hazards model revealed nuclear c-myc oncoprotein to be an independent prognostic marker (log-rank test, chi2 = 8.82, p = 0.005). These results support the authors' previous studies of the prognostic value of c-myc expression in melanoma and suggest that estimation of c-myc oncoprotein may be of clinical importance in identifying high-risk patients.

Transcriptional repression by blimp-1 (PRDI-BF1) involves recruitment of histone deacetylase

B-lymphocyte-induced maturation protein (Blimp-1) is a transcriptional repressor that is considered to be a master regulator of terminal B-cell development because it is sufficient to trigger differentiation in the BCL(1)-cell model. Transcription of the c-myc gene is repressed by Blimp-1 during B-cell differentiation. In this study, we have explored the mechanism by which Blimp-1 represses transcription by using Gal4-fusion protein assays and assays in which Blimp-1 represses the natural c-myc promoter. The results show that Blimp-1 represses the c-myc promoter by an active mechanism that is independent of the adjacently bound activator YY1. Blimp-1 contains two regions that independently associate with histone deacetylase (HDAC) and endogenous Blimp-1 in nuclear extracts binds in vitro to the c-myc Blimp-1 site in a complex containing HDAC. The functional importance of recruiting HDAC for Blimp-1-dependent repression of c-myc transcription is supported by two experiments. First, the HDAC inhibitor tricostatin A inhibits Blimp-1-dependent repression in cotransfection assays. Second, a chromatin immunoprecipitation assay shows that expression of Blimp-1 causes deacetylation of histone H3 associated with the c-myc promoter, and this deacetylation depends on the Blimp-1 binding site in the c-myc promoter.

Differential RNA display identifies novel genes associated with decreased vitamin D receptor expression

To characterize further the function of the intracellular vitamin D receptor (VDR), we have developed stable transfectant variants of a vitamin D-responsive cell line (U937) which express either decreased or increased numbers of VDR. In this study we have analyzed changes in gene expression associated with this variable VDR expression. Initial experiments indicated that a 50% decrease in VDR levels was associated with a 2-fold increase in cell proliferation and a similar rise in c-myc mRNA expression. Further studies were carried out using differential RNA display (DD). Sequence analysis of DD products revealed two cDNAs with identity to known gene products: the catalytic sub-unit of DNA-protein kinase (DNA-PK(CS)), and the peroxisomal enzyme 17beta-hydroxysteroid dehydrogenase type IV (17beta-HSD IV). Northern analysis confirmed that expression of both mRNAs was reduced in cells with decreased numbers of VDR. Down-regulation of 17beta-HSD IV mRNA expression was associated with enhanced estradiol inactivation by U937 cells, suggesting a link between estrogenic pathways and cell proliferation. Further Northern analyses indicated that there was no significant change in 17beta-HSD IV or DNA-PK(CS) mRNA levels following treatment with 1,25(OH)2D3, although expression of both genes varied with changes in cell proliferation. These data suggest that, in addition to its established role as a hormone-dependent trans-activator, VDR may influence gene expression by ligand-independent mechanisms.

The clinical significance of c-myc oncogene expression in melanomas of the scalp

Melanomas of the scalp are rare and have a poorer prognosis than melanomas arising at other cutaneous sites. In order to study the biology of this disease, the activity of the c-myc oncogene was studied in tumours from 25 patients with scalp melanoma using the technique of flow cytometry. Survival analysis revealed that stratification of patients according to oncogene activity provided a prognostic marker with shorter overall survival (log rank test, chi 2 = 3.9, P = 0.05) in tumours with high nuclear c-myc oncoprotein positivity. These results support our previous studies of the prognostic value of c-myc expression in melanoma and suggest that estimation of c-myc oncoprotein may be of clinical significance in identifying high risk patients.

Expression of c-myc oncoprotein represents a new prognostic marker in cutaneous melanoma

BACKGROUND

Overexpression of the c-myc oncoprotein has been observed to be of prognostic significance in several human cancers. This study was undertaken to establish whether c-myc has any prognostic significance in melanoma.

METHODS

Expression of c-myc oncoprotein was studied in a prospective series of primary and secondary malignant melanomas. Ethanol-fixed tissue was digested into nuclei and stained for flow cytometric analysis using a pan-myc antibody.

RESULTS

Overexpression of the c-myc oncoprotein was prevalent in both disease stages. In primary disease, overexpression was associated with tumour thickness and with the presence of ulceration, age and reduced disease-free interval. Of most importance, high expression of c-myc predicted poor outcome in both primary and metastatic disease. In addition, c-myc was able to discriminate prognosis in melanomas uniformly stratified as thick (greater than 3 mm) lesions.

CONCLUSION

These results indicate an important role for c-myc in melanoma which may lead to better prognostic information and identification of new therapeutic strategies.

