Overexpression Of C-myc Protein Research Articles

ATM suppresses c-Myc overexpression in the mammary epithelium in response to estrogen

ATM gene mutation carriers are predisposed to estrogen-receptor-positive breast cancer (BC). ATM prevents BC oncogenesis by activating p53 in every cell; however, much remains unknown about tissue-specific oncogenesis after ATM loss. Here, we report that ATM controls the early transcriptional response to estrogens. This response depends on topoisomerase II (TOP2), which generates TOP2-DNA double-strand break (DSB) complexes and rejoins the breaks. When TOP2-mediated ligation fails, ATM facilitates DSB repair. After estrogen exposure, TOP2-dependent DSBs arise at the c-MYC enhancer in human BC cells, and their defective repair changes the activation profile of enhancers and induces the overexpression of many genes, including the c-MYC oncogene. CRISPR/Cas9 cleavage at the enhancer also causes c-MYC overexpression, indicating that this DSB causes c-MYC overexpression. Estrogen treatment induced c-Myc protein overexpression in mammary epithelial cells of ATM-deficient mice. In conclusion, ATM suppresses the c-Myc-driven proliferative effects of estrogens, possibly explaining such tissue-specific oncogenesis.

Bisquinolinium-fluorescein conjugates as specific fluorescence probes of c-myc Pu22 G-quadruplex and their bioimaging and anticancer activities

Targeting G-quadruplexes in the c-myc gene promoter region is one of the few therapeutic opportunities through inhibiting the c-myc protein overexpression. However, the design of probes recognizing c-myc G-quadruplexes with high selectivity and specificity and the more evaluation of cellular system still remain a big challenge. Here, two novel bisquinolinium-fluorescein conjugates (1a and 1b) with the alkyl linkers, have been designed and evaluated their selectivity and specificity for parallel c-myc Pu22 G-quadruplex. Compared with conjugate 1b with longer alkyl linker, conjugate 1a with shorter alkyl linker, exhibited higher fluorescence response and specificity towards c-myc Pu22 G-quadruplex than other wild-type c-myc G-quadruplexes, human telomere G-quadruplexes, other G-quadruplexes in the promoter regions and double-stranded DNA. According to the binding mode, the interaction of compound 1a with the 2nd loop around the 3′-end G-quartet through regulating the length of the alkyl linker, excited its fluorescence and enhanced its selectivity towards c-myc Pu22 G-quadruplex. Furthermore, conjugate 1a could selectively recognize c-myc Pu22 G-quadruplex DNA in cells through microscopy experiments, and inhibit cell proliferation possibly by reducing c-myc protein expression in cancer cells. This study provides guidance to design the high-performance fluorescence probes towards c-myc G-quadrulex by regulating the alkyl linker in the conjugate of G-quadruplex binder and fluorescence ligand, and develop anticancer drugs targeting c-myc G-quadruplex.

C-MYC G-quadruplex binding by the RNA polymerase I inhibitor BMH-21 and analogues revealed by a combined NMR and biochemical Approach

BackgroundPyridoquinazolinecarboxamides have been reported as RNA polymerase I inhibitors and represent a novel class of potential antitumor agents. BMH-21, was reported to intercalate with GC-rich rDNA, resulting in nucleolar stress as a primary mechanism of cytotoxicity. MethodsThe interaction of BMH-21 and analogues with DNA G-quadruplex structures was studied by NMR and molecular modelling. The cellular response was investigated in a panel of human tumor cell lines and protein expression was examined by Western Blot analysis. Results and conclusionsWe explored the ability of BMH-21 and its analogue 2 to bind to G-quadruplex present in the c-MYC promoter, by NMR and molecular modelling studies. We provide evidence that both compounds are not typical DNA intercalators but are effective binders of the tested G-quadruplex. The interaction with c-MYC G-quadruplex was reflected in down-regulation of c-Myc expression in human tumor cells. The inhibitory effect was almost complete in lymphoma cells SUDHL4 characterized by overexpression of c-Myc protein. This downregulation reflected an early and persistent modulation of cMyc mRNA. Given the relevance of c-MYC in regulation of ribosome biogenesis, it is conceivable that the inhibition of c-MYC contributes to the perturbation of nuclear functions and RNA polymerase I activity. Similar experiments with CX-5461, another RNA polymerase I transcription inhibitor, indicate the same behaviour in G-quadruplex stabilization. General significanceOur results support the hypothesis that BMH-21 and analogue compounds share the same mechanism, i.e. G-quadruplex binding as a primary event of a cascade leading to inhibition of RNA polymerase I and apoptosis.

