Down-regulation of protein kinase C activity in 1,2-dimethylhydrazine-induced rat colonic tumors

PKCalpha tumor suppression in the intestine is associated with transcriptional and translational inhibition of cyclin D1

Alterations in PKC isozyme expression and aberrant induction of cyclin D1 are early events in intestinal tumorigenesis. Previous studies have identified cyclin D1 as a major target in the antiproliferative effects of PKCalpha in non-transformed intestinal cells; however, a link between PKC signaling and cyclin D1 in colon cancer remained to be established. The current study further characterized PKC isozyme expression in intestinal neoplasms and explored the consequences of restoring PKCalpha or PKCdelta in a panel of colon carcinoma cell lines. Consistent with patterns of PKC expression in primary tumors, PKCalpha and delta levels were generally reduced in colon carcinoma cell lines, PKCbetaII was elevated and PKCepsilon showed variable expression, thus establishing the suitability of these models for analysis of PKC signaling. While colon cancer cells were insensitive to the effects of PKC agonists on cyclin D1 levels, restoration of PKCalpha downregulated cyclin D1 by two independent mechanisms. PKCalpha expression consistently (a) reduced steady-state levels of cyclin D1 by a novel transcriptional mechanism not previously seen in non-transformed cells, and (b) re-established the ability of PKC agonists to activate the translational repressor 4E-BP1 and inhibit cyclin D1 translation. In contrast, PKCdelta had modest and variable effects on cyclin D1 steady-state levels and failed to restore responsiveness to PKC agonists. Notably, PKCalpha expression blocked anchorage-independent growth in colon cancer cells via a mechanism partially dependent on cyclin D1 deficiency, while PKCdelta had only minor effects. Loss of PKCalpha and effects of its re-expression were independent of the status of the APC/beta-catenin signaling pathway or known genetic alterations, indicating that they are a general characteristic of colon tumors. Thus, PKCalpha is a potent negative regulator of cyclin D1 expression and anchorage-independent cell growth in colon tumor cells, findings that offer important perspectives on the frequent loss of this isozyme during intestinal carcinogenesis.

Diets enriched with cereal brans or inulin modulate protein kinase C activity and isozyme expression in rat colonic mucosa

The role of dietary fibres in colon carcinogenesis is controversial. To elucidate the mechanisms by which different dietary fibre sources may affect colonic tumour development, we studied the effects of diets enriched with cereal brans or inulin on protein kinase C (PKC) activity and isozyme expression in rat colon. Male Wistar rats (twelve per group) were fed one of the following AIN-93G-based diets () for 4 weeks: a non-fibre high-fat diet or one of the four high-fat diets supplemented with either rye, oat or wheat bran or inulin at 100 g/kg diet. The fat concentration (20 g/100 g) and fatty acid composition of the non-fibre high-fat diet was designed to approximate that in a typical Western-type diet. In the proximal colon, rats fed the inulin diet had a significantly higher membrane PKC activity and a higher membrane PKC δ level than rats fed the non-fibre diet (P<0·05). In the distal colon, rats fed the inulin and oat bran diets had a higher total PKC activity and a higher membrane PKC β2 level than rats fed the wheat-bran diet. Rats in the non-fibre and wheat-bran groups had the lowest concentrations of luminal diacylglycerol. In conclusion, feeding of wheat bran resulted in low distal PKC activity and expression of PKC β2, a PKC isozyme related to colonic cell proliferation and increased susceptibility for colon carcinogenesis, which may explain in part the protective effect of wheat bran against tumour development in a number of experimental colon cancer studies. The increase in PKC activity and PKC β2 expression by feeding inulin may be a drawback of inulin as a functional food.

Defective human MutY phosphorylation exists in colorectal cancer cell lines with wild-type MutY alleles

