Expression of ras oncogene leads to down-regulation of protein kinase C

Histological and proteomic analysis of reversible H-RasV12G expression in transgenic mouse skin

We have used a two-transgene tetracycline system to reversibly express oncogenic H-Ras(V12G) in mouse skin and primary keratinocytes culture using the bovine keratin 5 promoter. Induction of H-Ras(V12G) expression in skin at 30 days after birth causes epidermal basal cell hyperplasia, an eruption of keratinous cysts and loss of hair follicles by 3 weeks. Subsequent H-Ras(V12G) de-induction for 3 days results in massive apoptosis in the non-H-Ras(V12G)-expressing stroma as well as in the suprabasal cells of the epidermis. Several procaspases such as CASP3, 1alpha, 5 and 12 disappeared, whereas the pro-apoptotic proteins AIF, Bax and Fas ligand were induced in H-Ras(V12G) de-induction skin. This process is followed by a wave of cell division at 14 days as hair follicles regrew, returning to near normal histology and skin appearance by 30 days. Using Kinetworkstrade mark multi-immunoblotting screens, the phosphorylation status of 37 proteins and expression levels of 75 protein kinases in the skin were determined in three samples: (i) wild-type skin, (ii) hyperplastic H-Ras(V12G)-expressing skin and (iii) skin where H-Ras(V12G) expression was suppressed for 7 days. Following H-Ras(V12G) induction, 16 kinases were increased over 2-fold, and 2 kinases were reduced over 50%. This included increased phosphorylation of both known downstream H-Ras(V12G) targets and unknown H-Ras(V12G) targets. After H-Ras(V12G) suppression, many but not all protein changes were reversed. These results from skin and primary keratinocytes are organized to reflect the molecular events that cause the histological changes observed. These proteomic changes identify markers that may mediate the oncogenic addiction paradigm.

Identifying the factors and signal pathways necessary for anchorage-independent growth of Ha-ras oncogene-transformed NIH/3T3 cells

Ha-ras(Val 12) overexpression was positively correlated with colony formation by NIH/3T3 derivative "2-12" cells harboring an inducible Ha-ras(Val 12) transgene. The ras-farnesylation inhibitor, Lovastatin, completely suppressed colony formation at higher dosages. However, Ha-ras oncogene overexpression alone could not stimulate colony formation under serum-deprived conditions, suggesting that ras is required but not sufficient for supporting colony formation. Substituting cow colostrum (AC-2) for serum did not result in colony formation from 2-12 cells in soft agar, suggesting the colostrum lacked or contained insufficient amounts of factors that stimulate colony formation. Supplementation of AC-2-containing medium with growth factors, such as insulin-like growth factor-1 (IGF-1), partially restored the capability of anchorage-independent cell growth induced by Ha-ras overexpression. Consistently, antibodies specific for IGF-1 receptors only partially blocked colony formation from 2-12 cells. The data indicate that multiple factors, including IGF-1, are required for Ha-ras-dependent colony formation. Signal transduction studies revealed that, under Ha-ras overexpression conditions, IGF-1 utilizes phosphatidyl inositol 3-kinase and NF-kappaB to transduce colony formation-related signaling.