Favorable prognosis in colorectal cancer patients with co-expression of c-MYC and ß-catenin.

BackgroundThe purpose of our research was to determine the prognostic impact and clinicopathological feature of c-MYC and β-catenin overexpression in colorectal cancer (CRC) patients.MethodsUsing immunohistochemistry (IHC), we measured the c-MYC and β-catenin expression in 367 consecutive CRC patients retrospectively (cohort 1). Also, c-MYC expression was measured by mRNA in situ hybridization. Moreover, to analyze regional heterogeneity, three sites of CRC including the primary, distant and lymph node metastasis were evaluated in 176 advanced CRC patients (cohort 2).ResultsIn cohort 1, c-MYC protein and mRNA overexpression and ß-catenin nuclear expression were found in 201 (54.8 %), 241 (65.7 %) and 221 (60.2 %) of 367 patients, respectively, each of which was associated with improved prognosis (P = 0.011, P = 0.012 and P = 0.033, respectively). Moreover, co-expression of c-MYC and ß-catenin was significantly correlated with longer survival by univariate (P = 0.012) and multivariate (P = 0.048) studies. Overexpression of c-MYC protein was associated with mRNA overexpression (ρ, 0.479; P < 0.001) and nuclear ß-catenin expression (ρ, 0.282; P < 0.001). Expression of c-MYC and ß-catenin was heterogeneous depending on location in advanced CRC patients (cohort 2). Nevertheless, both c-MYC and ß-catenin expression in primary cancer were significantly correlated with improved survival in univariate (P = 0.001) and multivariate (P = 0.002) analyses. c-MYC and ß-catenin expression of lymph node or distant metastatic tumor was not significantly correlated with patients’ prognosis (P > 0.05).ConclusionsCo-expression of c-MYC and ß-catenin was independently correlated with favorable prognosis in CRC patient. We concluded that the expression of c-MYC and ß-catenin might be useful predicting indicator of CRC patient’s prognosis.Electronic supplementary materialThe online version of this article (doi:10.1186/s12885-016-2770-7) contains supplementary material, which is available to authorized users.

C-MYC Copy-Number Gain Is an Independent Prognostic Factor in Patients with Colorectal Cancer.

BackgroundThe aim of this study was to determine the incidence and clinicopathological significance of c-MYC gene copy-number (GCN) gain in patients with primary colorectal cancer (CRC).MethodsThe c-MYC GCN was investigated in 367 consecutive CRC patients (cohort 1) by using dual-color silver in situ hybridization. Additionally, to evaluate regional heterogeneity, we examined CRC tissue from 3 sites including the primary cancer, distant metastasis, and lymph-node metastasis in 152 advanced CRC patients (cohort 2). KRAS exons 2 and 3 were investigated for mutations.ResultsIn cohort 1, c-MYC gene amplification, defined by a c-MYC:centromere of chromosome 8 ratio ≥ 2.0, was detected in 31 (8.4%) of 367 patients. A c-MYC GCN gain, defined by ≥ 4.0 c-MYC copies/nucleus, was found in 63 (17.2%) patients and was associated with poor prognosis (P = 0.015). Multivariate Cox regression analysis showed that the hazard ratio for c-MYC GCN gain was 2.35 (95% confidence interval, 1.453–3.802; P < 0.001). In a subgroup of stage II-III CRC patients, c-MYC GCN gain was significantly associated with poor prognosis by univariate (P = 0.034) and multivariate (P = 0.040) analyses. c-MYC protein overexpression was observed in 201 (54.8%) out of 367 patients and weakly correlated with c-MYC GCN gain (ρ, 0.211). In cohort 2, the c-MYC genetic status was heterogenous in advanced CRC patients. Discordance between GCN gain in the primary tumor and either distant or lymph-node metastasis was 25.7% and 30.4%, respectively. A similar frequency for c-MYC GCN gain and amplification was observed in CRC patients with both wild-type and mutated KRAS.Conclusions c-MYC GCN gain was an independent factor for poor prognosis in consecutive CRC patients and in the stage II-III subgroup. Our findings indicate that the status of c-MYC may be helpful in predicting the patients’ outcome and for managing CRC patients.

Gemcitabine-based regimen for primary ovarian angiosarcoma with MYC amplification.

