Expression of p21WAF1 is dependent on the activation of ERK during vitamin E-succinate-induced monocytic differentiation

Vitamin E-succinate (VES) induced monocvtic differentiation of HL-60 human leukemia cells. Treatment with VES increased the nitroblue tetrazolium reduction activity, and the expression of monocyte specific cell surface antigen, CD14 and c-fms. During the monocytic differentiation of HL-60 cells that were induced by VES, the phosphorylation of extracellular signal-regulated protein kinase (ERK) was increased by 12 h and then gradually decreased to a level that was similar to that of the control. However, the phosphorylation levels of p38 and JNK, as well as the expression levels of ERK, p38, and JNK, were unchanged by the VES treatment. Treatment with VES also induced hypophosphorylation of the retinoblastoma protein and an increase of the p21WAF1 protein level. VES-induced ERK phosphorylation was abolished by the ERK inhibitor, PD98059, which resulted in a remarkable prevention of VES-induced monocytic differentiation. Inhibition of the ERK activity by PD98059 also diminished the VES-induced p21WAF1 protein expression, but did not change the phosphorylation state of the retinoblastoma protein. Collectively, these data suggest that the ERK signaling pathway mediates the up-regulation of the p21xWAF1 expression that is induced by VES, which is required for monocytic differentiation of HL 60 cells.     

Akt/AMPK/mTOR pathway was involved in the autophagy induced by vitamin E succinate in human gastric cancer SGC-7901 cells

Vitamin E succinate (VES), a derivative of vitamin E, is a promising cancer chemopreventive agent that inhibits tumor promotion by inducing apoptotic cell death. The effects of VES on autophagy, an intricate programmed process which helps cells survive in some stressed situations by degrading some cytoplasmic material, are unclear. When human gastric cancer cells SCG-7901 were exposed to VES, both the level of microtubule-associated protein 1 light chain 3 and the yeast ATG6 homolog Beclin-1 increased, and related autophagy genes were activated, thereby suggesting that autophagy was induced by VES. We also observed that VES-induced autophagy was accompanied by the activation of AMP-activated protein kinases (AMPK). VES-induced autophagy decreased when AMPK was inhibited by using small interfering RNA (siRNA), thereby suggesting that VES-induced autophagy is mediated by AMPK. Moreover, further studies revealed that the decreased activity of mammalian target of rapamycin (mTOR) and its downstream targets P70S6K and 4EBP-1 were involved in VES-activated autophagy associated with AMPK activation. The experiments also showed that the activity of protein kinases B (Akt)-mTOR axis was inhibited by VES. VES-induced AMPK activation could be attenuated by Akt activation. Overall, our studies demonstrated that AMPK was involved in the VES-induced autophagy. Crosstalk exists between AMPK and the Akt/mTOR axis. The results elucidated the mechanism of VES-induced autophagy in human gastric cancer cells.     

Estradiol modulates breast cancer cell apoptosis: a novel nongenomic steroid action relevant to carcinogenesis.

It is known that steroids can induce cell surface receptor aggregation followed by activation of receptor and nonreceptor tyrosine kinases. It has been shown recently that 17beta-estradiol (E2) can stimulate the Src/p21ras/mitogen-activated protein kinase pathway in breast cancer cells, and this effect is supposed to mediate the E2-induced stimulation of breast cancer cell proliferation, possibly via activation of the c-fos and c-jun early genes or of genes involved in cell cycle control. Here we demonstrate the existence of an alternative mechanism of the cancer-promoting effect of E2. Human breast cancer cells (MCF-7) were exposed to the known proapoptotic agent vitamin E succinate (VES), added alone or together with different concentrations of E2. E2 conjugated with bovine serum albumin (E2-BSA), which cannot cross the plasma membrane of living cells, was also used in some experiments to assess whether E2 acted on the cell surface or at intracellular receptors. Apoptosis was analyzed by fluorescence-activated cell sorting after cell staining with propidium iodide and FITC-labeled annexin V. E2 showed a concentration-dependent stimulatory effect on spontaneous apoptosis but inhibited the VES-induced apoptosis. However, effects produced by the same molar concentrations of E2 were different when the hormone was free and when it was used in the form of the E2-BSA conjugate. The effects of E2 and E2-BSA were sensitive to genistein, a tyrosine kinase inhibitor. These data show that E2 modulates apoptosis of breast cancer cells, probably acting both at the cell surface and inside the cells. Tyrosine phosphorylation is involved in the signaling pathways mediating this E2 effect.     