Oxidative DNA damage can generate a variety of cytotoxic DNA lesions such as 8-oxoguanine (8-oxoG), which is one of the most mutagenic bases formed from oxidation of genomic DNA because 8-oxoG can readily mispair with either cytosine or adenine. If unrepaired, further replication of A.8-oxoG mispairs results in C:G to A:T transversions, a form of genomic instability. We reported previously that repair of A.8-oxoG mispairs was defective and that 8-oxoG levels were elevated in several microsatellite stable human colorectal cancer cell lines lacking MutY mutations (human MutY homolog gene, hmyh, MYH MutY homolog protein). In this report, we provide biochemical evidence that the defective repair of A.8-oxoG may be due, at least in part, to defective phosphorylation of the MutY protein in these cell lines. In MutY-defective cell extracts, but not extracts with functional MutY, A.8-oxoG repair was increased by incubation with protein kinases A and C (PKA and PKC) and caesin kinase II. Treatment of these defective cells, but not cells with functional MutY, with phorbol-12-myristate-13-acetate also increased the cellular A.8-oxoG repair activity and decreased the elevated 8-oxoG levels. We show that MutY is serine-phosphorylated in vitro by the action of PKC and in the MutY-defective cells by phorbol-12-myristate-13-acetate but that MutY is already phosphorylated at baseline in proficient cell lines. Finally, using antibody-isolated MutY protein, we show that MutY can be directly phosphorylated by PKC that directly increases the level of MutY catalyzed A.8-oxoG repair.

Dietary copper affects azoxymethane-induced intestinal tumor formation and protein kinase C isozyme protein and mRNA expression in colon of rats

Previous studies have show that changes in protein kinase C (PKC) isoform expression may be related to increased susceptibility of copper-deficient rats to aberrant crypt formation. The purpose of this study was to determine whether dietary copper would affect azoxymethane-induced intestinal tumor formation and PKC isozyme expression in normal colonic mucosa and tumor samples. Eighty weanling Fischer-344 rats were randomly assigned to diets that contained either 0.8 or 5.3 microg Cu/g diet. After 24 and 31 d of diet consumption, 30 rats/diet were administered azoxymethane (15 mg/kg i.p.) and 10 rats/diet were administered saline. Rats continued to consume their respective diets for an additional 38 wk. Rats injected with azoxymethane and fed the low copper diet had a significantly (P < 0.0001) greater small intestinal and total tumor incidence compared with rats fed adequate dietary copper. However, dietary copper did not affect colon tumor incidence. Low dietary copper significantly (P < 0.004) decreased PKC alpha protein expression in normal but not in tumor tissue. In contrast, low dietary copper did not affect PKC delta or zeta protein expression in either the normal or tumor tissue. PKC alpha and delta protein and mRNA expression were lower in tumor tissue than in normal tissue. These results along with previous observations suggest that dietary copper-mediated changes in PKC alpha, delta and zeta protein expression are not as important for colon tumor promotion/progression as they are for tumor initiation.

Dietary copper and dimethylhydrazine affect protein kinase C isozyme protein and mRNA expression and the formation of aberrant crypts in colon of rats

Low dietary copper has been shown to decrease the expression of various protein kinase C (PKC) isozymes and increase the risk of colon cancer development in experimental animals. The purpose of this study was to investigate the relationship between dietary copper and carcinogen administration on PKC isozyme accumulation and aberrant crypt foci (ACF) formation in rats fed 0.9 and 7.7 microg Cu/g diet. After 24 and 31 d on the diets, the rats were injected with either dimethylhydrazine (DMH) (25 mg/kg i.p.) or saline and killed at two time points (2 wk and 8 wk after DMH). Rats fed low dietary copper had significantly lower (p<0.0001) hematocrits, hemoglobin, ceruloplasmin activity and plasma and liver copper concentrations than rats fed adequate dietary copper. Ingestion of low dietary copper significantly (p<0.005) increased the formation of DMH-induced ACF (116.8 vs 59.6). Low dietary copper significantly (p<0.05) decreased the concentration of PKC alpha, delta, and zeta in the colon at 2 wk but not at 8 wk. Thus, changes in PKC isoform protein concentration may be related to increased susceptibility of copper-deficient animals to colon cancer.