Histone H1(S)-3 phosphorylation in Ha-ras oncogene-transformed mouse fibroblasts

Phosphorylation of linker histone H1(S)-3 (previously named H1b) and core histone H3 is elevated in mouse fibroblasts transformed with oncogenes or constitutively active mitogen-activated protein kinase (MAPK) kinase (MEK). H1(S)-3 phosphorylation is the only histone modification known to be dependent upon transcription and replication. Our results show that the increased amounts of phosphorylated H1(S)-3 in the oncogene Ha-ras-transformed mouse fibroblasts was a consequence of an elevated Cdk2 activity rather than the reduced activity of a H1 phosphatase, which our studies suggest is PP1. Induction of oncogenic ras expression results in an increase in H1(S)-3 and H3 phosphorylation. However, in contrast to the phosphorylation of H3, which occurred immediately following the onset of Ras expression, there was a lag of several hours before H1(S)-3 phosphorylation levels increased. We found that there was a transient increase in the levels of p21(cip1), which inhibited the H1 kinase activity of Cdk2. Cdk2 activity and H1(S)-3 phosphorylated levels increased after p21(cip1) levels declined. Our studies suggest that persistent activation of the Ras-MAPK signal transduction pathway in oncogene-transformed cells results in deregulated activity of kinases phosphorylating H3 and H1(S)-3 associated with transcribed genes. The chromatin remodelling actions of these modified histones may result in aberrant gene expression.

Increased Ser-10 phosphorylation of histone H3 in mitogen-stimulated and oncogene-transformed mouse fibroblasts

When the Ras mitogen-activated protein kinase (MAPK) signaling pathway of quiescent cells is stimulated with growth factors or phorbol esters, the early response genes c-fos and c-myc are rapidly induced, and concurrently there is a rapid phosphorylation of histone H3. Using an antibody specific for phosphorylated Ser-10 of H3, we show that Ser-10 of H3 is phosphorylated, and we provide direct evidence that phosphorylated H3 is associated with c-fos and c-myc genes in stimulated cells. H3 phosphorylation may contribute to proto-oncogene induction by modulating chromatin structure and releasing blocks in elongation. Previously we reported that persistent stimulation of the Ras-MAPK signaling pathway in oncogene-transformed cells resulted in increased amounts of phosphorylated histone H1. Here we show that phosphorylated H3 is elevated in the oncogene-transformed mouse fibroblasts. Further we show that induction of ras expression results in a rapid increase in H3 phosphorylation. H3 phosphatase, identified as PP1, activities in ras-transformed and parental fibroblast cells were similar, suggesting that elevated H3 kinase activity was responsible for the increased level of phosphorylated H3 in the oncogene-transformed cells. Elevated levels of phosphorylated H1 and H3 may be responsible for the less condensed chromatin structure and aberrant gene expression observed in the oncogene-transformed cells.

Carcinogen and dietary lipid regulate ras expression and localization in rat colon without affecting farnesylation kinetics

Epidemiological and experimental data suggest that dietary fiber and fat are major determinants of colorectal cancer. However, the mechanisms by which these dietary constituents alter the incidence of colon cancer have not been elucidated. Evidence indicates that dominant gain-of-function mutations short-circuit protooncogenes and contribute to the pathogenesis of cancer. Therefore, we began to dissect the mechanisms whereby dietary fat and fiber, fed during the initiation, promotion and progression stages of colon tumorigenesis, regulate ras p21 localization, expression and mutation frequency. Male Sprague-Dawley rats (140) were provided with corn oil or fish oil and pectin or cellulose plus or minus the carcinogen azoxymethane (AOM) in a 2 x 2 x 2 factorial design and killed after 34 weeks. We have previously shown adenocarcinoma incidence in these animals to be 70.3% (52/74) for corn oil + AOM and 56.1% (37/66) for fish oil + AOM (P < 0.05). Total ras expression as well as ras membrane:cytosol ratio was 4- to 6-fold higher in colon tumors than in mucosa from AOM- or saline-injected rats. Expression of ras in the mucosal membrane fraction was 13% higher for animals fed corn oil compared with fish oil feeding (P < 0.05), which is noteworthy since ras must be localized at the plasma membrane to function. The elevated ras membrane:cytosol ratio in tumors was not due to increased farnesyl protein transferase activity or prenylation state, as nearly all detectable ras was in the prenylated form. Phosphorylated p42 and p44 mitogen activated protein kinase (ERK) expression was two-fold higher in tumor extracts compared with uninvolved mucosa from AOM- and saline-injected rats (P < 0.05). The frequency of K-ras mutations was not significantly different between the various groups, but there was a trend toward a greater incidence of mutations in tumors from corn oil fed rats (85%) compared with fish oil fed rats (58%). Our results indicate that the carcinogen-induced changes in ras expression and membrane localization are associated with the in vivo activation of the ERK pathway. In addition, suppression of tumor development by dietary n-3 polyunsaturated fatty acids may be partly due to a combined effect on colonic ras expression, membrane localization, and mutation frequency.