Angiosarcoma is a rare and aggressive type of sarcoma, and primary angiosarcoma of the ovary is extremely rare. We report the case of a 29-year-old woman who was diagnosed with ovarian angiosarcoma and possible bone metastases. We treated this patient with a gemcitabine-based regimen as postoperative adjuvant chemotherapy, after which she achieved at least 7 years of progression-free survival, an extremely long duration given the aggressive features of this tumour. We retrospectively performed immunohistochemical analyses and fluorescence in situ hybridization to make a pathology diagnosis and to investigate the tumour features. MYC amplification and c-Myc protein overexpression were positively detected. It might be possible to correlate the effectiveness of the gemcitabine-based chemotherapeutic regimen with MYC gene amplification and c-Myc protein overexpression.

Evaluation of the antitumor effects of c-Myc-Max heterodimerization inhibitor 100258-F4 in ovarian cancer cells.

Epithelial ovarian carcinoma is the most lethal gynecological cancer due to its silent onset and recurrence with resistance to chemotherapy. Overexpression of oncogene c-Myc is one of the most frequently encountered events present in ovarian carcinoma. Disrupting the function of c-Myc and its downstream target genes is a promising strategy for cancer therapy. Our objective was to evaluate the potential effects of small-molecule c-Myc inhibitor, 10058-F4, on ovarian carcinoma cells and the underlying mechanisms by which 10058-F4 exerts its actions. Using MTT assay, colony formation, flow cytometry and Annexin V FITC assays, we found that 10058-F4 significantly inhibited cell proliferation of both SKOV3 and Hey ovarian cancer cells in a dose dependent manner through induction of apoptosis and cell cycle G1 arrest. Treatment with 10058-F4 reduced cellular ATP production and ROS levels in SKOV3 and Hey cells. Consistently, primary cultures of ovarian cancer treated with 10058-F4 showed induction of caspase-3 activity and inhibition of cell proliferation in 15 of 18 cases. The response to 10058-F4 was independent the level of c-Myc protein over-expression in primary cultures of ovarian carcinoma. These novel findings suggest that the growth of ovarian cancer cells is dependent upon c-MYC activity and that targeting c-Myc-Max heterodimerization could be a potential therapeutic strategy for ovarian cancer.

C-MYC aberrations as prognostic factors in diffuse large B-cell lymphoma: a meta-analysis of epidemiological studies.

ObjectivesVarious studies have investigated the prognostic value of C-MYC aberrations in diffuse large B-cell lymphoma (DLBCL). However, the role of C-MYC as an independent prognostic factor in clinical practice remains controversial. A systematic review and meta-analysis were performed to clarify the clinical significance of C-MYC aberrations in DLBCL patients.MethodsThe pooled hazard ratios (HRs) for overall survival (OS) and event-free survival (EFS) were calculated as the main effect size estimates. The procedure was conducted according to the Cochrane handbook and PRISMA guidelines, including the use of a heterogeneity test, publication bias assessment, and meta-regression, as well as subgroup analyses.ResultsTwenty-four eligible studies enrolling 4662 patients were included in this meta-analysis. According to the nature of C-MYC aberrations (gene, protein, and mRNA), studies were divided into several subgroups. For DLBCL patients with C-MYC gene abnormalities, the combined HR was 2.22 (95% confidence interval, 1.89 to 2.61) for OS and 2.29 (95% confidence interval, 1.81 to 2.90) for EFS, compared to patients without C-MYC gene abnormalities. For DLBCL patients with overexpression of C-MYC protein and C-MYC mRNA, pooled HRs for OS were 2.13 and 1.62, respectively. C-MYC aberrations appeared to play an independent role among other well-known prognostic factors in DLBCL. Addition of rituximab could not overcome the inferior prognosis conferred by C-MYC.ConclusionThe present systematic review and meta-analysis confirm the prognostic value of C-MYC aberrations. Screening of C-MYC should have definite prognostic meaning for DLBCL stratification, thus guaranteeing a more tailored therapy.

Common pathways for primary hypertrophic and dilated cardiomyopathy.

