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

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.

Mechanisms involved in responses to the poroxisome proliferator WY-14,643 on gap junctional intercellular communication in V79 hamster fibroblasts

WY-14,643, a potent hepatic peroxisome proliferator, decreased gap junctional intercellular communication when used at low and intermediate concentrations (1 nM to 10 microM) in the V79 Chinese hamster fibroblast cell line. It did not decrease intercellular communication in early passage Syrian hamster embryo fibroblasts. The mechanism of WY-14,643-induced suppression of intercellular communication was studied by preexposure of V79 cells to inhibitors of protein kinase C, mitogen-activated protein kinases, phosphatidylinositol 3-kinase, or mammalian target-of-rapamycin before addition of WY-14,643. Only protein kinase C, particularly the delta isoform, appeared involved in the inhibition of communication by WY-14,643. Also clofibrate-induced suppression of GJIC in V79 cells appeared to involve protein kinase Cdelta. Furthermore, WY-14,643 did not cause any activation of the mitogen-activated protein kinases p44/42, p38, or Jun N-terminal kinase. When WY-14,643 was used at a higher concentration (100 microM), intercellular communication was increased both in V79 and Syrian hamster embryo cells. This effect was inhibited by preexposure of V79 cells to brefeldin A. Thus, there may be a pool of connexins in the Golgi complex that could be recruited to the plasma membrane upon exposure to high concentrations of WY-14,643.

Apoptosis, mitosis and cyclophilin-40 expression in regressing peroxisome proliferator-induced adenomas

Chronic exposure to peroxisome proliferators (PP), including certain industrial and pharmaceutical chemicals, causes liver cancer in rodents. Continuous exposure to PP is needed for tumor development since the frequency of hepatocellular neoplasms is decreased in animals returned to control diet. To determine cellular and molecular events responsible for enhanced growth in PP-induced liver tumors, we evaluated the relationships of WY-14,643 levels, apoptosis, mitosis and cyclophilin-40 (Cyp-40) expression in regressing tumors induced by WY-14,643, a potent PP. Male F344 rats were fed WY-14,643 (0.1%) in the diet for 43 weeks and then switched to control diet for 2, 3, 5 or 36 days. Mean serum and hepatic concentrations of WY-14,643 were decreased as early as 2 days following removal of WY-14,643 as compared with rats continuously fed WY-14,643. Adenomas from rats maintained on WY-14,643 markedly compressed surrounding parenchyma. Evidence of adenoma regression was observed by 3 days of WY-14,643 withdrawal and was characterized by loss of compression. Decreased compression corresponded to increases in the apoptotic index and decreases in the mitotic index in regressing adenomas at 2, 3, and 5 days following the switch to control diet. Cyclophilins are multifunctional receptor proteins involved in numerous signal transduction pathways, including those mediated by cyclosporin, a liver tumor promoter in rats. Cyp-40 expression was markedly increased in adenomas from continuously exposed rats, but expression returned to levels similar to surrounding parenchyma in adenomas after 5 days of WY-14,643 withdrawal. Taken together, these results indicate that WY-14, 643-induced adenomas regress rapidly following withdrawal of the PP in association with declining liver WY-14,643 levels, suggesting that peroxisome proliferator-activated receptor alpha may mediate PP-induced alterations in mitogenic and/or apoptotic regulation in growing tumors, in conjunction with alterations in Cyp-40 signal transduction.

Novel role of oxidants in the molecular mechanism of action of peroxisome proliferators

Peroxisome proliferators are nongenotoxic rodent carcinogens that act as tumor promoters by increasing cell proliferation; however, their precise mechanism of action is not well understood. Oxidative DNA damage caused by leakage of hydrogen peroxide (H2O2) from peroxisomes was hypothesized initially as the mechanism by which these compounds cause liver tumors. It seems unlikely that oxidants of peroxisomal origin explain the mechanism of action of peroxisome proliferators because treatment with these compounds in vivo does not lead to increased H2O2 production. On the other hand, Kupffer cell-derived oxidants, such as superoxide, may play a role in initiating tumor nerosis factor-alpha (TNF-alpha) production that leads to hepatocyte proliferation. Peroxisome proliferators have been shown to activate Kupffer cells both in vitro and in vivo, and the use of Kupffer cell inhibitors such as methyl palmitate and dietary glycine have demonstrated that Kupffer cells are responsible for hepatocyte proliferation by mechanisms involve TNF-alpha. Moreover, peroxisome proliferators activate the transcription factor NF-kappaB, one of the major regulators of TNF-alpha expression, in Kupffer cells. Importantly, activation of NF-kappaB by peroxisome proliferators was shown to be oxidant-dependent, leading to the hypothesis that oxidants of Kupffer cell origin are involved in the mechanism of action. Many of the effects of peroxisome proliferators, including peroxisome induction and hepatomegaly, involve the peroxisome proliferator-activated receptor-alpha (PPARalpha). Recently, it was shown that peroxisome proliferator-induced cell proliferation and tumors require the PPARalpha. However, PPARalpha is not involved in TNF-alpha production by Kupffer cells because it is not expressed in this cell type. How it is involved in liver tumor remains unclear and one possible explanation is that both Kupffer cell TNF-alpha and parenchymal cell PPARalpha are required. Collectively, recent data are consistent with the hypothesis that oxidants play a role in signaling hepatocellular proliferation due to peroxisome proliferators via activation of NF-kappaB and incrase in mitogenic cytokines such as TNF-alpha.