Differential expression of protein kinase C isoforms in diethylnitrosamine-initiated rat liver

The role of PKC isoforms in the inhibition of NF-κB activation by vitamin K2 in human hepatocellular carcinoma cells

Vitamin K (VK) has diverse protective effects against osteoporosis, atherosclerosis and carcinogenesis. We recently reported that menatetrenone, a VK2 analogue, suppressed nuclear factor (NF)-κB activation in human hepatoma cells. Although NF-κB is regulated by isoforms of protein kinase C (PKC), the involvement of PKCs in VK2-mediated NF-κB inhibition remains unknown. Therefore, the effects of VK2 on the activation and the kinase activity of each PKC isoform were investigated. The human hepatoma Huh7 cells were treated with PKC isoform-specific inhibitors and/or siRNAs against each PKC isoform with or without 12-O-tetradecanoylphorbol-13-acetate (TPA). VK2 inhibited the TPA-induced NF-κB activation in Huh7 cells. NF-κB activity was inhibited by the pan-PKC inhibitor Ro-31-8425, but not by the PKCα-specific inhibitor Gö6976. The knockdown of individual PKC isoforms including PKCα, δ and ɛ showed only marginal effects on the NF-κB activity. However, the knockdown of both PKCδ and PKCɛ, together with treatment with a PKCα-specific inhibitor, depressed the NF-κB activity. VK2 suppressed the PKCα kinase activity and the phosphorylation of PKCɛ after TPA treatment, but neither the activation nor the enzyme activity of PKCδ was affected. The knockdown of PKCɛ abolished the TPA-induced phosphorylation of PKD1, and the effects of PKD1 knockdown on NF-κB activation were similar to those of PKCɛ knockdown. Collectively, all of the PKCs, including α, δ and ɛ, and PKD1 are involved in the TPA-mediated activation of NF-κB. VK2 inhibited the NF-κB activation through the inhibition of PKCα and ɛ kinase activities, as well as subsequent inhibition of PKD1 activation.

Regulation of tumor suppressor PDCD4 by novel protein kinase C isoforms

Transforming growth factor-beta1 (TGF-beta1) induces apoptosis in normal hepatocytes and hepatoma cells. PDCD4 is involved in TGF-beta1-induced apoptosis via the Smad pathway. The tumor promoter 12-O-tetradecanoylphorbor-13-acetate (TPA), a protein kinase C stimulator, inhibits TGF-beta1-induced apoptosis. However, the mechanisms of TPA action on PDCD4 expression remain to be elucidated. Therefore. the regulatory mechanism of PDCD4 expression by PKC was investigated. The treatment of the human hepatoma cell line, Huh7 with TPA suppressed PDCD4 protein expression and TGF-beta1 failed to increase the PDCD4 protein expression. PKC inhibitors Ro-31-8425 or bisindolylmaleimide-1-hydrocholoride (pan-PKC inhibitors) and rottlerin (PKCdelta inhibitor), but not Go6976 (PKCalpha inhibitor), enhanced the induction of PDCD4 protein by TGF-beta1. Furthermore, siRNA-mediated knockdown of PKCdelta and epsilon, but not PKCalpha, augmented the TGF-beta1-stimulated PDCD4 protein expression. However, TPA or pan-PKC inhibitor did not alter the PDCD4 mRNA expression either under basal- and TGF-beta1-treated conditions. The down-regulation of PDCD4 by TPA was restored by treatment with the proteasome inhibitor MG132. These data suggest that two isoforms of PKCs are involved in the regulation of the PDCD4 protein expression related to the proteasomal degradation pathway.