Activation of PKC but not of ERK is required for vitamin E-succinate-induced apoptosis of HL-60 cells.

Vitamin E-succinate (VES) induced HL-60 human leukemia cells to undergo apoptosis. Treatment with VES induced membrane translocation of Fas; cleavages of caspase-3, PARP, and lamin B; hypophosphorylation of retinoblastoma protein; and increase of p21(WAF1) protein level. During the induction of apoptosis, activity of PKC was gradually increased with downregulation of VES-induced ERK activity and accompanied by activation of caspase-3. Inhibition of PKC by GF109203X blocked VES-mediated membrane translocation of PKC-alpha and cleavage of caspase-3 cascade, resulting in prevention of VES-induced apoptosis. On the contrary, PKC activation by cotreatment with LPC or thapsigargin and VES synergistically increased VES-mediated apoptosis. However, inhibition of ERK activity by PD98059 showed no significant effect on VES-induced PKC activity and apoptosis. Taken together, our data suggest that VES induces activation of PKC and PKC-dependent hypophosphorylation of retinoblastoma protein, which results in induction of apoptosis, and that VES-induced early activation of ERK and ERK-dependent induction of p21(WAF1) are not required for apoptosis.     

PKC-zeta is required for angiotensin II-induced activation of ERK and synthesis of C-FOS in MCF-7 cells

We examined the signalling pathways responsible for the Ang II induction of growth in MCF-7 human breast cancer cells. Ang II in MCF-7 cells induced: (a) the translocation from the cytosol to membrane and nucleus of atypical protein kinase C-zeta (PKC-zeta) but not of PKC-alpha, -delta, - epsilon and -eta; (b) the expression of c-fos mRNA and protein; (c) the phosphorylation of the extracellular signal-regulated protein kinases 1 and 2 (ERK1/2). All these effects were due to the activation of the Ang II type I receptor (AT1) since they were blocked by the AT1 antagonist losartan. The Ang II-stimulated ERK1/2 phosphorylation was blocked by (a) high doses of staurosporine, inhibitor of PKC-zeta, and by a synthetic myristoylated peptide with sequences based on the endogenous PKC-zeta pseudosubstrate region (zeta-PS); (b) PD098059, a mitogen-activated protein kinase kinase inhibitor (MAPKK/MEK); and, moreover, (c) the inhibitors of phosphoinositide 3-kinases (PI3K), LY294002 and wortmannin, thus indicating that PI3K may act upstream of ERK1/2. The Ang II-evoked c-fos induction was blocked only by high doses of staurosporine and by zeta-PS whilst PD098059, LY294002 and wortmannin were ineffective, thus indicating that c-fos induction is not due to ERK1/2 activity. When the epidermal growth factor-receptor (EGFR) tyrosine kinase activity was inhibited by the use of its inhibitor AG1478, Ang II was still able to induce ERK1/2 phosphorylation and c-fos expression, therefore proving that the transactivation of EGFR was not required for these Ang II effects in MCF-7 cells. The previously reported proliferation of MCF-7 cells induced by Ang II was blocked by PD098059 and by wortmannin in a dose-dependent manner, thereby indicating that in MCF-7 cells the PI3K and ERK pathways mediate the mitogenic signalling of AT1. Our results suggest that in MCF-7 cells Ang II activates multiple signalling pathways involving PKC-zeta, PI3K and MAPK; of these pathways only PKC-zeta appears responsible for the induction of c-fos.     

PRIMA-1 induces apoptosis by inhibiting JNK signaling but promoting the activation of Bax

