Hypoxic resistance of KRAS mutant tumor cells to 3-Bromopyruvate is counteracted by Prima-1 and reversed by N-acetylcysteine

Background

The metabolic inhibitor 3-bromopyruvate (3-BrPA) is a promising anti-cancer alkylating agent, shown to inhibit growth of some colorectal carcinoma with KRAS mutation. Recently, we demonstrated increased resistance to 3-BrPA in wt p53 tumor cells compared to those with p53 silencing or mutation. Since hypoxic microenvironments select for tumor cells with diminished therapeutic response, we investigated whether hypoxia unequally increases resistance to 3-BrPA in wt p53 MelJuso melanoma harbouring (Q61L)-mutant NRAS and wt BRAF, C8161 melanoma with (G12D)-mutant KRAS (G464E)-mutant BRAF, and A549 lung carcinoma with a KRAS (G12S)-mutation. Since hypoxia increases the toxicity of the p53 activator, Prima-1 against breast cancer cells irrespective of their p53 status, we also investigated whether Prima-1 reversed hypoxic resistance to 3-BrPA.

Results

In contrast to the high susceptibility of hypoxic mutant NRAS MelJuso cells to 3-BrPA or Prima-1, KRAS mutant C8161 and A549 cells revealed hypoxic resistance to 3-BrPA counteracted by Prima-1. In A549 cells, Prima-1 increased p21CDKN1mRNA, and reciprocally inhibited mRNA expression of the SLC2A1-GLUT1 glucose transporter-1 and ALDH1A1, gene linked to detoxification and stem cell properties. 3-BrPA lowered CAIX and VEGF mRNA expression. Death from joint Prima-1 and 3-BrPA treatment in KRAS mutant A549 and C8161 cells seemed mediated by potentiating oxidative stress, since it was antagonized by the anti-oxidant and glutathione precursor N-acetylcysteine.

Conclusions

This report is the first to show that Prima-1 kills hypoxic wt p53 KRAS-mutant cells resistant to 3-BrPA, partly by decreasing GLUT-1 expression and exacerbating pro-oxidant stress.

Abstract 3049: BRAF status and low glutamine/glucose influence glutamine addiction modulated by oxidative stress in human melanoma cells

BACKGROUND.- Cancer cells from solid tumors become metabolically stressed, when nutrients are insufficient within poorly vascularized regions. Metabolic stress results from severe deprivation of oxygen, glutamine and glucose, partly through excessive reactive oxygen species (ROS) production. Mutant BRAF can stimulate the accumulation of hypoxia-inducible factor-1alpha (HIF-1α) in physiological glucose and supplementation with supra-physiological glutamine in melanoma cells (Kumar SM et al, Cancer Res. 2007;67:3177-84).Although HIF1α reprograms melanomas to become highly glycolytic, this can be impaired in a microenvironment with sub-physiological levels of glutamine and glucose (Kwon SJ, Lee YJ.Clin Cancer Res. 2005;11:4694-700).,because melanomas require glycolysis and mitochondrial glutamine utilization. PURPOSE.- To compare proliferation and survival in response to glutamine and glucose restriction in wt-BRAF or mutant BRAF melanoma. EXPERIMENTAL. Determination of metabolic activity/proliferation and cell survival by fluorometric assays and crystal violet staining. RESULTS.- Glutamine depletion diminished survival even in physiological 5 mM glucose, particularly during hypoxia. However, 2.5 mM glucose or exogenous pyruvate when added together with another anti-oxidant N-acetylcysteine (NAC) partly substituted for glutamine in normoxic wt-BRAF cells. A much lower attenuation was seen in glutamine-depleted mutant BRAF cells when supplemented with NAC and pyruvate, particularly, under hypoxia. CONCLUSIONS.-1) glutamine restriction decreases metabolic activity and tumor cell survival, irrespective of BRAF status, being more toxic to mut-BRAF cells, either under hypoxia or in physiological glucose and normoxia,. 2) Antagonizing glutamine addiction may decrease tumor recurrence if combined with anti-BRAF targeted therapy.