c-Ha-rasEJ transfection in vascular smooth muscle cells circumvents PKC requirement during mitogenic signaling

In view of the prominent role of protein kinase C (PKC) in the regulation of vascular smooth muscle cell (VSMC) growth and differentiation, the present studies were conducted to assess the impact of c-Ha-rasEJ transfection on PKC-dependent growth programming. PKC activity was elevated in the cytosolic and particulate compartments of c-Ha-rasEJ VSMC, relative to naive or pSV2neo vector controls. Constitutive and 12-O-tetradecanoyl phorbol 13-acetate (TPA)-inducible binding to a TPA-responsive element (TRE) was also enhanced in c-Ha-rasEJ VSMC. Fetal bovine serum (FBS) did not increase TRE-binding activity in serum-starved c-Ha-rasEJ VSMC but increased TRE-binding activity in pSV2neo VSMC. FBS-mediated TRE-binding activity was dramatically decreased in serum-starved pSV2neo VSMC pretreated with 100 ng/ml TPA for 24 h to downregulate PKC activity. c-Ha-rasEJ VSMC exhibited a marked proliferative advantage over controls under both restrictive and growth-permissive serum conditions. PKC downregulation did not influence the mitogenic response to serum in c-Ha-rasEJ VSMC but ablated [3H]thymidine incorporation into DNA in naive or pSV2neo vector counterparts. Western blot analysis demonstrated increased expression of extracellular signal-regulated kinase 2 (ERK2), but not ERK1, in c-Ha-rasEJ VSMC, relative to pSV2neo control. Immunoblots of serum-starved and PKC-depleted c-Ha-rasEJ VSMC demonstrated a dramatic increase in the phosphorylated form of ERK2, relative to pSV2neo controls. These data suggest that oncogenic c-Ha-rasEJ circumvents a requirement for a TPA-responsive PKC isoform(s) during mitogenic stimulation of VSMC.

ras downregulation of protein kinase C mRNA in C3H 10T1/2 fibroblasts

C3H 10T1/2 fibroblasts transformed by oncogenic ras have lower levels of protein kinase C (PKC) activity and protein. It was previously suggested that elevated levels of diacylglycerol in ras-transformed fibroblasts lead to activation-induced proteolysis of cellular PKC. We found that stable expression of T24ras in C3H 10T1/2 fibroblasts resulted in a significant decrease in levels of PKC alpha and PKC epsilon mRNA. Using C3H 10T1/2 cell lines in which the levels of activated ras can be exogenously regulated (by addition of zinc to induce the expression of a metallothionein-promoted human Ha-ras oncogene), we examined the temporal dependence of oncogenic ras expression on PKC downregulation. In these cells, downregulation of PKC protein and activity was induced but was not preceded by activation of PKC. The downregulation of PKC levels correlated with the appearance of a highly transformed morphology and was seen only at high levels of ras expression. In the inducible cells, the decrease in levels of PKC alpha mRNA had the same dependence on the levels of ras expression as did protein downregulation. These experiments provide evidence that downregulation of PKC protein levels by expression of oncogenic Ha-ras in C3H 10T1/2 fibroblasts is primarily due to altered transcriptional regulation. Because the downregulation of PKC was coupled with the onset of morphological transformation, the data suggest that this downregulation is involved in or facilitates the maintenance of a ras-transformed phenotype in C3H 10T1/2 fibroblasts.