Reduction of PKC alpha decreases cell proliferation, migration, and invasion of human malignant hepatocellular carcinoma

Protein kinase C (PKC) superfamily play key regulatory roles on the development of cancer. However, the exact role of these enzymes in human hepatocellular carcinoma (HCC) has not been well established. Using the RT-PCR and Western blotting to analyze the levels of PKC isoforms mRNA and protein in the five different differentiated hepatoma cell lines, we found that PKC alpha was highly expressed in the poor-differentiated HCC cell lines (SK-Hep-1 and HA22T/VGH) as compared with that in the well-differentiated HCC cell lines (PLC/PRF/5, Hep3B, and HepG2). When treated with PKC alpha antisense oligonucleotides (ODN), both HA22T/VGH and SK-Hep-1 cells lines showed the reduction of PKC alpha expression, as well as a deceleration in the growth rate and in the level of cyclin D1, but the increase in the levels of p53 and p21(WAF1/CIP1). Moreover, the reduction of PKC alpha expression also inhibited the migratory and invasive potential of both HA22T/VGH and SK-Hep-1 cells lines, and revealed a down-regulation of several migration/invasion-related genes (MMP-1, u-PA, u-PAR, and FAK). These phenomenon were also confirmed by DNA-based small interfering RNA (siRNA) PKC alpha and PKC alpha/beta specific inhibitor Go6976. Thus, the results indicated that PKC alpha may be associated with regulation of cell proliferation/migration/invasion in human poorly differentiated HCC cells, suggesting a role for the PKC alpha in the malignant progression of human HCC.

Subcellular redistribution of protein kinase C isozymes is associated with rat liver cirrhotic changes induced by carbon tetrachloride or thioacetamide

BACKGROUND AND AIMS

Protein kinase C (PKC) plays a key role in the alteration of signal transduction in the liver, which may contribute to the development of liver cirrhosis. The aim of the present study was to examine the subcellular redistribution of PKC isozymes in rat liver cirrhosis, which is induced by two different cirrhotic chemical agents, carbon tetrachloride (CCl4) and thioacetamide (TAA).

METHODS AND RESULTS

Thioacetamide and CCl4 were administered to rats for 8 and 30 weeks, respectively before rats were killed and autopsies performed at 9, 20 and 30 weeks later. The TAA induced a fibrotic pattern in the liver that differed from that produced by CCl4, notably in the formation of fibrous connective tissue and the proliferation of bile ductule cells. Cholangiofibrosis and clear-cell foci were also observed in TAA-treated rats at 30 weeks. Histological examination revealed that severe cirrhotic changes were present 9 weeks after the commencement of CCl4 treatment and 30 weeks after TAA treatment.

DISCUSSION

When the subcellular redistribution of PKC isozymes (PKCalpha, -beta1, -delta, and -epsilon) was examined, all the PKC isozymes in CCl4-treated rats were found to be translocated to the membrane fraction, which may mean PKC activation, and then downregulated by proteolytic degradation after 9 weeks of treatment, which coincided with peak cirrhotic changes. All rats treated with CCl4 recovered to the control level after 20 weeks of treatment. In the case of TAA-treated rats, PKC isozymes were translocated to the particulate fraction of the liver after 9 weeks of treatment and this persisted in most of the rats for the duration of the experiment.

CONCLUSIONS

From these results, it would appear that PKC translocation preceded morphologic changes, and that an altered subcellular distribution of the PKC isozyme may be associated with the response to liver damage and carcinogenesis.

Alteration of protein kinase C isoform-specific expression during rat hepatocarcinogenesis after exposure to the peroxisome proliferator WY-14,643

The role of protein kinase C (PKC) isoforms in mediating peroxisome proliferator chemical- (PPC) induced hepatocarcinogenesis was examined. After an acute gavage exposure to WY-14,643 (WY) membrane-bound PKCdelta and cytosolic PKCbeta decreased, whereas the expression of the other isoforms was not altered. After a 13-week chronic exposure, membrane-bound PKCbeta, delta and zeta levels decreased. In WY-induced hepatocellular adenomas, PKCalpha was increased, and PKCbeta was further decreased in membrane fractions. These results, taken together with previous studies, indicate that alterations in PKCalpha, beta and delta isoforms, which regulate mitogenesis, could play important roles in perpetuating the high cell proliferative rate in PPC-induced hepatocellular adenomas.