The p53 protein plays a major role in the maintenance of genome stability in mammalian cells. Mutations of p53 occur in over 40% of breast cancers and are indicative of tumor resistance to chemotherapeutic agents. Recently, there has been a high degree of interest in pharmacological approaches for restoring the normal function to mutant p53. The low molecular weight compound p53 reactivation and induction of massive apoptosis (PRIMA-1) was shown to induce cytotoxic effects and apoptosis in human tumor cells with mutant p53. Here, we studied the molecular mechanisms of PRIMA-1-induced apoptosis in human breast cancer cells with p53 mutations such as MDA-231 and GI-101A as compared to MCF-7 cells. We show that PRIMA-1 selectively induces apoptosis in human breast cancer cells MDA-231 and GI-101A compared to the MCF-7. This effect was paralleled by an increase in total p53 level in the nucleus and the induction of its phosphorylation at Ser-15 site. Using the chromatin immunoprecipitation (ChIP) assays, we show that PRIMA-1 restored p53 DNA binding activity to the promoters of the proapoptotic genes such as Bax and PUMA, but inhibited the binding activity to the promoters of the MAP4K4 gene. Knockdown of p53 protein in breast cancer cells using siRNA followed by PRIMA-1 treatment resulted in decline of Bax and PUMA proteins expression. Cell incubation with either PRIMA-1 or SP600125 (c-Jun NH2-terminal kinase inhibitor) resulted in the abrogation of adriamycin-induced c-Jun NH2-terminal kinase (JNK) activation, whereas Bax activation was not inhibited. We conclude that both Bax and PUMA but not JNK signaling are involved in PRIMA-1-induced apoptosis in breast cancer cells with p53 mutation.     

Functional analysis of Csk and CHK kinases in breast cancer cells.

In this report, we analyzed the expression and kinase activities of Csk and CHK kinases in normal breast tissues and breast tumors and their involvement in HRG-mediated signaling in breast cancer cells. Csk expression and kinase activity were abundant in normal human breast tissues, breast carcinomas, and breast cancer cell lines, whereas CHK expression was negative in normal breast tissues and low in some breast tumors and in the MCF-7 breast cancer cell line. CHK kinase activity was not detected in human breast carcinoma tissues (12 of 12) or in the MCF-7 breast cancer cell line (due to the low level of CHK protein expression), but was significantly induced upon heregulin (HRG) stimulation. We have previously shown that CHK associates with the ErbB-2/neu receptor upon HRG stimulation via its SH2 domain and that it down-regulates the ErbB-2/neu-activated Src kinases. Our new findings demonstrate that Csk has no effect on ErbB-2/neu-activated Src kinases upon HRG treatment and that its kinase activity is not modulated by HRG. CHK significantly inhibited in vitro cell growth, transformation, and invasion induced upon HRG stimulation. In addition, tumor growth of wt CHK-transfected MCF-7 cells was significantly inhibited in nude mice. Furthermore, CHK down-regulated c-Src and Lyn protein expression and kinase activity, and the entry into mitosis was delayed in the wt CHK-transfected MCF-7 cells upon HRG treatment. These results indicate that CHK, but not Csk, is involved in HRG-mediated signaling pathways, down-regulates ErbB-2/neu-activated Src kinases, and inhibits invasion and transformation of breast cancer cells upon HRG stimulation. These findings strongly suggest that CHK is a novel negative growth regulator of HRG-mediated ErbB-2/neu and Src family kinase signaling pathways in breast cancer cells.     

Differentiation induction in human breast tumor cells by okadaic acid and related inhibitors of protein phosphatases 1 and 2A.

Okadaic acid (OA), an inhibitor of protein phosphatases 1 and 2A, induces differentiation in human MCF-7, AU-565, and MB-231 breast tumor cells. In MCF-7 cells, OA elicited within 5 min an increase in the levels of a set of phosphorylated cellular proteins, within hours expression of the early response genes junB, c-jun, and c-fos, and within days manifestation of differentiation. Differentiation was also induced by two related protein phosphatase inhibitors, but not by an inactive OA derivative or by an inhibitor that penetrates epithelial cells poorly. These results indicate that OA and related agents can induce tumor breast cell differentiation, and this induction is correlated with their ability to inhibit PPH 1 and 2A.     

p53-mediated up-regulation of CD95 is not involved in genotoxic drug-induced apoptosis of human breast tumor cells