Measurement of c-myc oncoprotein provides an independent prognostic marker for regional metastatic melanoma

Patients with melanoma who develop nodal metastatic disease represent a group with heterogeneous clinical outcome. Nodal positivity remains the most accurate prognostic marker for regional melanoma although it fails to predict outcome in a significant number of patients. Recent studies have illustrated the prognostic potential of c-myc oncogene expression in melanoma. The aim of this study was to measure c-myc oncoprotein in a series of regional metastatic specimens from 48 patients, and evaluate its use as a marker of clinical outcome. Oncoprotein expression was detected in 46 (96%) of the tumours with a median positivity of 68% (range 0-98%). Survival analysis revealed a significant association between oncoprotein positivity and survival (Long-Rank test, chi 2 = 15.2, P < 0.001). Multivariate analysis of outcome showed c-myc oncoprotein to be an independent prognostic marker more accurate than all other clinicopathological parameters including nodal positivity (chi 2 = 8.34, P = 0.003). Estimation of c-myc oncoprotein is therefore recommended as a powerful prognostic marker for regional metastatic melanoma.

Repression of c-myc transcription by Blimp-1, an inducer of terminal B cell differentiation

Transcription of c-myc in plasma cells, which are terminally differentiated B cells, is repressed by plasmacytoma repressor factor. This factor was identified as Blimp-1, known for its ability to induce B cell differentiation. Blimp-1 repressed c-myc promoter activity in a binding site-dependent manner. Treatment of BCL1 lymphoma cells with interleukin-2 (IL-2) plus IL-5 induced Blimp-1 and caused a subsequent decline in c-Myc protein. Ectopic expression of Blimp-1 in Abelson-transformed precursor B cells repressed endogenous c-Myc and caused apoptosis; Blimp-1-induced death was partially overcome by ectopic expression of c-Myc. Thus, repression of c-myc is a component of the Blimp-1 program of terminal B cell differentiation.

c-myc oncogene expression and cell proliferation in mixed oligo-astrocytoma

Mixed gliomas (oligo-astrocytomas) are brain tumours with an admixture of 2 different cell populations: astrocytes and oligodendroglia. On the basis of histological features and behaviour, these tumours are classified as low-grade mixed gliomas (MG) and malignant mixed gliomas (MMG). We have studied the relationship between c-myc protein expression and cellular proliferation in this class of tumours. Using antibody c-33 for c-myc and PC-10 for the proliferating cell nuclear antigen (PCNA), immunohistochemistry was performed on 14 MG and 9 MMG. PCNA was increased in MMG as compared to MG in both astrocytic and oligodendroglial areas. However, more c-myc-positive cells were seen only in the astrocyte areas of MMG. Analysis of the relationship of c-myc and PCNA suggests that the correlation of c-myc with cellular proliferation is dependent on tissue type and differentiation status.

Stabilization of c-myc protein in human glioma cells

The regulation of c-myc protein, product of c-myc/genes, was studied in four glioma cell lines by Northern blot, pulse-chase dot blot, immunoblot and immunoprecipitation analyses. Northern blot analysis revealed no overexpression of c-myc transcript, and pulse-chase dot blot analysis showed normal turnover rate of c-myc transcript, suggestive of no evidence of aberrant regulation of c-myc at post-transcriptional level. The synthesis levels of c-myc protein were shown by immunoprecipitation and closely associated with the c-myc transcript levels demonstrated by Northern blot, suggestive of no evidence of aberrant translational control of c-myc, whereas they were dissociated from the accumulation levels of c-myc protein shown by immunoblot, suggestive of an evidence of aberrant regulation of c-myc at post-translational level. The mean (+/- standard deviation) half-lives of c-myc protein in four glioma cell lines were calculated from the pulse-chase immunoprecipitation analysis, and being 98 +/- 8 to 143 +/- 11 min, were about four- to sixfold longer than normal. In surgical specimens, the immunostain of c-myc protein was not found in normal astrocytes but localized heterogenously in nuclei of reactive astrocytes and glioma cells, and increased in stained cell number in proportion to malignancy. Although this study was limited to four glioma cell lines, it suggests that the c-myc protein in glioma cells may be accumulated due to its prolonged half-life contributing to an uncontrolled proliferation.

c-myc oncoprotein expression in oral precancerous and early cancerous lesions

Squamous cell carcinoma of the oral cavity is often preceded by clinically evident precancerous lesions. These lesions exhibit progressive cytomorphologic changes connoting cellular transformation to malignant neoplasia. Molecular events underlie the microscopically identifiable cytological changes that herald transformation. Various proto-oncogenes are activated, overexpressed or mutated in the process of transformation. This investigation was designed to determine whether the nuclear binding c-myc oncoprotein, an activator of cell division, parallels or precedes cytomorphological changes at various histologically defined stages in oral precancerous lesions. Employing immunohistological methods, it was determined that c-myc nuclear labelling paralleled the progressive histological changes among various stages of transformation. It was also noted that some, yet not all, instances of benign keratoses which lacked any cytologic evidence of atypia harboured c-myc oncoprotein positive nuclei. It is concluded that c-myc expression is correlated with progressive cell transformation in oral cancerous lesions.