Protein kinase C signaling mediates a program of cell cycle withdrawal in the intestinal epithelium

Members of the protein kinase C (PKC) family of signal transduction molecules have been widely implicated in regulation of cell growth and differentiation, although the underlying molecular mechanisms involved remain poorly defined. Using combined in vitro and in vivo intestinal epithelial model systems, we demonstrate that PKC signaling can trigger a coordinated program of molecular events leading to cell cycle withdrawal into G(0). PKC activation in the IEC-18 intestinal crypt cell line resulted in rapid downregulation of D-type cyclins and differential induction of p21(waf1/cip1) and p27(kip1), thus targeting all of the major G(1)/S cyclin-dependent kinase complexes. These events were associated with coordinated alterations in expression and phosphorylation of the pocket proteins p107, pRb, and p130 that drive cells to exit the cell cycle into G(0) as indicated by concomitant downregulation of the DNA licensing factor cdc6. Manipulation of PKC isozyme levels in IEC-18 cells demonstrated that PKCalpha alone can trigger hallmark events of cell cycle withdrawal in intestinal epithelial cells. Notably, analysis of the developmental control of cell cycle regulatory molecules along the crypt-villus axis revealed that PKCalpha activation is appropriately positioned within intestinal crypts to trigger this program of cell cycle exit-specific events in situ. Together, these data point to PKCalpha as a key regulator of cell cycle withdrawal in the intestinal epithelium.

Morphodensitometric analysis of protein kinase C beta(II) expression in rat colon: modulation by diet and relation to in situ cell proliferation and apoptosis

We have recently demonstrated that overexpression of PKC beta(II) renders transgenic mice more susceptible to carcinogen-induced colonic hyperproliferation and aberrant crypt foci formation. In order to further investigate the ability of PKC beta(II) to modulate colonocyte cytokinetics, we determined the localization of PKC beta(II) with respect to cell proliferation and apoptosis along the entire colonic crypt axis following carcinogen and diet manipulation. Rats were provided diets containing either corn oil [containing n-6 polyunsaturated fatty acids (PUFA)] or fish oil (containing n-3 PUFA), cellulose (non-fermentable fiber) or pectin (fermentable fiber) and injected with azoxymethane (AOM) or saline. After 16 weeks, an intermediate time point when no macroscopic tumors are detected, colonic sections were utilized for immunohistochemical image analysis and immunoblotting. Cell proliferation was measured by incorporation of bromodeoxyuridine into DNA and apoptosis by terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labeling. In the distal colon, PKC beta(II) staining was localized to the upper portion of the crypt. In comparison, proximal crypts had more (P < 0.05) staining in the lower tertile. AOM enhanced (P < 0.05) PKC beta(II) expression in all regions of the distal colonic crypt (upper, middle and lower tertiles). There was also an interaction (P < 0.05) between dietary fat and fiber on PKC beta(II) expression (corn/pectin > fish/cellulose, fish/pectin > corn/cellulose) in all regions of the distal colonic crypt. With respect to colonic cell kinetics, proliferation paralleled the increase in PKC beta(II) expression in carcinogen-treated animals. In contrast, apoptosis at the lumenal surface was inversely proportional to PKC beta(II) expression in the upper tertile. These results suggest that an elevation in PKC beta(II) expression along the crypt axis in the distal colon is linked to enhancement of cell proliferation and suppression of apoptosis, predictive intermediate biomarkers of tumor development. Therefore, select dietary factors may confer protection against colon carcinogenesis in part by blocking carcinogen-induced PKC beta(II) expression.

Protein kinase C isoforms during the development of deciduomata in pregnant rats

In this study, we determined the expression of protein kinase C (PKC) isoforms during pregnancy. At pregnant duration, PKC alpha was down-modulated in the deciduomata but not in the myometrium. Down-modulation was compatible with the increase in cell mitosis, which reached a maximum at 8-9 days. On the other hand, PKC zeta was not down-modulated. It was increased both in the cytosolic and particulate fractions of the deciduomata, and paralleled the frequency of decidual cell mitosis. The other PKC isoform of delta was also increased, but it was associated with the cell regression. Therefore, these findings confirmed that the variable expression of PKC isoforms in decidualizing tissue may be involved in the modulation of decidual cell growth.

Overexpression of protein kinase C betaII induces colonic hyperproliferation and increased sensitivity to colon carcinogenesis

Protein kinase C betaII (PKC betaII) has been implicated in proliferation of the intestinal epithelium. To investigate PKC betaII function in vivo, we generated transgenic mice that overexpress PKC betaII in the intestinal epithelium. Transgenic PKC betaII mice exhibit hyperproliferation of the colonic epithelium and an increased susceptibility to azoxymethane-induced aberrant crypt foci, preneoplastic lesions in the colon. Furthermore, transgenic PKC betaII mice exhibit elevated colonic beta-catenin levels and decreased glycogen synthase kinase 3beta activity, indicating that PKC betaII stimulates the Wnt/adenomatous polyposis coli (APC)/beta-catenin proliferative signaling pathway in vivo. These data demonstrate a direct role for PKC betaII in colonic epithelial cell proliferation and colon carcinogenesis, possibly through activation of the APC/beta-catenin signaling pathway.