Citation Format: Manuel Rieber, Valery Chavez-Perez, Mary Strasberg-Rieber. BRAF status and low glutamine/glucose influence glutamine addiction modulated by oxidative stress in human melanoma cells. [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 3049. doi:10.1158/1538-7445.AM2015-3049

Abstract 2907: AMPK induction, lysosomal acidification and melanoma survival increased by p53 dysfunction are counteracted by inhibiting autophagy

BACKGROUND. In contrast to the majority of human tumors which harbour p53 mutations, melanomas preferentially have BRAF V600E or NRAS(Q61K) mutations accompanied by dysfunction of the p53 tumor suppressor gene, which may help progression from nevi to invasive melanoma. Although ultraviolet radiation accelerates BRAF-driven melanomagenesis by targeting TP53 (Viros A. et al, Nature. 2014;511:478-82), some evidences for an anomalous wt p53 response to radiation in melanomas are a dysfunctional pro-apoptotic DNA damage response which can be enhanced by radiation sensitizers like halogenated pyrimidines (Rieber M, Rieber MS.Cancer Biol Ther. 2008;7:1561-6), and a report that p53 knockdown in melanoma cells resulted in decreased proliferation (Avery-Kiejda KA1, Bowden NA, Croft AJ, Scurr LL, Kairupan CF, Ashton KA, Talseth-Palmer BA, Rizos H, Zhang XD, Scott RJ, Hersey P.BMC Cancer. 2011;11:203) .PURPOSE. Since activation of AMPK by AMP inhibits ATP-consuming metabolic pathways during metabolic stress which may cause autophagy, we investigated the possible role of melanoma p53 status in modulating response to AMPK activators and autophagy inhibitors, in physiological 5mM glucose. EXPERIMENTAL. Genetically-matched NRAS-mutant human melanoma cells with unequal p53 mutational status were exposed to different glucose levels in the presence of AMPK inducers including autophagy modulators. Formation of autophagic vesicles were quantitated by cytometric red/gree acridine orange fluorescence Apoptosis-associated PARP cleavage was studied by specific immune blotting. RESULTS. AMPK induction in response to AICAR was greater than that of metformin and was increased in mutant p53 cells, which also showed greater resistance to autophagy inhibition. Greater AMPK activation correlated with increased lysosomal acidification in mutant p53 cells. CONCLUSIONS. Our results identify a role for a) p53 dysfunction in enhancing the response to AMPK activators and autophagy to favour tumor cell survival, b) autophagy inhibitors in counteracting pro-survival AMPK signaling.

Citation Format: Valery Chavez-Perez, Mary Strasberg-Rieber, Manuel Rieber. AMPK induction, lysosomal acidification and melanoma survival increased by p53 dysfunction are counteracted by inhibiting autophagy. [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 2907. doi:10.1158/1538-7445.AM2015-2907

Abstract 4331: Lowering glucose increases nutlin-3 toxicity against melanomas irrespective of BRAF,NRAS or p53 mutations

BACKGROUND: Melanomas are commonly characterized by mutually exclusive BRAF V600Emutations (∼60%) or NRASQ61 mutations (∼15%). Although targeted therapy with vemurafenib is initially efficient in melanoma with BRAF mutations, tumor recurrence occurs including emergence of resistant melanoma cells with secondary N-RASQ61K mutations. Recently it was demonstrated that nutlin-3, an HDM2 antagonist capable of restoring p53 tumor suppressor function synergized with vemurafenib against BRAF V600E wt p53 melanomas. Although nutlin-3 was initially believed to be non-genotoxic, it can originate p53 mutations which are infrequent in primary melanomas.

OBJECTIVES: Since tumor growth increases metabolic demand and glucose consumption, we investigated whether lowering glucose with glycolytic inhibitors increases nutlin-3 toxicity against melanomas irrespective of BRAF, NRAS or p53 status. EXPERIMENTAL: Metabolic activity/proliferation of melanoma cells harbouring BRAF, NRAS or p53 mutations was quantitated by fluorometric studies, crystal violet staining was used to determine differential cell survival, and mechanisms of cell death were studied by protein array immune blotting.