Enhanced expression of the protooncogenes c-myc and c-fos in normal and malignant renal growth

The protooncogenes c-myc and c-fos play an important role in growth and differentiation of renal tissue. They are highly expressed during embryogenesis in the mitotically active tubule epithelium, while in terminally differentiated tubule cells of the kidney the expression is completely shut off. Furthermore, induction of cell proliferation in cultured renal cells by addition of growth factors is preceded by enhanced expression of c-myc and c-fos. Increased expression of these protooncogenes is also obtained by treatment of kidney cells in culture with the potent nephrocarcinogen N-dimethylnitrosamine and also with the nephrotoxin and possibly nephrocarcinogen S-(1,2-dichlorovinyl)-L-cysteine. Finally, the expression of c-myc and c-fos is induced after unilateral nephrectomy during compensatory renal growth in the remaining kidney and also during regenerative cell proliferation after in vivo application of the strong nephrotoxins folic acid and mercury chloride.

The nephrotoxin dichlorovinylcysteine induces expression of the protooncogenes c-fos and c-myc in LLC-PK1 cells--a comparative investigation with growth factors and 12-O-tetradecanoylphorbolacetate

Previous studies in kidney cells showed that S-(1,2-dichlorovinyl)-L-cysteine (DCVC) induces both direct DNA damage and DNA double-strand breaks by activation of Ca(2+)-dependent endonucleases. The objective of this study was to investigate the effects of DCVC on the expression of the protooncogenes c-fos and c-myc in cultured kidney cells (LLC-PK1). Supplementation of the incubation medium with 10% FCS after 24 hr incubation in 0.2% FCS resulted in a clear, but comparatively weak induction of the expression of c-fos and c-myc in LLC-PK1 cells. Addition of 500 microns DCVC to the high serum incubation medium induced a further three-fold increase of the transcript levels. A similar increase in the absolute amount of c-fos mRNA was induced by a mixture of growth factors (epidermal growth factor/insulin/transferrin) and of c-myc mRNA with 12-O-tetradecanoylphorbolacetate. However, the kinetics of gene expression were different. In the presence of DCVC the expression of c-fos and c-myc increased continuously in a time-dependent manner during the entire incubation period. In contrast, with growth factors and 12-O-tetradecanoyl-phorbolacetate the maximum transcript levels were detected after 0.5 hr (c-fos) and 1 hr (c-myc), respectively; thereafter, a slight decrease was observed up to the end of the incubation time.

The proto-oncogene c-myc is involved in cell differentiation as well as cell proliferation: studies on growth plate chondrocytes in situ

A combination of immunocytochemistry and microdensitometry has been used to localize and quantify the expression of the proto-oncogene c-myc within chondrocytes of the proximal growth plates of rat and chick long bones. Although the c-myc protein was localized in all chondrocytes of the growth plate of both species the most intense staining was restricted to the proliferating and differentiating chondrocytes. These were identified by their ability to synthesize DNA (bromodeoxyuridine positive) and the presence of alkaline phosphatase activity, respectively. Species differences did exist with the c-myc concentration of the chick proliferating and differentiating chondrocytes being higher (128% and 240%, respectively) than the respective chondrocytes of the rat. The higher c-myc concentration in the chick proliferating chondrocytes paralleled the differences in the bromodeoxyuridine labelling index between the two species. In the rat, the concentration of c-myc protein present in the differentiating chondrocytes was 74% higher than in the respective proliferating chondrocytes, while in the chick it was 146% higher. The data not only provides further evidence for a role of the c-myc protein in cell proliferation but also suggests involvement of this protein in chondrocyte differentiation and/or hypertrophy.

Down-regulation of cellular proto-oncogenes during inhibition of rat parotid acinar cell proliferation

The role of cell surface galactosyltransferase in mediating isoproterenol-induced parotid gland hypertrophy and hyperplasia was examined in rat parotid gland acinar cells. Introduction of the transferase modifier, alpha-lactalbumin, or galactosyltransferase-associated kinase inhibitor trifluoperazine, into beta-agonist-treated rats prevented acinar cell proliferation as determined by [3H]thymidine incorporation after 96 h of treatment. However, [3H]thymidine incorporation into DNA after 24 h of treatment, with injection of a combination of isoproterenol/alpha-lactalbumin or isoproterenol/trifluoperazine, was similar to injections of isoproterenol alone; suggesting that acinar cells could be stimulated to undergo a single round of DNA synthesis. Northern blot analysis of myc and fos expression followed a similar pattern of down-regulation to control levels after 96 h but not after 24 h. Hybridization with erb B showed little change with proliferation, confirming previous observations on protein levels of the EGF-receptor in acinar cells. Western blot analysis of nuclear protein expression of myc revealed that isoproterenol caused an increase in a 62-kDa protein which was again down-regulated with inhibition of cell proliferation. Analysis of protein levels of Rb110 protein showed no change in protein level in the nucleus with cell proliferation, but did show an associated increase in protein phosphorylation in response to growth stimulation.