Activation of the epidermal growth factor receptor signal transduction pathway stimulates tyrosine phosphorylation of protein kinase C delta

The expression of an oncogenic rasHa gene in epidermal keratinocytes stimulates the tyrosine phosphorylation of protein kinase C delta and inhibits its enzymatic activity (Denning, M. F., Dlugosz, A. A., Howett, M. K., and Yuspa, S. H. (1993) J. Biol. Chem. 268, 26079-26081). Keratinocytes expressing an activated rasHa gene secrete transforming growth factor alpha (TGFalpha) and have an altered response to differentiation signals involving protein kinase C (PKC). Because the neoplastic phenotype of v-rasHa expressing keratinocytes can be partially mimicked in vitro by chronic treatment with TGF alpha and the G protein activator aluminum fluoride (AlF4-), we determined if TGF alpha or AlF4- could induce tyrosine phosphorylation of PKCdelta. Treatment of primary keratinocyte cultures for 4 days with TGFalpha induced tyrosine phosphorylation of PKCdelta, whereas AlF4- only slightly stimulated PKCdelta tyrosine phosphorylation. The PKCdelta that was tyrosine-phosphorylated in response to TGFalpha had reduced activity compared with the nontyrosine-phosphorylated PKCdelta. Treatment of keratinocytes expressing a normal epidermal growth factor receptor (EGFR) with TGFalpha or epidermal growth factor for 5 min induced PKCdelta tyrosine phosphorylation. This acute epidermal growth factor treatment did not induce tyrosine phosphorylation of PKCdelta in keratinocytes isolated from waved-2 mice that have a defective epidermal growth factor receptor. In addition, the level of PKCdelta tyrosine phosphorylation in v-rasHa-transduced keratinocytes from EGFR null mice was substantially lower than in v-rasHa transduced wild type cells, suggesting that activation of the EGFR is important for PKC delta tyrosine phosphorylation in ras transformation. However, purified EGFR did not phosphorylate recombinant PKC delta in vitro, whereas members of the Src family (c-Src, c-Fyn) and membrane preparations from keratinocytes did. Furthermore, clearing c-Src or c-Fyn from keratinocyte membrane lysates decreased PKCdelta tyrosine phosphorylation, and c-Src and c-Fyn isolated from keratinocytes treated with TGFalpha had increased kinase activity. Acute or chronic treatment with TGFalpha did not induce significant PKCdelta translocation in contrast to the phorbol ester 12-O-tetradecanoylphorbol-13-acetate, which induced both translocation and tyrosine phosphorylation of PKCdelta. This suggests that TGFalpha-induced tyrosine phosphorylation of PKC delta results from the activation of a tyrosine kinase rather than physical association of PKCdelta with a membrane-anchored tyrosine kinase. Taken together, these results indicate that PKCdelta activity is inhibited by tyrosine phosphorylation in response to EGFR-mediated signaling and activation of a member of the Src kinase family may be the proximal tyrosine kinase acting on PKCdelta in keratinocytes.

Selective drug efflux in multidrug-resistant immunoblastic B lymphoma cells with overexpressed P-glycoprotein

Multidrug-resistant (MDR) sublines of the immunoblastic B lymphoma cell line were established by sequentially selecting in increasing concentrations of vincristine or adriamycin. The vincristine- and adriamycin-resistant cell lines, HOB1/VCR and HOB1/ADR, respectively, demonstrated resistance to a wide spectrum of chemotherapeutic agents including MDR drugs (Vinca alkaloids and anthracycline), antimicrotubule drugs (colchicine), and DNA-damaging agents (cisplatin and mitomycin C). The expression of human mdrl gene, as analyzed by Western blotting and reverse transcription-polymerase chain reaction (RT-PCR), revealed a 10-15-fold overexpression in both drug-resistant cell lines. Drug accumulation analysis demonstrated reduced accumulation of vincristine but not adriamycin in HOB1/VCR and HOB1/ADR cell lines. Inhibition of vincristine resistance was observed in both cell lines by verapamil, associated with restoration of drug accumulation, suggesting that acquired resistance in these cells is mainly due to P-glycoprotein. The drug accumulation was also examined in two series of previously characterized adriamycin-selected MDR colon adenocarcinoma cells and vincristin-selected non-MDR lung cancer cells. These studies demonstrated that immunoblastic B lymphoma cells selected for vincristine or adriamycin resistance preferentially develop P-glycoprotein-mediated vincristine efflux which plays a pivotal role in vincristine resistance. In contrast, these cells did not elevate adriamycin efflux, suggesting an additional mechanism responsible for adriamycin resistance.