RESULTS: Restricting glucose increases nutlin-3 toxicity against wt p53 melanoma cells harbouring BRAF or NRAS, even if the latter have p53 mutations. Responses occur through mechanisms dependent and independent of p53 serine 15-phosphorylation and p21/CIP1/CDNK1A.

CONCLUSION: Elimination of melanoma cells irrespective of their BRAF, NRAS or p53 mutational status may be helped by joint use of glucose-lowering agents together with nutlin-3.

Citation Format: Manuel Rieber, Valery Chavez-Perez, Mary Strasberg-Rieber. Lowering glucose increases nutlin-3 toxicity against melanomas irrespective of BRAF,NRAS or p53 mutations. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4331. doi:10.1158/1538-7445.AM2014-4331

p53 inactivation decreases dependence on estrogen/ERK signalling for proliferation but promotes EMT and susceptility to 3-bromopyruvate in ERα+ breast cancer MCF-7 cells

BACKGROUND

Most breast cancers express the estrogen receptor alpha (ERα(+)), harbor wt TP53, depend on estrogen/ERK signalling for proliferation, and respond to anti-estrogens. However, concomittant activation of the epidermal growth factor receptor (EGFR)/MEK pathway promotes resistance by decreasing estrogen dependence. Previously, we showed that retroviral transduction of mutant p53 R175H into wt TP53 ERα(+) MCF-7 cells induces epidermal growth factor (EGF)-independent proliferation, activation of the EGF receptor (p-EGFR) and some characteristics of epithelial-mesenchymal transition (EMT).

PURPOSE

To investigate whether p53 inactivation augments ERα(+) cell proliferation in response to restrictive estradiol, chemical MEK inhibition or metabolic inhibitors.

RESULTS

Introduction of mutant p53 R175H lowered expression of p53-dependent PUMA and p21WAF1, decreased E-cadherin and cytokeratin 18 associated with EMT, but increased the % of proliferating ERα(+)/Ki67 cells, diminishing estrogen dependence. These cells also exhibited higher proliferation in the presence of MEK-inhibitor UO126, reciprocally correlating with preferential susceptibility to the pyruvate analog 3-bromopyruvate (3-BrPA) without a comparable response to 2-deoxyglucose. p53 siRNA silencing by electroporation in wt TP53 MCF-7 cells also decreased estrogen dependence and response to MEK inhibition, while also conferring susceptibility to 3-BrPA.

CONCLUSIONS

(a) ERα(+) breast cancer cells dysfunctional for TP53 which proliferate irrespective of low estrogen and chemical MEK inhibition are likely to increase metabolic consumption becoming increasingly susceptible to 3-BrPA; (b) targeting the pyruvate pathway may improve response to endocrine therapy in ERα(+) breast cancer with p53 dysfunction.

Hypoxia and hypoxia mimetic cooperate to counteract tumor cell resistance to glucose starvation preferentially in tumor cells with mutant p53

We demonstrated that exogenous pyruvate promotes survival under glucose depletion in aerobic mutant p53 (R175H) human melanoma cells. Others subsequently indicated that mutant p53 tumor cells undergo p53 degradation and cell death under aerobic glucose-free conditions. Since glucose starvation occurs in hypoxic gradients of poorly vascularized tumors, we investigated the role of p53 siRNA under hypoxia in wt p53 C8161 melanoma using glucose starvation or 5mM physiological glucose. p53 Silencing decreased survival of glucose-starved C8161 melanoma with pyruvate supplementation under hypoxia (≤1% oxygen), but increased resistance to glycolytic inhibitors oxamate and 2-deoxyglucose in 5mM glucose, preferentially under normoxia. Aiming to counteract hypoxic tumor cell survival irrespective of p53 status, genetically-matched human C8161 melanoma harboring wt p53 or mutant p53 (R175H) were used combining true hypoxia (≤1% oxygen) and hypoxia mimetic CoCl2. No significant decrease in metabolic activity was evidenced in C8161 melanoma irrespective of p53 status in 2.5mM glucose after 48h of physical hypoxia. However, combining the latter with 100μM CoCl2 was preferentially toxic for mutant p53 C8161 melanoma, and was enhanced by catalase in wt p53 C8161 cells. Downregulation of MnSOD and LDHA accompanied the toxicity induced by hypoxia and CoCl2 in 5mM glucose, and these changes were enhanced by oxamate or 2-deoxyglucose. Our results show for the first time that survival of malignant cells in a hypoxic microenvironment can be counteracted by hypoxia mimetic co-treatment in a p53 dependent manner.