Protein kinase C isoforms during the development of deciduomata in pseudopregnant rats

In this study, we determined the expression of protein kinase C (PKC) isoforms during trauma-induced decidualization. The findings revealed that at least five PKC isoforms (alpha, delta, zeta, iota and lambda) were present in both control and decidualized tissues. After trauma-stimulation, PKC alpha was down-modulated in the deciduomata but not in the myometrium. Down-modulation was compatible with the increase in cell mitosis which reached a maximum at 2-3 days. On the other hand, PKC zeta was not down-modulated. It was increased both in the deciduomata and myometrium, and paralleled the frequency of decidual cell mitosis. The PKC isoforms of delta, iota and lambda were also increased, but they were associated with the depression of cell mitosis. Therefore, these findings suggested that the variable expression of PKC isoforms in trauma-induced decidualizing tissue in pseudopregnant rats may be involved in the modulation of decidual cell growth.

Colonocyte differentiation is associated with increased expression and altered distribution of protein kinase C isozymes

BACKGROUND & AIMS

Colon cancer cells express reduced levels of protein kinase C (PKC). This study examines the regulation of PKC isozymes in normal colonic epithelium, as a basis for understanding the significance of alterations in this enzyme system in colon carcinogenesis.

METHODS

The expression and localization of PKC isozymes in mouse and rat colonocytes at different developmental stages were determined using a combined morphological and biochemical approach. PKC alpha expression was compared in colonic adenocarcinomas and adjacent normal mucosa by immunoblot analysis.

RESULTS

Mouse and rat colonocytes express PKC alpha, beta II, delta, epsilon, and zeta. Relatively low levels of these isozymes were detected in proliferating cells of the crypt base, predominantly in the cytosolic compartment. Coincident with colonocyte growth arrest/differentiation, PKC isozyme expression markedly increased in both the cytosolic and, more significantly, in the membrane/cytoskeletal fraction. Colonic tumors express reduced levels of PKC alpha, an isozyme that has been implicated in negative control of intestinal cell growth.

CONCLUSIONS

These findings are supportive of a role for certain PKC isozyme(s) in signaling pathways mediating postmitotic events in colonocytes in situ, and suggest that diminished activity of these pathway(s) may contribute to the alterations in growth control/differentiation associated with colonic neoplasia.

Protein kinase C isozyme-mediated cell cycle arrest involves induction of p21(waf1/cip1) and p27(kip1) and hypophosphorylation of the retinoblastoma protein in intestinal epithelial cells

The molecular mechanisms underlying protein kinase C (PKC) isozyme-mediated control of cell growth and cell cycle progression are poorly understood. Our previous analysis of PKC isozyme regulation in the intestinal epithelium in situ revealed that multiple members of the PKC family undergo changes in expression and subcellular distribution precisely as the cells cease proliferating in the mid-crypt region, suggesting that activation of one or more of these molecules is involved in negative regulation of cell growth in this system (Saxon, M. L., Zhao, X., and Black, J. D. (1994) J. Cell Biol. 126, 747-763). In the present study, the role of PKC isozyme(s) in control of intestinal epithelial cell growth and cell cycle progression was examined directly using the IEC-18 immature crypt cell line as a model system. Treatment of IEC-18 cells with PKC agonists resulted in translocation of PKC alpha, delta, and epsilon from the soluble to the particulate subcellular fraction, cell cycle arrest in G1 phase, and delayed transit through S and/or G2/M phases. PKC-mediated cell cycle arrest in G1 was accompanied by accumulation of the hypophosphorylated, growth-suppressive form of the retinoblastoma protein and induction of the cyclin-dependent kinase inhibitors p21(waf1/cip1) and p27(kip1). Reversal of these cell cycle regulatory effects was coincident with activator-induced down-regulation of PKC alpha, delta, and epsilon. Differential down-regulation of individual PKC isozymes revealed that PKC alpha in particular is sufficient to mediate cell cycle arrest by PKC agonists in this system. Taken together, the data implicate PKC alpha in negative regulation of intestinal epithelial cell growth both in vitro and in situ via pathways which involve modulation of Cip/Kip family cyclin-dependent kinase inhibitors and the retinoblastoma growth suppressor protein.