Induction of CD95 (Fas/APO-1) and CD95 ligand during chemotherapeutic treatment may contribute to the death by apoptosis of some tumor cells. In this study, we have analyzed the role of the CD95 system in genotoxic drug-induced death of human breast tumor cells. Incubation of the breast tumor cell lines MCF-7 and EVSA-T with doxorubicin or methotrexate caused apoptosis after 48 h of treatment. These drugs induced a marked increase in the level of CD95 mRNA and protein in wild-type p53-expressing MCF-7 cells. On the contrary, the breast cancer cell line EVSA-T that expresses high levels of an inactive form of p53, did not up-regulate CD95 upon drug treatment. Elevation of CD95 expression by DNA-damaging drugs was notably blocked in MCF-7 cells expressing the human papillomavirus type 16 E6 protein (E6 cells) which prevented p53 accumulation upon DNA damage. However, E6 cells were still killed by the drugs. Furthermore, the genotoxic drugs did not induce the expression of CD95 ligand in MCF-7 cells at doses that caused apoptosis in these breast tumor cells. Moreover, drug-induced apoptosis of breast tumor cells was not prevented in the presence of either a CD95 antagonistic antibody or a CD95 ligand blocking antibody. We also observed a strong synergism between lower doses of DNA-damaging drugs and CD95 agonistic antibody in the induction of apoptosis in MCF-7 cells. In summary, our data indicate that drug-induced apoptosis of breast tumor cells occurs by a CD95/CD95L-independent mechanism although by elevating the tumor suppressor proteins p53 and CD95, genotoxic drugs may sensitize breast tumor cells to CD95-mediated apoptosis.     

Vitamin E succinate inhibits human prostate cancer cell growth via modulating cell cycle regulatory machinery

Several epidemiological studies have demonstrated that vitamin E is a chemopreventative agent for prostate cancer. alpha-Tocopheryl succinate (VES), a derivative of vitamin E, effectively modulates prostate cancer cell growth. However, little is known about the mechanisms regarding this action. Here we show that VES causes human prostate cancer cell LNCaP arrest at G1 phase. This effect is accomplished through VES significantly decreasing expression of the cell cycle regulatory proteins cyclin D1, D3, and E, cdk2 and 4, but not cdk6. Furthermore, VES reduces cdk4 kinase activity, Rb phosphorylation, and cyclin E mRNA expression. Recently there is increasing interest in the protective effect of the VES and selenium combination on prostate cancer. Here we show that VES and selenium work through different mechanisms to exert their inhibitory effects on prostate cancer cells. Taken together, our studies suggest that VES-mediated prostate cancer cell G1/S arrest is a consequence of the regulation of multiple molecules of the cell cycle regulatory machinery.     

Distinct mechanisms mediate the initial and sustained phases of cell migration in epidermal growth factor receptor-overexpressing cells

Elevated levels of epidermal growth factor receptor (EGFR) are predictive of increased invasion and metastasis in many human cancers. In the present study, we have shown that two distinct pathways regulate cell migration in EGFR-overexpressing invasive cells such as MDA 468 breast cancer cells: mitogen-activated protein kinase (MAPK or ERK 1 and 2) pathways play a major role in early stages to cell migration; and protein kinase C delta isoforms (PKC-delta) play a significant role in later stages of sustained cell migration. Inhibition of MAPK activity with MAP kinase kinase (MEK) inhibitor PD98059 blocks early stages of cell migration (up to 4 h); however, cells revert back to enhanced cell migration after 4 h. While inhibition of PKC-delta activity with rottlerin or dominant-negative PKC-delta expression blocks sustained cell migration after 4 h and up to 12 h, the combination of MAPK and PKC inhibitors completely blocked transforming growth factor alpha (TGF-alpha)-induced cell migration in EGFR-overexpressing breast cancer cells. However, inhibition of MAPK activity completely blocked cell migration in low EGFR-expressing non-invasive breast cancer cells such as MCF-7 cells. Forced overexpression of EGFR in MCF-7 cells (EGFR/MCF-7 cells) resulted in cell migration patterns seen in MDA 468 cells, that is, MAPK pathways play a major role in early stages to cell migration, and PKC-delta plays a major role in later stages of sustained cell migration. The above data demonstrate that EGFR-overexpressing invasive cells have the ability to compensate the loss of MAPK-mediated signaling through activation of PKC-delta signaling for cell migration, which plays a major role in invasion and metastasis. In addition, data suggest that inhibition of MAPK and PKC-delta signaling pathways should abrogate cell migration and invasion in EGFR-overexpressing human breast cancer cells.     

A ligand for the erbB-2 oncogene product (gp30) induces differentiation of human breast cancer cells.