Lack of elevated drug efflux in adriamycin-resistant immunoblastic B lymphoma cells with mdr1 overexpression

A multidrug-resistant (MDR) subline of the immunoblastic B lymphoma cell line was established by sequentially selecting in increasing concentrations of adriamycin. The adriamycin-resistant cell line (HOB1/ADR) demonstrated resistance to a wide spectrum of chemotherapeutic agents including MDR drugs (Vinca alkaloids and anthracycline), antimicrotubule drug (colchicine), and DNA-damaging agents (cisplatin and mitomycin C). The expression of human mdr1 gene, as analyzed by RT-PCR and Western blotting, revealed a 13-15-fold increase in resistant cells. Unexpectedly, HOB1/ADR cells demonstrated a lack of reduced accumulation and of enhanced efflux of adriamycin. More than 60% adriamycin was effluxed at the same rate in both cell lines within 10 min. In contrast, the initial rate of vincristine accumulation was reduced by 3 fold in this resistant cell line. The maximal level of vincristine accumulation was 50% lower in the resistant cells than the parental cells. The maximal efflux rate was enhanced by 5 fold in the resistant cells. Inhibition of vincristine resistance by verapamil associated with restoration of drug accumulation, suggesting that acquired resistance in these cells is due to P-glycoprotein. These studies demonstrated that immunoblastic B lymphoma cells selected for adriamycin resistance preferentially developed P-glycoprotein-mediated vincristine efflux which plays an important role in vincristine resistance. In contrast, the resistant cells did not elevate adriamycin efflux, suggesting an additional mechanism responsible for adriamycin resistance.

Adriamycin-inducible proteins associated with drug sensitivity in resistant immunoblastic B lymphoma cells

We have previously established an immunoblastic B lymphoma cell line, designated HOB1. This cell line is hypersensitive to a wide spectrum of chemotherapeutic agents. Two co-regulated polypeptides around 64 kDa (termed p64) were induced 10-30-fold in response to adriamycin and some other drugs at the IC50 (the concentration inhibiting cell growth by 50%). These inducible proteins are localized as monomeric forms in the cytosolic fraction, with isoelectric points of pH = 6.2 (major protein) and pH = 7.0 (minor protein). An adriamycin-resistant cell line was established from HOB1 cells. The p64 inducibility was dramatically reduced in resistant HOB1 cells or unrelated cell lines which show phenotypic resistance to adriamycin toxicity. The loss of p64 inducibility in resistant cells is not due to a failure of cells to take up adriamycin since drug accumulation kinetics remained the same as in the parental cells.

A role for myn in Ha-ras transformation of mouse 10T1/2 fibroblasts

The human max protein has been shown to form heterodimers with myc family proteins. The ability of myc+max heterodimers to act as sequence-specific transcriptional activators appears to be essential for the oncogenic activity of myc. max (also called myn in murine cells) can homodimerize to form a transcriptionally inactive complex. We previously showed that in mouse 10T1/2 cells, a combination of activated ras, c-myc, and a mutant form of p53 can cooperate in the induction of cellular transformation and metastasis. In the study presented here, we tested the hypothesis that expression of the myn gene may play a role in this cooperative process. Analysis of myn mRNA in these cell lines revealed the presence of a major 2.0-kb RNA species. This message and the 21- and 22-kDa myn polypeptides it encodes were significantly overexpressed in cells transformed by activated ras alone, by ras in combination with c-myc or mutant p53, or by ras plus myc plus mutant p53, in comparison with untransformed parental 10T1/2 cells. We also found that induction of ras expression in a cell line harboring an inducible ras gene was accompanied by an increase in myn mRNA expression. Interestingly, cotransfection of 10T1/2 cells with ras and myn inhibited cellular transformation in a focus-forming assay when compared with transfection with ras alone. These results suggest a role for ras in the regulation of myn gene expression and suggest a model of oncogene cooperativity in which the relative levels of myc and myn gene expression can influence ras transformation of mouse 10T1/2 cells.