Selective preservation of protein kinase C-zeta in the chemoprevention of azoxymethane-induced colonic tumors by piroxicam

While nonsteroidal anti-inflammatory drugs have been shown to exert preventive effects against the development of colonic tumors in humans and in chemically-induced tumors in animal models, the mechanism(s) involved in this phenomenon is unclear. We have recently demonstrated that one such agent, piroxicam, when supplemented (75 ppm) in the diets of rats administered azoxymethane, reduced the incidence of rats bearing tumors. To date, the effects of piroxicam on protein kinase C, a family of serine/threonine kinases which may be intimately involved in the colonic malignant transformation process, have not been examined. It was, therefore, of interest to determine whether piroxicam altered the expression of one or more isoforms of this kinase in these tumors. The present studies demonstrate that dietary piroxicam selectively preserved the expression of protein kinase C-zeta in azoxymethane-induced tumors; suggesting that this is at least one mechanism involved in this agent's chemopreventive actions in this organ.

Effect of dietary fat on colonic protein kinase C and induction of aberrant crypt foci

A major objective of the present study was to determine whether a high-fat diet affects early events during colon carcinogenesis. Female Sprague-Dawley rats were injected with saline or azoxymethane (20 mg/kg) and fed either a normal (5% corn oil w/w) or a high (5% corn oil and 15% beef tallow w/w) fat diet. To assess the effect of a known tumor-promoting diet on the early events of neoplastic transformation, Study 1 examined the induction and growth of aberrant crypt foci (ACF) as well as of proliferative indices. The total number of ACF were similar in both groups even after 8 wk of dietary treatment; however, ACF with accelerated growth characteristics (> or = 4 crypts/focal lesion) were more prevalent (P < or = 0.05) in the colons of animals fed the high-fat diet. Metaphase arrest cells and 5'-bromo-2'-deoxyuridine labelled cells showed no appreciable response to dietary changes. To determine whether changes in colonic signal transduction pathways represent an early response to dietary modification, Study 2 evaluated the activity of protein kinase C (PKC), proliferative indices and changes in phospholipid fatty acid profiles. In comparison to the normal fat group, the colons of high-fat fed animals exhibited higher (P < or = 0.05) membranes and lower soluble PKC activity; however, proliferation patterns of these colons were not altered. Changes in the membrane lipid composition were minor; however, an increase in the phosphatidylcholine/phosphatidylethanolamine ratio and in 20:4n-6 was noted.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of bile acids on phospholipase C activity in extracts of normal human colon mucosa and primary colon tumors

Phospholipase C (PLC) activity and its response to stimulation by bile acids was assayed in cellular extracts from 16 primary human colon tumors of various Duke's stages and paired adjacent normal mucosal samples. In the absence of bile acid, there was negligible degradation of phosphatidylinositol (PI) 1-stearoyl-2-[14C]-arachiodonoyl by tumor or normal tissue, but the addition of deoxycholic acid (DCA) or taurocholic acid (TCA) resulted in concentration-dependent and time-dependent stimulation of diacylglycerol (DAG) formation at optimal concentrations of 2 mM DCA and 4 mM TCA. Triton X-100 (0.125-1.0%) inhibited rather than enhanced the PI-degrading activity of these extracts, indicating that the stimulatory effects of DCA and TCA were not simply due to a detergent effect. Under the same assay conditions there was only a small amount of labeled monoacylglycerol or free arachidonic acid produced by extracts incubated in the absence or presence of DCA or TCA. No major differences in DAG production from PI were seen between paired samples of normal colon mucosa and primary colon tumors, in assays done in the presence of 2 mM TCA. Extracts from tumors in the distal part of the colon had higher activity than those from the proximal colon. This was also true for the extent of release of free arachidonic acid from labeled PI. Under the same conditions, labeled phosphatidylcholine or phosphatidylethanolamine did not serve as substrates for the colon mucosa or tumor extracts. Nor was there significant hydrolysis of the labeled DAG (1-stearoyl-2-14C-arachidonoylglycerol) by normal colon mucosa or tumor extracts, in the absence or presence of DCA or TCA. On the other hand, a low level of DAG lipase activity was detected in the presence of Triton X-100. These findings provide the first evidence that normal human colon mucosa and primary colon tumors contain a PI-specific PLC activity that is markedly stimulated by bile acids. Our results also suggest that bile acids may enhance colon carcinogenesis by acting on this enzyme system, thereby influencing signal transduction pathways in the target cells.

