I. B. Weinstein

Novel Roles of Specific Isoforms of Protein Kinase C in Activation of the c-fos Serum Response Element

ABSTRACT Protein kinase C (PKC) is a multigene family of enzymes consisting of at least 11 isoforms. It has been implicated in the induction of c-fos and other immediate response genes by various mitogens. The serum response element (SRE) in the c-fospromoter is necessary and sufficient for induction of transcription of c-fos by serum, growth factors, and the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). It forms a complex with the ternary complex factor (TCF) and with a dimer of the serum response factor (SRF). TCF is the target of several signal transduction pathways and SRF is the target of the rhoA pathway. In this study we generated dominant-negative and constitutively active mutants of PKC-α, PKC-δ, PKC-ɛ, and PKC-ζ to determine the roles of individual isoforms of PKC in activation of the SRE. Transient-transfection assays with NIH 3T3 cells, using an SRE-driven luciferase reporter plasmid, indicated that PKC-α and PKC-ɛ, but not PKC-δ or PKC-ζ, mediate SRE activation. TPA-induced activation of the SRE was partially inhibited by dominant negative c-Raf, ERK1, or ERK2, and constitutively active mutants of PKC-α and PKC-ɛ activated the transactivation domain of Elk-1. TPA-induced activation of the SRE was also partially inhibited by a dominant-negative MEKK1. Furthermore, TPA treatment of serum-starved NIH 3T3 cells led to phosphorylation of SEK1, and constitutively active mutants of PKC-α and PKC-ɛ activated the transactivation domain of c-Jun, a major substrate of JNK. Constitutively active mutants of PKC-α and PKC-ɛ could also induce a mutant c-fos promoter which lacks the TCF binding site, and they also induce transactivation activity of the SRF. Furthermore, rhoA-mediated SRE activation was blocked by dominant negative mutants of PKC-α or PKC-ɛ. Taken together, these findings indicate that PKC-α and PKC-ɛ can enhance the activities of at least three signaling pathways that converge on the SRE: c-Raf–MEK1–ERK–TCF, MEKK1-SEK1-JNK-TCF, and rhoA-SRF. Thus, specific isoforms of PKC may play a role in integrating networks of signal transduction pathways that control gene expression.

Overexpression of protein kinase C in HT29 colon cancer cells causes growth inhibition and tumor suppression.

By using a retrovirus-derived vector system, we generated derivatives of the human colon cancer cell line HT29 that stably overexpress a full-length cDNA encoding the beta 1 isoform of rat protein kinase C (PKC). Two of these cell lines, PKC6 and PKC7, displayed an 11- to 15-fold increase in PKC activity when compared with the C1 control cell line that carries the vector lacking the PKC cDNA insert. Both of the overexpresser cell lines exhibited striking alterations in morphology when exposed to the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA). Following exposure to TPA, PKC6 and PKC7 cells displayed increased doubling time, decreased saturation density, and loss of anchorage-independent growth in soft agar; but these effects were not seen with the C1 cells. Also, in contrast to the control cells, the PKC-overproducing cells failed to display evidence of differentiation, as measured by alkaline phosphatase activity, when exposed to sodium butyrate. In addition, the PKC-overexpresser cells displayed decreased tumorigenicity in nude mice, even in the absence of treatment with TPA. These results provide the first direct evidence that PKC can inhibit tumor cell growth. Thus, in some tumors, PKC might act as a growth-suppressor gene.

Cells that overproduce protein kinase C are more susceptible to transformation by an activated H-ras oncogene.

We recently developed rat fibroblast cell lines that stably overproduce high levels of the beta 1 form of protein kinase C (PKC). These cells display several disorders in growth control and form small microscopic colonies in agar. In the present study we demonstrate that one of these cell lines, R6-PKC3, is extremely susceptible to transformation by an activated human bladder cancer c-H-ras oncogene (T24). Compared with control cell line R6-C1, T24-transfected R6-PKC3 cells yielded a 10-fold increase in the formation of large colonies in agar. Cell lines established from these colonies displayed a highly transformed morphology, expressed the T24-encoded p21 ras protein, continued to express high levels of PKC, and were highly tumorigenic in nude mice. These results provide genetic evidence that PKC mediates some of the effects of the c-H-ras oncogene on cell transformation. Data are also presented suggesting that optimum synergistic effects between c-H-ras and PKC require critical levels of their respective activities. These findings may be relevant to the process of multistage carcinogenesis in tissues containing cells with an activated c-H-ras oncogene.