The human erbB-2 oncogene encodes a tyrosine kinase receptor. A ligand for the erbB-2 receptor (gp30), with an apparent molecular weight of 30,000, was reported to modulate the growth of cells overexpressing erbB-2. Whereas low concentrations of gp30 induced proliferation of these cells, higher concentrations inhibited their growth. To elucidate the cellular mechanisms underlying cell growth inhibition by gp30, we tested the effect of this ligand on cell growth and differentiation of the human breast cancer cells AU-565 and MDA-MB-453 (which overexpress erbB-2) and MCF-7 cells (which express low levels of this protooncogene). Ligand concentrations that inhibited growth in cells overexpressing erbB-2 induced apparent differentiation of cells with a more mature phenotype, i.e., with characteristics such as inhibited cell growth, altered cytoplasmic and nuclear morphology, and increased synthesis of milk components (casein and lipids). No significant effect of the ligand was observed in the human breast cancer cell line MCF-7. Concomitant with the induction of differentiation in AU-565 and MDA-MB-453 cells, the erbB-2 protein was translocated from membrane to the cytoplasm and perinuclear sites. These findings indicate that ligand-induced growth inhibition in cells overexpressing erbB-2 is associated with an apparent induction of differentiation.     

Mechanism of alpha-tocopheryl succinate-induced apoptosis of promyelocytic leukemia cells

Selective induction of apoptosis in tumor cells is important for treating patients with cancer. Because oxidative stress plays an important role in the process of apoptosis, we studied the effect of alpha-tocopheryl succinate (VES) on the fate of cultured human promyelocytic leukemia cells (HL-60). The presence of fairly low concentrations of VES inhibited the growth and DNA synthesis of HL-60 cells, and also induced their apoptosis via a mechanism that was inhibited by z-VAD-fluoromethylketone (z-VAD-fmk), an inhibitor of pan-caspases. VES activated various types of caspases, including caspase-3, 6, 8, and 9, but not caspase-1. VES triggered the reaction leading to the cleavage of Bid, a member of the death agonist Bcl-2 family, and released cytochrome c (Cyt.c) from the mitochondria into the cytosol by a z-VAD-fmk-inhibitable mechanism. VES transiently increased the intracellular calcium level [Ca2+]i and stimulated the release of Cyt.c in the presence of inorganic phosphate (Pi). However, high concentrations of VES (approximately 100 microM) hardly induced swelling of isolated mitochondria but depolarized the mitochondrial membrane potential by a cyclosporin A (CsA)-insensitive mechanism. These results indicate that VES-induced apoptosis of HL-60 cells might be caused by activation of the caspase cascade coupled with modulation of mitochondrial membrane function.     

Activation of MAP kinase by muscarinic cholinergic receptors induces cell proliferation and protein synthesis in human breast cancer cells

Carbachol (Cch), a muscarinic acetylcholine receptor (mAChR) agonist, increases intracellular-free Ca(2+) mobilization and induces mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) phosphorylation in MCF-7 human breast cancer cells. Pretreatment of cells with the selective phospholipase C (PLC) inhibitor U73122, or incubation of cells in a Ca(2+)-free medium did not alter Cch-stimulated MAPK/ERK phosphorylation. Phosphorylation of MAPK/ERK was mimicked by phorbol 12-myristate acetate (PMA), an activator of protein kinase C (PKC), but Cch-evoked MAPK/ERK activation was unaffected by down-regulation of PKC or by pretreatment of cells with GF109203X, a PKC inhibitor. However, Cch-stimulated MAPK/ERK phosphorylation was completely blocked by myristoylated PKC-zeta pseudosubstrate, a specific inhibitor of PKC-zeta, and high doses of staurosporine. Pretreatment of human breast cancer cells with wortmannin or LY294002, selective inhibitors of phosphoinositide 3-kinase (PI3K), diminished Cch-mediated MAPK/ERK phosphorylation. Similar results were observed when MCF-7 cells were pretreated with genistein, a non-selective inhibitor of tyrosine kinases, or with the specific Src tyrosine kinase inhibitor PP2. Moreover, in MCF-7 human breast cancer cells mAChR stimulation induced an increase of protein synthesis and cell proliferation, and these effects were prevented by PD098059, a specific inhibitor of the mitogen activated kinase kinase. In conclusion, analyses of mAChR downstream effectors reveal that PKC-zeta, PI3K, and Src family of tyrosine kinases, but not intracellular-free Ca(2+) mobilization or conventional and novel PKC activation, are key molecules in the signal cascade leading to MAPK/ERK activation. In addition, MAPK/ERK are involved in the regulation of growth and proliferation of MCF-7 human breast cancer cells.     