The hepatitis B virus transactivator HBx causes elevation of diacylglycerol and activation of protein kinase C

Chronic infection with hepatitis B virus (HBV) is accompanied by an increasing risk of developing hepatocellular carcinoma. There are indications that the HBx protein of HBV is involved in the process of tumour formation. HBx also transactivates several transcription factor binding sites. Recently, we reported that HBx can use a tumour promotor pathway for transactivation. In particular, we found that transactivation of the binding motif for transcription factor AP-1 (JUN/FOS) by HBx is dependent on functional protein kinase C (PKC), as indicated by abolition of the transcriptional stimulation following downregulation or inhibition of the enzyme. Moreover, HBx activates PKC, probably via increasing the endogenous PKC activator sn-1,2-diacylglycerol (DAG). Here we extend these data and report on the time course of PKC activation. We found that activation of PKC by HBx is transient and differs from activation of PKC by the ras oncogene product or phorbol ester in that it does not lead to rapid downregulation of the enzyme subsequent to the activation. Moreover, we provide evidence that an increase in cellular DAG is observable not only as an early event in response to HBx but also in cell lines transformed after transfection with HBV DNA and stably expressing HBx. Besides its important role in the regulation of cellular genes, PKC is also the intracellular receptor for tumour-promoting agents and an activator of proto-oncogenes, suggesting that our observations might provide an explanation for the oncogenic properties of HBx.

p21ras and protein kinase C function in distinct and interdependent signaling pathways in C3H 10T1/2 fibroblasts

Both p21ras and protein kinase C (PKC) are believed to function downstream of plasma membrane-associated tyrosine kinases in cellular signal transduction pathways. However, it has remained controversial whether they function in the same pathway and, if so, what their relative position and functional relationship in such a pathway are. We investigated the possibilities that p21ras and PKC function either upstream or downstream of each other in a common linear pathway or that they function independently in colinear signal pathways. Either decreased expression of endogenous normal ras in fibroblasts transfected with an inducible antisense ras construct or overexpression of a mutant ras gene reduced the capacity of the phorbol ester tetradecanoyl phorbol acetate to trigger expression of the tetradecanoyl phorbol acetate-responsive and ras-dependent reporter gene osteopontin (OPN). PKC depletion decreased basal OPN mRNA levels, and the overexpression of ras restored OPN expression to the level of non-PKC-depleted cells. We propose a model in which ras and PKC function in distinct and interdependent signaling pathways.

p21ras and protein kinase C function in distinct and interdependent signaling pathways in C3H 10T1/2 fibroblasts

Both p21ras and protein kinase C (PKC) are believed to function downstream of plasma membrane-associated tyrosine kinases in cellular signal transduction pathways. However, it has remained controversial whether they function in the same pathway and, if so, what their relative position and functional relationship in such a pathway are. We investigated the possibilities that p21ras and PKC function either upstream or downstream of each other in a common linear pathway or that they function independently in colinear signal pathways. Either decreased expression of endogenous normal ras in fibroblasts transfected with an inducible antisense ras construct or overexpression of a mutant ras gene reduced the capacity of the phorbol ester tetradecanoyl phorbol acetate to trigger expression of the tetradecanoyl phorbol acetate-responsive and ras-dependent reporter gene osteopontin (OPN). PKC depletion decreased basal OPN mRNA levels, and the overexpression of ras restored OPN expression to the level of non-PKC-depleted cells. We propose a model in which ras and PKC function in distinct and interdependent signaling pathways.