Molecular cloning of gene sequences regulated by tumor promoters and mitogens through protein kinase C.

cDNA clones representing genes whose expression is modulated by treatment with mitogens and tumor promoters were isolated and characterized. TPA-S1 corresponds to an mRNA species whose abundance was increased markedly within 1 h of exposure to the tumor promoter 12-O-tetradecanoyl phorbol-13-acetate (TPA), and TPA-R1 represents an mRNA that was decreased in TPA-treated cells. The induction of TPA-S1 was blocked by actinomycin D but was not affected by cycloheximide, and it was specific for phorbol esters with tumor-promoting activity. The role of protein kinase C in the induction of TPA-S1 is supported by the following lines of evidence. (i) Agents that activated protein kinase C (TPA, platelet-derived growth factor, and diacylglycerol) also increased TPA-S1 mRNA levels. (ii) A potent PKC inhibitor blocked the induction of TPA-S1. (iii) Down-regulation of PKC activity, by treatment of cells with TPA for 24 h, resulted in a loss of responsiveness to TPA-S1 induction by subsequent TPA treatment. DNA sequence analysis of TPA-S1 predicts a cysteine-rich, secreted protein with a molecular weight of 22.6 X 10(3) that exhibits homology with sequences representing a protein with human erythroid-potentiating activity and protease inhibitory activity.

Effects of 5-azacytidine on methylation and expression of specific dna sequences in c3h 10t1/2 cells.

The present study indicates that the transient exposure of C3H 10T1/2 mouse embryo fibroblasts to 5-azacytidine leads to extensive loss of methylation of the protooncogene c-mos and the beta-globin locus at the cell population level and in at least 40 isolated subclones. These changes persisted, even when the cells were serially passaged for many generations without further exposure to the drug. Even though the amount of demethylation, assessed through differential digestion by the restriction enzymes HpaII and MspI, was quite extensive, neither locus was transcribed at a detectable level. This nonselective analysis suggests, therefore, that loss of DNA methylation is not sufficient per se to induce the expression of certain loci. Presumably, the expression of these loci requires additional factors, some of which may be related to cell lineage and differentiation.

Oncogene-induced transformation of a rat embryo fibroblast cell line is enhanced by tumor promoters.

Rat embryo fibroblast cell line 6 was transfected with plasmid pT24, which contains the activated human bladder c-Ha-ras oncogene, and the cells were grown continuously in the absence or presence of the tumor promoters 12-O-tetradecanoyl phorbol-13-acetate (TPA) or teleocidin. The presence of TPA or teleocidin led to a 6- to 14-fold increase in the number of morphologically transformed foci. No transformed foci were seen when rat 6 cells were transfected with the normal c-Ha-ras oncogene in the absence or presence of TPA, or in cells simply treated with TPA or teleocidin. Enhancement of pT24-induced foci was seen even when the addition of TPA was delayed until day 16. In transfection studies with the drug resistance genes gpt and neo, TPA and teleocidin did not increase the number of Gpt+ or Neo+ colonies. When rat 6 cells were cotransfected with pT24 and neo genes and grown in the absence or presence of TPA, the presence of TPA did not increase the yield of Neo+ colonies but caused a fivefold increase in the number of Neo+ colonies that displayed a transformed morphology. Southern blot analyses of DNAs obtained from these clones indicated that TPA treatment did not influence the extent of integration of either the pT24 or neo gene. DNA samples from all of the morphologically transformed cells displayed a characteristic 2-kilobase SacI fragment homologous to pT24 DNA and expressed relatively high levels of the corresponding mRNA. Our findings indicate that in this system tumor promoters do not simply enhanced the process of DNA transfection per se. Thus, this model system may be useful for analyzing synergistic interactions between tumor promoters and activated oncogenes during multistage carcinogenesis. It may also serve as a simple screening test for detecting new tumor promoters.

Effects of 5-azacytidine on the progressive nature of cell transformation.

C3H 10T1/2 mouse embryo fibroblasts were exposed to 3 microM 5-azacytidine for 24 h and then serially passaged in the absence of 5-azacytidine and examined for subsequent changes in growth properties. The treated cells showed changes in morphology, saturation density, growth rate, and serum dependence. By the 5th passage they acquired the ability to grow in 0.3% agarose, and by the 30th passage they gave rise to fully transformed foci that grew in agarose, in agar, and in liquid suspension. This progression was rapidly accelerated if the cultures derived from 5-azacytidine-treated cells were exposed for 48 h to the carcinogen benzo[a]pyrene. Results of these studies provide evidence that aberrations in DNA methylation may be one of a series of critical events during the course of multistage carcinogenesis and thus enhance the evolution of tumor cells.

Novel revertants of H-ras oncogene-transformed R6-PKC3 cells.