Abstract 1890: Superoxide modulates tumor cell survival in response to hypoxia and metabolic stress: role of p53 status

Background: Cancer cells from solid tumors become metabolically stressed, when nutrients are insufficient within poorly vascularized regions. Metabolic stress resulting from severe deprivation of glucose and serum-derived growth factors can promote cell death, partly through excessive reactive oxygen species (ROS) production. As possible sources of oxidative stress, NADPH oxidases (NOX) are centrally involved in the control of the malignant state: activated NOX enzymes use NADPH as an electron donor to reduce molecular oxygen generating superoxide anion radical (O2•−), which can act extracellularly as an autocrine/paracrine regulator or intracellularly as a signaling messenger in a variety of mammalian cells. Excessive superoxide can be dismutated to H2O2. by superoxide dismutases (SOD). Objectives: To investigate whether chemical inhibition of NOX or removal of extracellular superoxide by exogenous SOD influences tumor cell survival depending on p53 status or metabolic stress. Experimental: Genetically-matched human melanoma cells with unequal p53 mutational status were exposed to glucose or serum starvation under normoxia, physical hypoxia and/or chemical hypoxia .Differential cell survival was assesed by live-dead fluorometric assays or crystal violet staining. Mechanisms influencing response to glucose limitation were studied by specific immune blotting. Results: Lower cell proliferation under normoxia in 1 % serum was attenuated by chemically or enzymatically decreasing superoxide or by exposing them to hypoxia. Diphenylene iodonium preferentially restored the metabolic activity of mutant p53 cells in 1 % serum, whereas SOD acted preferentially on wt p53 cells under comparable conditions.

Apoptosis-associated PARP cleavage under normoxia in 2.5 mM glucose and 5 % dialyzed serum was counteracted by SOD only in wt p53 cells. Cell death induced by CoCl2 preconditioning in the absence of physical hypoxia was preferential for mut p53 C8161 melanoma and reversed by SOD. Conclusions: Our results support a cytotoxic role for Nox-mediated free radical production in cancer cell death under metabolic stress, and suggests that p53 mutation increases superoxide-mediated toxicity during limitation of intracellular glucose and/ serum-derived growth factors.

Citation Format: Valery Chavez-Perez, Mary Strasberg-Rieber, Manuel Rieber. Superoxide modulates tumor cell survival in response to hypoxia and metabolic stress: role of p53 status. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1890. doi:10.1158/1538-7445.AM2013-1890

Abstract 3212: N-acetylcysteine protects melanoma cells from glucose depletion through unequal modulation of SOD1 and Copper chaperone for SOD1 depending on p53 mutational status

BACKGROUND.-Cancer cells from solid tumors undergo metabolic stress, when nutrients become limiting, within poorly vascularized regions. Severe deprivation of glucose and serum-derived growth factors can promote cell death by metabolic stress, partly through excessive reactive oxygen species (ROS) production.OBJECTIVES.- To investigate whether survival in response to metabolic stress is influenced by N-acetylcysteine (NAC), an anti-oxidant glutathione precursor, and whether it is linked to modulation of superoxide dismutases (SOD1 and SOD2) that catalyze intracellularly the conversion of O2− to H2O2. EXPERIMENTAL- matched human melanoma cells with unequal p53 mutational status were exposed to overnight depletion of glucose with either 0.5% or 10% serum supplementation. Differential cell survival was assesed by crystal violet staining. Mechanisms influencing response to glucose limitation were studied by specific immune blotting. RESULTS.- A 24 hour glucose depletion suppressed cell survival in 0.5% dialyzed serum but this was counteracted by 2 mM NAC irrespective of cellular p53 status. A shorter glucose depletion in 0.5% serum revealed normoxic induction of hypoxia-inducible factor (HIF1α) reciprocal with concomitant down-regulation of SOD1 together with the Copper chaperone for SOD1 (CCS) and intact PARP, with no comparable effect on mitochondrial SOD2. Under these restrictive conditions, NAC addition partly restored SOD1 and PARP but not CCS expression in mutant p53(R175H) cells. No comparable NAC restoration of these proteins was evidenced in matched wt p53 melanoma under glucose depletion in 0.5 % dialyzed serum. However, SOD1 but not CCS was highest following glucose deprivation with 10% dialyzed serum in wt p53 melanoma cells, compared to identically treated mutant p53 cells. NAC addition increased CCS irrespective of p53 status with glucose depletion in 10% serum, and preferentially induced SOD1 in mutant p53 cells under comparable conditions. CONCLUSIONS.-human melanoma cells are protected from transient glucose depletion by anti-oxidants like NAC through differential modulation of SOD1 and CCS depending on serum availability and p53 mutational status.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3212. doi:1538-7445.AM2012-3212