Testing the hypothesis that hypertrophic and dilated cardiomyopathy as well as viral myocarditis share a common mitogenic growth response pathway with mitotically competent cell types are the aims of this study. The expression of the c-fos, H-ras and c-myc genes was immunohistochemically determined in biopsies from 12 patients with dilated cardiomyopathy, 24 patients with hypertrophic cardiomyopathy, and 4 patients with myocarditis. Normal myocardium from 9 subjects was used as the control group. Staining results were correlated with patient's demographic data. C-fos, H-ras and c-myc protein overexpression was seen in 15 patients (62.5%) with primary hypertrophic and 4 patients (33.3%) with dilated cardiomyopathy. The majority of hypertrophic and dilated cardiomyopathy patients expressed at least one of the genes studied compared with the control group (p = 0.006). Primary cardiomyopathy patients also showed a statistically significant difference in the gene co-expression compared with the control group (p = 0.042). C-fos, H-ras, and C-myc protein expression did not differ substantially between patients with hypertrophic and dilated cardiomyopathy. Patients with myocarditis expressed only the C-fos protein (n = 2, 50%). C-fos, h-ras and c-myc genes are overexpressed in patients with cardiac hypertrophy and cardiac dilation. Cardiac myocytes respond to biomechanical stress by initiating several different processes. One of them is oncogene expression. This results in a hypertrophy of the myocytes proportional in length and width (hypertrophic cardiomyopathy or with a relatively greater increase in length than in the width (dilated cardiomyopathy).

C-MYC Activation in Primary and Metastatic Ductal Adenocarcinoma of the Pancreas: Incidence, Mechanisms, and Clinical Significance

Amplification and overexpression of c-MYC is a common event in various neoplasias. Recently, comparative genomic hybridization (CGH) of primary pancreatic adenocarcinomas revealed a distinct high-level amplification of 8q23-qter, suggesting that c-MYC located on 8q24 may be a candidate oncogene. To evaluate the biological significance and prognostic value of c-MYC activation in pancreatic carcinoma, we performed interphase fluorescence in situ hybridization (FISH) and immunohistochemistry on a series of 69 primary pancreatic adenocarcinomas, 19 corresponding lymph node metastases, and 5 pancreatic intraductal lesions. Dual color FISH using a probe for c-MYC (8q24) and a centromeric probe for chromosome 8 revealed amplification of c-MYC in 32.3% and 29.4% of primary and metastatic tumors, respectively. Immunostaining identified c-MYC protein overexpression in 43.5% of primaries and 31.6% of metastases. Low concordance between positive FISH and immunostaining (13.4%) suggests multiple independent regulatory pathways of c-MYC activation. Statistical evaluation revealed significant correlation (α = 0.033) between c-MYC protein overexpression and histopathological tumor grade but absence of correlation with tumor stage or lymph node status. Analysis of pancreatic intraductal lesions showed c-MYC amplification and protein overexpression in two of five cases in which invasive carcinoma exhibited identical aberrations. We conclude that deregulation of c-MYC protein is common in pancreatic cancer and that it may be involved in early neoplastic development and progression rather than in locoregional spread of invasive cancer.

Protein expression and molecular analysis of c-myc gene in primary breast carcinomas using immunohistochemistry and differential polymerase chain reaction

Overexpression of c-myc protein and amplification of c-myc were investigated by immunohistochemistry and differential polymerase chain reaction (dPCR) in 440 formalin-fixed primary breast carcinoma tissues, respectively. Overexpression of c-myc was detected in 45% (199/440) and amplification of c-myc was observed in 25% (112/440) of the primary breast carcinomas. Immunolocalization of c-myc oncoprotein was demonstrated in 35% (8/23) of the comedo subtype, 17% (3/18) of the non-comedo subtype, 37% (15/41) of the comedo DCIS and 49% (20/41) of the adjacent invasive ductal carcinomas, 21% (4/19) of the non-comedo DCIS and 37% (7/19) of the adjacent invasive lesions, 49% (133/270) of the invasive ductal carcinomas, 33% (11/33) of the invasive lobular carcinomas, 29% (6/21) of the colloid carcinomas and 47% (7/15) of the medullary carcinomas. C-myc was amplified in 13% (3/23) of the comedo DCIS, 17% (7/41) of the comedo DCIS and 24% (10/41) of the adjacent invasive ductal carcinomas, 30% (82/270) of the invasive ductal carcinomas, 21% (7/33) of the invasive lobular carcinomas, 14% (3/21) of the colloid carcinomas and 24% (4/15) of the medullary carcinomas. Amplification of c-myc was noted in 16% (3/9) of the invasive ductal carcinomas but not in the adjacent non-comedo DCIS lesions. A significant association (P<0.05) was observed between in situ components and adjacent invasive lesions for c-myc expression and amplification. Overexpression of c-myc protein was significantly correlated with poorly differentiated (P<0.05) and high proliferation index (Ki-67) (P<0.05) tumors but not with lymph node metastases (P>0.05), patient age (P>0.05) and estrogen receptor status (P>0.05). Significant relationship was also noted between amplification of c-myc and absence of estrogen receptor (P<0.05), high histological grade (P<0.05) and high proliferation index (Ki-67) (P<0.05). No relationship was seen with nodal status (P>0.05) and patient age (P>0.05). Majority of the Malaysian female patients are from younger age group (<50 years old) but overexpression and amplification of c-myc was not statistically associated with patient age (P>0.05) indicating that these alterations may be independent events of patient age. The above observations suggest that overexpression and amplification of c-myc could play an important role in tumor progression from non-invasive to invasive and, also, it may have the potential as a marker of poor prognosis of breast cancer.