Rat 6 fibroblasts that overproduce protein kinase C beta 1 (R6-PKC3 cells) are hypersensitive to complete transformation by the T24 H-ras oncogene; yet T24 H-ras-transformed R6-PKC3 cells are killed when exposed to 12-O-tetradecanoylphorbol-13-acetate (TPA) (W.-L. W. Hsiao, G. M. Housey, M. D. Johnson, and I. B. Weinstein, Mol. Cell. Biol. 9:2641-2647, 1989). Treatment of an R6-PKC3 subclone that harbors a T24 H-ras gene under the control of an inducible mouse metallothionein I promoter with ZnSO4 and TPA is extremely cytocidal. This procedure was used to isolate rare revertants that are resistant to this toxicity. Two revertant lines, R-1a and ER-1-2, continue to express very high levels of protein kinase C enzyme activity but, unlike the parental cells, do not grow in soft agar. Furthermore, these revertants are resistant to the induction of anchorage-independent growth by the v-src, v-H-ras, v-raf, and, in the case of the R-1a line, v-fos oncogenes. Both revertant lines, however, retain the ability to undergo morphological alterations when either treated with TPA or infected with a v-H-ras virus, thus dissociating anchorage independence from morphological transformation. The revertant phenotype of both R-1a and ER-1-2 cells is dominant over the transformed phenotype in somatic cell hybridizations. Interestingly, the revertant lines no longer induce the metallothionein I-T24 H-ras construct or the endogenous metallothionein I and II genes in response to three distinct agents: ZnSO4, TPA, and dexamethasone. The reduction in activity of metallothionein promoters seen in these revertants may reflect defects in signal transduction pathways that control the expression of genes mediating specific effects of protein kinase C and certain oncogenes in cell transformation.

A factor present in fetal calf serum enhances oncogene-induced transformation of rodent fibroblasts.

Our previous studies indicated that addition of the tumor promoters 12-O-tetradecanoylphorbol-13-acetate (TPA) or teleocidin to Dulbecco modified Eagle medium supplemented with calf serum enhanced T24-induced focus formation in both the murine C3H 10T1/2 and rat 6 embryo fibroblast cell lines. In the present studies we have found that fetal calf serum (FCS) is more potent than 12-O-tetradecanoylphorbol-13-acetate in enhancing T24-induced focus formation, in terms of the number and size of the foci, in both C3H 10T1/2 and rat 6 cells. Time course studies indicate that FCS can exert this enhancing effect when it is added several days after the transfection with T24 DNA. In rat 6 cells, an 11-fold increase in T24-induced focus formation occurred when the transfected cultures were maintained for only 1 day in 5% FCS, starting 4 days after the transfection. Several known growth factors, including epidermal growth factor, transforming growth factors alpha and beta, insulin, and platelet-derived growth factor, did not enhance T24-induced transformation in these cell systems. Fractionation studies indicate that the factor present in FCS has a molecular weight of about 1,300, is not lipid soluble, and is acid, base, and heat stable. These findings suggest that a factor(s) normally present in serum may enhance the emergence of tumor cells in vivo, by acting in concert with an activated oncogene, during the multistage carcinogenic process.

Tumor promoters alter the temporal program of adenovirus replication in human cells.

In this study we evaluated the effect of phorbol ester tumor promoters on the kinetics of adenovirus type 5 (Ad5) replication in human cells. When added at the time of infection, 12-O-tetradecanoyl-phorbol-13-acetate (TPA) accelerated the appearance of an early virus antigen (72,000-molecular-weight [72K] deoxyribonucleic acid-binding protein), the onset of viral deoxyribonucleic acid synthesis, and the production of infectious virus. The appearance of an Ad5-specific cytopathic effect (CPE) was also accelerated in infected cultures exposed to TPA, whereas phorbol, 4 alpha-phorbol-12,13-didecanoate and 4-OmeTPA, which are inactive as tumor promoters, were ineffective in inducing this morphological change. The acceleration of the CPE seen in TPA-treated Ad5-infected cells was not caused by TPA induction of the protease plasminogen activator, since the protease inhibitors leupeptin and antipain do not inhibit the earlier onset of this CPE and, in contrast, epidermal growth factor, which induces plasminogen activator in HeLa cells, does not induce an earlier CPE. Evidence for a direct effect of TPA on viral gene expression was obtained by analyzing viral messenger ribonucleic acid (mRNA) synthesis. TPA accelerated the appearance of mRNA from all major early regions of Ad5, transiently stimulated the accumulation of region III mRNA, and accelerated the appearance of late Ad5 mRNA. Thus, TPA altered the temporal program of Ad5 mRNA production and accelerated the appearance of at least some Ad5-specific polypeptides during lytic infection of human cells. These effects presumably explain the earlier onset of the Ad5-specific CPE in TPA-treated cells and may have relevance to the effects of TPA on viral gene expression in nonpermissive cells carrying integrated viral deoxyribonucleic acid sequences.