Abstract 1265: Survival under glucose and serum deprivation is increased by DN-mutant p53 R175H in human melanoma: A possible gain of oncogenic function

BACKGROUND.- To help counteract tumor development, the p53 tumor suppressor protein functions to preserve genomic stability, when inhibiting cell cycle progression or inducing senescence or apoptosis. However, cancer-associated dominant-negative (DN) p53 mutations can override the tumor suppressor functions of wt p53, through possible gain-of oncogenic functions including those increasing resistance to metabolic stress. OBJECTIVES.- Since hypoglycemia and growth factor shortage may intermittently occur in the microenvironment of poorly vascularized tumors, this report investigated whether metabolic restriction (glucose deprivation and low serum supplementation), unequally influence the survival of genetically-matched human C8161 melanoma harbouring wt or DN-R175H mutant p53.(Strasberg-Rieber, M. et al. Clin. Cancer Res.7(2001)1446). RESULTS.- Cell death by prolonged metabolic restriction increased oxidative stress, since it was counteracted by the anti-oxidant N-acetylcysteine. Melanoma cells harbouring the DN-mutant p53 R175H were more resistant to cell death induced by shorter metabolic restriction. DPI, a NADPH oxidase inhibitor of superoxide generation counteracted the greater resistance of glucose-deprived mutant p53 cells, by altering expression of mitochondrial Mn-SOD∼SOD2. Pyruvate preferentially protected from DPI treatment under metabolic restriction, restoring levels of MnSOD. CONCLUSIONS. our results suggests that tumor cell death in response to anti-glycolytic treatment, may be antagonized by DN-mutant p53R175H.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1265. doi:10.1158/1538-7445.AM2011-1265

H(2)O(2) preferentially synergizes with nitroprusside to induce apoptosis associated with superoxide dismutase dysregulation in human melanoma irrespective of p53 status: Antagonism by o-phenanthroline

The pro-oxidant hydrogen peroxide (H(2)O(2)) is converted to a reactive oxygen species by transition metals like iron. Since mutations in the p53 tumor suppressor gene contribute to drug resistance, we used genetically-matched human C8161 melanoma harbouring wt or DN-R175H mutant p53, to investigate the influence of p53 status on the potentiation of H(2)O(2) toxicity by: (a) intact sodium nitroprusside or nitroferricyanide (SNP), (b) its light-exhausted NO-depleted form (lex-SNP), (c) potassium ferricyanide, or (d) ferric ammonium sulphate. Whereas single treatments with SNP or H(2)O(2) were partly cytotoxic, preferentially potentiation of H(2)O(2) toxicity was evidenced with intact or lex-SNP. No comparable increase of H(2)O(2) toxicity was induced by ferricyanide, ferric ammonium sulphate or S-nitroso-N-acetyl penicillamine (SNAP), a known NO donor lacking iron. Immune blotting revealed apoptosis-associated PARP cleavage induced by [SNP+H(2)O(2)] irrespective of p53 status. This correlated with an eightfold induction of [Mn-SOD; SOD2] in wt p53 melanoma cells, and with a super-induction of the same enzyme reciprocal with loss of [Cu,Zn-SOD; SOD1], in mutant p53 cells. All these changes were antagonized by the anti-oxidant N-acetylcysteine or the iron chelator o-phenanthroline. We hypothesize that superoxide dismutase imbalance and iron-dependent redox changes involving OH species generated from a Fenton reaction between [SNP+H(2)O(2)], may be important in this anti-tumor activity. Although tumor drug resistance is frequently associated with DN-p53 mutations, our data shows for the first time the preferential ability of SNP to enhance H(2)O(2) toxicity, irrespective of p53 status.