Amplification of c- myc gene and overexpression of c-Myc protein in breast cancer and adjacent non-neoplastic tissue

Background: Deregulated c-Myc expression and alterations of c- myc oncogene have been reported to play an important role in breast cancer tumorigenesis. We examined the relationship between c-Myc protein level, amplification of c- myc oncogene and commonly used clinical and pathologic factors. Methods: The studies were conducted on 94 ductal and lobular cancers. Amplification of c- Myc was assessed by the semiquantitative multiplex PCR assay. The amount of c-Myc protein was estimated by the densitometry analysis of Western blots. Results: Amplification of c- Myc was found in 21% of examined cancers. There was no association of c- myc amplification with established risk factors. Overexpression of c-Myc protein without c- myc amplification was associated with negative status of axillary lymph node. The size of lobular carcinoma displaying overexpression of c-Myc and the normal copy number of c- myc gene was significantly smaller than the size of tumor with elevated c-Myc and amplification of c- myc gene ( p < 0.01). Within tumors displaying overexpression of c-Myc protein and c- myc gene amplification the size of ductal carcinoma was smaller than the size of lobular carcinoma ( p < 0.007). Conclusion: Data presented in this study suggest that alterations of c- myc gene and c-Myc protein level might be related to breast cancer progression. The prognostic utility of elevated level of c-Myc protein associated with normal status of c- myc gene for patients with lobular carcinoma requires further studies.

A Study of Biomarkers in Cervical Carcinoma and Clinical Correlation of the Novel Biomarker MN

The MN protein is a newly described biomarker found to be overexpressed in most cervical carcinomas. This study was an effort to evaluate the prognostic importance of tumor MN expression, HPV status, and the presence of other biomarkers in cervical cancers. Tumor DNA and protein for study were extracted from archived frozen tissue. Tumor tissues and controls were evaluated by Western blot analysis for MN, intestinal alkaline phosphatase (IAP), c-myc, and p53 protein overexpression. Immunohistochemistry was performed for MN quantification and the study of expression patterns in histologic subtypes of cervical cancer. HPV data were obtained by PCR amplification of extracted DNA using consensus and type-specific primers. Clinical data were obtained from the patients’ records and from the cancer registry. Clinical and molecular data were correlated by χ2, Fisher's exact test, and logistic regression. The results demonstrate that IAP is not overexpressed in clinical specimens of cervical carcinoma, although in somatic cell hybrid experiments, overexpression of IAP correlates with the malignant state. None of 47 tumors, including those which were HPV negative, overexpressed p53. c-myc protein overexpression occurred in 11 of 52 tumors, most of which contained HPV 16, but this was not significantly different from the tumors as a whole. There was no apparent association between MN protein expression and the overexpression of c-myc protein. MN was overexpressed in all cancers and quantitatively varied with the histologic subtype. Specifically, lower expression of MN correlated with adenosquamous and less-differentiated histology (P< 0.01 for grade 3 tumors). Low expression of MN protein also correlated with HPV negativity (P< 0.05). In stage IB and IIA cancers, low expression of MN was associated with deeper cervical stromal invasion (P< 0.03). Further, low expression of MN correlated with lymph node metastases in small (<3.5 cm) IB and IIA cervical cancers (P< 0.04). These data suggest that MN is emerging as a potentially important new biomarker for cervical carcinoma. The overexpression commonly seen in cervical cancer is possibly associated with loss of a critical tumor suppressor gene located on chromosome 11. Low expression of MN antigen appears to correlate with several adverse prognostic features and further prospective study is warranted.

Apoptotic cell death and its relationship to carcinogenesis in colorectal carcinoma.