Mechanism of α-tocopheryl succinate-induced apoptosis of promyelocytic leukemia cells

Selective induction of apoptosis in tumor cells is important for treating patients with cancer. Because oxidative stress plays an important role in the process of apoptosis, we studied the effect of α-tocopheryl succinate (VES) on the fate of cultured human promyelocytic leukemia cells (HL-60). The presence of fairly low concentrations of VES inhibited the growth and DNA synthesis of HL-60 cells, and also induced their apoptosis via a mechanism that was inhibited by z-VAD-fluoromethylketone (z-VAD-fmk), an inhibitor of pan-caspases. VES activated various types of caspases, including caspase-3, 6, 8, and 9, but not caspase-1. VES triggered the reaction leading to the cleavage of Bid, a member of the death agonist Bcl-2 family, and released cytochrome c (Cyt.c) from the mitochondria into the cytosol by a z-VAD-fmk-inhibitable mechanism. VES transiently increased the intracellular calcium level [Ca²⁺]i and stimulated the release of Cyt.c in the presence of inorganic phosphate (Pi). However, high concentrations of VES (～100 μM) hardly induced swelling of isolated mitochondria but depolarized the mitochondrial membrane potential by a cyclosporin A (CsA)-insensitive mechanism. These results indicate that VES-induced apoptosis of HL-60 cells might be caused by activation of the caspase cascade coupled with modulation of mitochondrial membrane function.     

Critical role of p53 upregulated modulator of apoptosis in benzyl isothiocyanate-induced apoptotic cell death

Benzyl isothiocyanate (BITC), a constituent of edible cruciferous vegetables, decreases viability of cancer cells by causing apoptosis but the mechanism of cell death is not fully understood. The present study was undertaken to determine the role of Bcl-2 family proteins in BITC-induced apoptosis using MDA-MB-231 (breast), MCF-7 (breast), and HCT-116 (colon) human cancer cells. The B-cell lymphoma 2 interacting mediator of cell death (Bim) protein was dispensable for proapoptotic response to BITC in MCF-7 and MDA-MB-231 cells as judged by RNA interference studies. Instead, the BITC-treated MCF-7 and MDA-MB-231 cells exhibited upregulation of p53 upregulated modulator of apoptosis (PUMA) protein. The BITC-mediated induction of PUMA was relatively more pronounced in MCF-7 cells due to the presence of wild-type p53 compared with MDA-MB-231 with mutant p53. The BITC-induced apoptosis was partially but significantly attenuated by RNA interference of PUMA in MCF-7 cells. The PUMA knockout variant of HCT-116 cells exhibited significant resistance towards BITC-induced apoptosis compared with wild-type HCT-116 cells. Attenuation of BITC-induced apoptosis in PUMA knockout HCT-116 cells was accompanied by enhanced G2/M phase cell cycle arrest due to induction of p21 and down regulation of cyclin-dependent kinase 1 protein. The BITC treatment caused a decrease in protein levels of Bcl-xL (MCF-7 and MDA-MB-231 cells) and Bcl-2 (MCF-7 cells). Ectopic expression of Bcl-xL in MCF-7 and MDA-MB-231 cells and that of Bcl-2 in MCF-7 cells conferred protection against proapoptotic response to BITC. Interestingly, the BITC-treated MDA-MB-231 cells exhibited induction of Bcl-2 protein expression, and RNA interference of Bcl-2 in this cell line resulted in augmentation of BITC-induced apoptosis. The BITC-mediated inhibition of MDA-MB-231 xenograft growth in vivo was associated with the induction of PUMA protein in the tumor. In conclusion, the results of the present study indicate that Bim-independent apoptosis by BITC in cancer cells is mediated by PUMA.     

Activation by phorbol esters of protein kinase C in MCF-7 human breast cancer cells

Exposure of MCF-7 human breast cancer cells to the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) leads to the inhibition of cell proliferation. We investigate here the short-term effects of TPA on subcellular distribution of protein kinase C, and on protein phosphorylation in cultured MCF-7 cells. We report a rapid and dramatic decrease in cytosolic protein kinase C activity after TPA treatment. Only 30% of the enzymatic activity lost in the cytosol was recovered in the particulate fraction. These data suggest that subcellular translocation of protein kinase C is accompanied by a rapid down-regulation of the enzyme (70%). Furthermore, TPA and other protein kinase C activators rapidly induce the phosphorylation of a 28 kDa protein in intact MCF-7 cells. Phorbol esters devoid of tumor-promoting activity are ineffective both for inducing these early biochemical events and for inhibiting cell proliferation.