Invasion-associated MMP-2 and MMP-9 are Up-regulated Intracellularly in Concert with Apoptosis Linked to Melanoma Cell Detachment

Matrix metalloproteinases, like MMP-2 and MMP-9 gelatinases, show multiple functions as extracellular/cell-surface enzymes, and are broadly recognised for their matrix-degrading ability and involvement in cell motility. Given that adherent cells have reduced attachment during migration and also detach from their substratum during apoptosis, we now investigated whether extracellular matrix-bound gelatinases and intracellular MMP-2 and MMP-9 are modified with progression of death-inducing stimuli. This report shows that melanoma cells undergoing death in response to 2-acetyl furanonaphtoquinone (FNQ) as evidenced by greater Annexin V binding, increased cytosolic expression of pro-MMP-2 and intracellular activation of particulate MMP-9. These changes were associated with early activation of a substrate-attached 40 kDa gelatinase reciprocal with changes in extracellular matrix-bound activated MMP-2. A subsequent activation of secreted MMP-9 and induction of apoptosis-associated fragmentation of poly ADP-Ribose polymerase (PARP) correlated with cell detachment. Our data suggests that intracellularly activated gelatinases may cleave survival-associated substrates other than gelatin that share the Gly-Leu/Iso-Pro like collagen-binding acetylcholinesterase, thereby linking them to apoptosis associated with cell detachment.

Relationship of Mcl-1 isoforms, ratio p21WAF1/cyclin A, and Jun kinase phosphorylation to apoptosis in human breast carcinomas

Full length Mcl-1 is an anti-apoptotic protein consisting of two closely migrating 42/40kDa species. We now investigated the relationship of these isoforms to the expression of cell cycle stimulatory (cyclin A) and inhibitory (p21WAF1) proteins and to the induction of apoptosis in wt p53 MCF-7 and mutant p53 SKBR3 human breast carcinomas. The latter cells exhibited lower 42kDa Mcl-1, higher expression of cyclin A relative to that of p21WAF1, and apoptosis in response to okadaic acid, a phosphatase 1/2A inhibitor. The proteasome inhibitor MG-115 selectively increased expression of the 40kDa Mcl-1 isoform and induced p21WAF1, but also promoted preferential apoptosis in SKBR3 cells. Neither okadaic acid nor MG-115 caused comparable effects in MCF-7 cells. However, vanadate or acetyl furanonaphthoquinone induced the 40kDa Mcl-1 and greater Jun kinase (JNK) phosphorylation without apoptosis-associated PARP fragmentation in MCF-7 cells. Our data suggest that the higher susceptibility of SKBR3 cells to undergo apoptosis may be partly due to their greater proliferative potential (cyclin A), low expression of the anti-apoptotic 42kDa Mcl-1 isoform, and suboptimal JNK activation in response to stress.

Induction of p53 and melanoma cell death is reciprocal with down-regulation of E2F, cyclin D1 and pRB

Tumor-suppressor-gene products such as p53 and retinoblastoma (Rb) play an important role as negative regulators of cell-cycle progression, which is reciprocally favored by the availability of cyclin D1 and the E2F transcription factor. We now show that UV irradiation of B16 melanoma after prior exposure to the radiation sensitizer, bromodeoxyuridine (BUdR) leads to induction of p53 and DNA fragmentation, and concomitant decreases in Rb, E2F, cyclin D1, and cell viability, with no comparable effects on irradiated unsensitized cells. Our data suggest that over-expression of p53 correlates with down-regulation of E2F, cyclin D1 in inducing apoptosis.