Apoptotic cell death plays an important role in the proliferation and turnover of cells in various tumors. The relationship between apoptosis and cell proliferation was studied to determine each of their roles in colorectal carcinogenesis.Apoptotic cells were identified by the terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling (TUNEL) method. The occurrence of apoptosis was examined in colorectal cancer that had invaded the submucosa. Specimens were obtained from 38 cases of cancer with adenoma and 29 cases of cancer de novo. Apoptotic indices (AIs) as percentages of TUNEL-positive cells relative to the number of tumor cells and Ki-67 labeling indices (PI) were investigated. The relationship between the frequency of apoptosis and the expression of p53 and c-myc proteins was also investigated.In cancer with adenoma, the ratio of AI/PI in adenoma cells was significantly higher than that of cancer cells (P < 0.0001). Mean AIs of cancer with adenoma were significantly higher than those of cancer de novo particularly in the flat-type cancers (P < 0.05). Among p53 negative tumors, the ratio of AI/PI for cancer de novo was significantly lower than that for cancer with adenoma (P < 0.05). AI of cancer de novo was lower than that of cancer with adenoma in cases with overexpression of c-myc protein (P < 0.05), whereas there was no significant difference in the ratio of AI/PI between cancer de novo and cancer with adenoma.Colorectal carcinogenesis is related to the inhibition of apoptosis and to the augmentation of proliferative activity both in cancer with adenoma and in cancer do novo. A reduction of the rate of apoptosis as compared with that of cell proliferation might explain the rapid-growing nature of cancer de novo particularly in cases with the flat-type appearance.

Apoptotic cell death and its relationship to carcinogenesis in colorectal carcinoma

Apoptotic cell death plays an important role in the proliferation and turnover of cells in various tumors. The relationship between apoptosis and cell proliferation was studied to determine each of their roles in colorectal carcinogenesis. Apoptotic cells were identified by the terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling (TUNEL) method. The occurrence of apoptosis was examined in colorectal cancer that had invaded the submucosa. Specimens were obtained from 38 cases of cancer with adenoma and 29 cases of cancer de novo. Apoptotic indices (AIs) as percentages of TUNEL-positive cells relative to the number of tumor cells and Ki-67 labeling indices (PI) were investigated. The relationship between the frequency of apoptosis and the expression of p53 and c-myc proteins was also investigated. In cancer with adenoma, the ratio of AI/PI in adenoma cells was significantly higher than that of cancer cells (P < 0.0001). Mean AIs of cancer with adenoma were significantly higher than those of cancer de novo particularly in the flat-type cancers (P < 0.05). Among p53 negative tumors, the ratio of AI/PI for cancer de novo was significantly lower than that for cancer with adenoma (P < 0.05). AI of cancer de novo was lower than that of cancer with adenoma in cases with overexpression of c-myc protein (P < 0.05), whereas there was no significant difference in the ratio of AI/PI between cancer de novo and cancer with adenoma. Colorectal carcinogenesis is related to the inhibition of apoptosis and to the augmentation of proliferative activity both in cancer with adenoma and in cancer do novo. A reduction of the rate of apoptosis as compared with that of cell proliferation might explain the rapid-growing nature of cancer de novo particularly in cases with the flat-type appearance.

Apoptosis Induced by c-Myc Overexpression Is Dependent on Growth Conditions

Recent reports have suggested a causal link between the expression of the c- myc gene and ensuing cell death by apoptosis, particularly in cells prevented from dividing or after withdrawal of growth factors. In contrast, other studies have suggested that cells constitutively expressing c- myc are actually more resistant to cell death induced by some chemotherapeutic drugs that block cell division. We have examined the frequency of cell death in several Chinese hamster ovary cell lines that contain 20 to 30 copies of the human c- myc gene and that express high levels of human c- myc mRNA and protein. We found that constitutive c- myc expression in cells incubated at low density in medium containing 0.1% serum correlates with increased cell death due to apoptosis, as indicated by oligonucleosomal DNA fragmentation and a requirement for ongoing protein synthesis. However, apoptosis was not enhanced in these cells when they were blocked in cell division in the presence of serum, nor when grown at moderate to high density under low-serum conditions, despite their continued accumulation of high levels of c-Myc protein. Our results show that overexpression of c-Myc protein can promote cell death under some but not all conditions that block cell division and further suggest that c-Myc may accelerate but does not necessarily initiate apoptosis.