Decreased levels of protein kinase C enzyme activity and protein kinase C mRNA in primary colon tumors

PURPOSE

We have previously reported decreased protein kinase C (PKC) enzyme activity in primary human colorectal carcinomas. The purpose of this study was to extend these findings to a larger number of cases and to also examine the levels of expression of mRNAs that encode specific isoforms of PKC in these tumors.

METHODS

Colorectal carcinomas and paired grossly normal adjacent mucosal samples were collected from 39 patients. Complete histopathologic analyses were performed on all samples. PKC enzyme activity in both the cytosolic and particulate fractions was quantitated by measuring the amount of 32P incorporated into histone Type III-S. Northern blot nucleic acid hybridization was performed using polyA+ RNA extracted from both the tumor and normal tissue samples and 32P-labeled probes for specific isoforms of PKC. The paired sample t-test was used to determine the statistical significance of tumor to normal ratios of both enzyme activity and mRNA levels.

RESULTS

The mean value for cellular PKC enzyme activity in the colon tumors from 39 patients was about 60 percent of that found in the paired adjacent grossly normal mucosa samples (P < 0.001). The subcellular distribution of PKC activity was similar in normal and tumor samples (about 70 percent in the particulate fraction). The abundance of PKC alpha mRNAs varied considerably among 28 tumor/normal pairs, with a mean tumor to normal (T:N) ratio of 1.0 +/- 0.6 for the 9.9-kb mRNA band and 1.4 +/- 0.7 for the 3.5-kb band. The abundance of PKC beta mRNAs was decreased in 30 of 39 tumors, with a mean T:N ratio of 0.6 +/- 0.4 for both the 9.4- and 3.5-kb bands for all 39 samples (P < 0.001). None of the parameters measured correlated with Dukes stage or the grade of the tumor.

CONCLUSIONS

These studies extend previous evidence that total PKC enzyme activity is frequently decreased in primary human colon tumors. Our finding that this is often associated with decreased levels of PKC beta mRNA suggest that this is not simply due to post-translational down-regulation of this enzyme system. Further studies are required to determine whether these changes in PKC alpha and PKC beta mRNAs are due to altered de novo transcription or mRNA stability. It will also be of interest to examine the expression of other isoforms of PKC in colon tumors.

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.

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.

Effect of dietary soybean and licorice on the male F344 rat: an integrated study of some parameters relevant to cancer chemoprevention

The individual and combined effects of dietary toasted soybean meal (3.13-25%) and dietary licorice root extract (0.38-3.0%) on selected liver and intestinal enzyme levels and on clinical chemistry and histopathological parameters were evaluated on male F344 rats. All parameters were measured one and three months after the 50-day-old rats were started on the diets. By use of newly developed high-performance liquid chromatography-based analytic methods, measurable levels of daidzein (2.67 micrograms/ml) and glycyrrhetinic acid (7.87 micrograms/ml) were detected in the sera of rats on the 25% soybean and 3% licorice diets, respectively. Histopathological evaluations of organs and tissues yielded only nonsignificant strain-related changes. At all dosages, there were no significant soybean- or licorice-related anatomic lesions or hematologic changes. In the clinical biochemistry profile, soybean meal caused moderate but significant dose-dependent decreases in serum cholesterol and increases in alkaline phosphatase, blood urea nitrogen, and phosphorus, which remained within the normal range. Liver glutathione transferase, catalase, and protein kinase C showed significant inductions (up to 50%) in response to increasing doses of soybean meal and licorice extract, with evidence for only marginal interaction between the two additives. Their effects on the intestinal mucosa were not significant. Ornithine decarboxylase levels, an indicator of promotional activity, were unchanged or repressed by the additives. The favorable effects of up to 25% toasted soybean meal and 3% licorice root extract on the levels of the four enzymes, without unfavorable changes in clinical parameters, might account in part for the chemopreventive activities of these additives. These effects would be in addition to direct inhibitory effects of known components in these additives on these or other enzymes or modulation of hormone activity that is not evaluated in this study.