Analysis of calcium-dependent protein kinase C isoforms in the early stages of diethylnitrosamine-induced rat hepatocarcinogenesis

The profiles of the calcium-dependent protein kinase C (PKC) isozymes alpha, beta, and gamma were examined in subcellular fractions from Fischer 344 rat liver during the early stages (48 h, 96 h, 7 d, and 60 d) of diethylnitrosamine (DEN)-induced carcinogenesis, using the Solt-Farber "resistant hepatocyte" model (DEN-2-acetylaminofluorene-partial hepatectomy; DEN-AAF-PH), and then related to the presence of focal or nodular gamma-glutamyl transpeptidase (GGT)-positive morphologic changes in the liver. After DEAE and hydroxyapatite column chromatography, two peaks, immunologically identified as PKC-alpha and -beta isoforms, were detected in the liver of normal (alpha/beta ratio = 4.0) and treated rats. In DEN-AAF-PH hepatocarcinogenesis an increase in PKC-alpha expression was found after PH (+43 +/- 19% at 48 h, alpha/beta ratio = 5.1; +125 +/- 25% at 96 h, alpha/beta ratio = 4.8), whereas the PKC-beta isoform appeared less significantly modified (+11 +/- 3% at 48 h and +89 +/- 17% at 96 h). Seven and 60 days after PH, a marked increase in the PKC-alpha (+96 +/- 20% and +150 +/- 48%, respectively) and PKC-beta isoforms (+158 +/- 41%, alpha/beta ratio = 3.1 and +130 +/- 26%, alpha/beta ratio = 4.4, respectively), occurred along with the appearance of GGT-positive altered hepatic foci and nodules in the liver sections. Sham hepatectomy caused PKC-alpha and -beta isoform activities similar to those of normal controls. In contrast, saline-AAF-PH-treated rats had downregulation of PKC-alpha after PH (alpha/beta ratio = 1.8 at 96 h), possibly due to the mitoinhibitory effect of the carcinogen AAF on normal uninitiated hepatocytes. Immunohistochemical analysis with monoclonal antibodies to PKC-alpha and -beta revealed diffuse positive cytoplasmic signals in GGT-positive foci and nodules in rat liver. Taken together, these preliminary results, using the Solt-Farber model of liver carcinogenesis, suggest a role for PKC in tumor promotion. They also suggest that the PKC-alpha isoform may play a specific role in clonal expansion of DEN-initiated hepatocytes after PH.

Induction of protein kinase C down-regulation by the phorbol ester TPA in a calpain/protein kinase C complex

Using a calpain/protein kinase C (PKC) complex, we were able to reproduce, in vitro, the induction of PKC down-regulation by the phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (TPA) which had been previously observed in cells. We show that TPA initiates this phenomenon by promoting a calpain-dependent conversion of PKC to the Ca2+ phospholipid-independent protein kinase M (PKM), at physiological calcium concentrations. This effect of TPA was dependent upon the presence of phosphatidylserine and was observed only when PKC was the substrate for the protease, inactivation of calpain by autolysis not being modified by the presence of TPA. Moreover, PKM generated from the calpain-PKC complex was resistant to calpain, even after addition of TPA. These results suggest that TPA induces a conformational change in PKC, increasing the affinity of the kinase for calpain and consequently permitting its proteolysis for the basal level of calcium in cells.

Alterations in protein kinase C system of colonic epithelium during fasting-refeeding. Evidence for protein kinase C independent pathway of enhanced proliferative activity

In the present study, we compared (1) incorporation of [3H]dThd into DNA, (2) total protein kinase C (PKC) activity, (3) the subcellular distribution of PKC, and (4) PKC isozyme (alpha, beta and gamma) mass in colonic mucosal scrapings and isolated superficial and proliferative colonic epithelial cells from 48-hr fasted, 48-hr fasted-refed, and ad libitum-fed rats. Total colonic mucosal PKC activity and PKC alpha mass were higher and the in vivo rate of [3H]dThd incorporation into mucosal DNA was markedly depressed in 48-hr fasted rats compared to ad libitum-fed or fasted-refed rats. These alterations were localized predominantly to the proliferative pool of colonic epithelial cells. Despite an 11-fold increase in mucosal DNA synthesis, no alterations in total mucosal PKC activity were detected in fasted-refed rats compared to rats fed ad libitum. Moreover, no differences in the subcellular distribution of PKC were noted among any of the dietary groups. Intrarectal instillation of deoxycholate activated PKC and increased DNA synthesis 1.5- to 2-fold. Deoxycholate-induced increases in DNA synthesis, but not those induced by refeeding, were inhibited by treatment of rats with the PKC inhibitors H-7, sphingosine, or staurospaurine. The results do not support a role for PKC in the mediation of increased proliferative activity of colonic mucosa induced by refeeding.

Regulation of natural antibody binding and susceptibility to natural killer cells through Zn(++)-inducible ras oncogene expression

Changes in the natural resistance phenotype were examined for the 2H1, 10T 1/2 cells expressing the activated human H-ras oncogene under the transcriptional regulation of the zinc-inducible mouse metallothionein-I promoter. Culture of the cells in 50 microM ZnSO4 induced an increase in ras protein p21 levels which were maximal within 1 day. Natural-antibody (NAb) binding was significantly increased following 2 days of cell culture in ZnSO4 and continued to increase up to 4 days. The increased NAb binding returned to uninduced levels within 2 days following the removal of added zinc ions from the culture medium. The cells also exhibited a significant increase in natural killer (NK) cell sensitivity following 2 days in ZnSO4. This was maintained as long as the zinc was in the medium, but returned to uninduced levels within 1 day following its removal. The results show that NAb binding and susceptibility to NK cells increased following ras oncogene expression in 10T 1/2 cells and that both parameters were regulated by p21 expression. Repeated i.v. administration of whole-serum NAb prior to tumor inoculation reduced the number of early tumors following s.c. injection of Zn(++)-induced 2Hl cells into Zn(++)-treated C3H/HeN mice, consistent with an in vivo role for NAb in the defense against ras-transformed cells. In contrast, small but statistically significant reductions in NAb binding were observed following v-H-ras transformation of NIH 3T3 cells or v-src transformation of 10T 1/2. The data argue for an NAb- and NK-cell-susceptible phase of ras-induced tumor development which is a prerequisite for these mediators to contribute to a first line of defense against incipient neoplasia, and suggest that characteristics of the recipient cell and the transforming oncogene are important in determining the natural resistance phenotype.

Cytokinesis is more rapid in Ha-T24-ras transfected rat embryo fibroblasts than in non-transfected control cells

It has long been known that neoplastic cells are characterized by increases in cell motility. Earlier studies from this laboratory indicated that mitotic events were also altered in many tumor and experimentally transformed cells and that this included increases in metaphase duration and a reduction in the duration of cytokinesis. The studies presented in this paper were done to determine whether or not transfection of normal rat embryo fibroblasts by the Ha-T24-ras oncogene could also produce such alterations in mitotic events. The results obtained with the use of time lapse video microscopy indicate that neither the duration of metaphase nor the rate of chromosome movement during anaphase was altered but that the rate of furrow progression during cytokinesis occurred at a significantly more rapid rate. Thus, the cellular alterations induced by transfection with Ha-T24-ras accelerate microfilament-dependent cytokinetic furrowing without significant effects on microtubule-dependent mitotic events. One of several possible mechanisms that could account for these observations involves a down regulation of protein kinase C which has been reported to occur in many neoplastic cells including those transformed by ras. Such a hypothesis could also have broader implications because it may be applicable to the increase in motility and metastatic activity generally observed in transformed cells.