The altered activity of membrane-bound protein kinase C in human liver cancer

Overexpression of protein kinase C alpha mRNA in human hepatocellular carcinoma: a potential marker of disease prognosis

BACKGROUND

Members of the protein kinase C (PKC) isoenzyme family play a central role in the tumorigenesis of several tissues. However, little is known about subtype specific intracellular expression of PKC in human hepatocellular carcinomas.

METHODS

We investigated PKC isoforms mRNA expression in 42 HCC specimens using reverse transcription polymerase chain reaction analysis, and the correlation between PKC isoforms expression and clinicopathologic parameters.

RESULTS

We found that PKCalpha, PKCdelta and PKCiota mRNA were significantly increased in HCCs as compared to the corresponding non-cancerous liver tissues. PKCalpha expression also significantly correlated with tumor size (P<0.05) and TNM stage (P<0.05), but PKCdelta and PKCiota did not. The log-rank analysis revealed that patients with higher PKCalpha mRNA expression in the HCC tissues had significantly shorter survival rate than patients with lower PKCalpha mRNA expression (P<0.01).

CONCLUSIONS

Our results suggested that the PKCalpha may be a prognostic factor for the survival of patients with HCC.

Opportunities for targeted therapies in hepatocellular carcinoma

Hepatocellular cancer (HCC) is the fifth most common solid tumor worldwide, accounting for 500,000 new cases annually. Although less common in the United States, HCC is expected to increase in incidence over the next two decades largely because of the prevalence of hepatitis C virus infection. A majority of patients present with advanced disease and are not candidates for liver transplantation, surgical resection, or regional therapy. In 60% to 80% of patients with HCC, treatment is complicated by underlying liver cirrhosis and hepatic dysfunction. Systemic treatments are minimally effective, can have significant toxicity, and have not been shown to improve patient survival. New approaches targeting molecular abnormalities specific to HCC are needed to improve patient outcome. This review summarizes the state of knowledge of those key aspects of the molecular pathogenesis of HCC that may represent rational therapeutic targets in this disease. Relevant preclinical and clinical information on novel compounds directed toward abnormalities in HCC is reviewed.

Down-regulation of particulate protein kinase Cepsilon and up-regulation of nuclear activator protein-1 DNA binding in liver following in vivo exposure of B6C3F1 male mice to heptachlor epoxide

The effects of in vivo administration of the cyclodiene tumor promoter heptachlor epoxide on mouse liver protein kinase C were studied in male B6C3F1 mice by protein kinase C activity assays and Western blotting under conditions known to increase the incidence of hepatocellular carcinoma because protein kinase C is thought to be critical in phorbol ester-induced tumor promotion. Under these test conditions, 20 ppm dietary heptachlor epoxide for 1-20 days increased cytosolic and decreased particulate total protein kinase C activities, while 10 ppm had no effect. Further, total cytosolic and particulate protein kinase C activities were decreased within 1 hour by 10 mg/kg intraperitoneal (i.p.) heptachlor epoxide. Western blotting showed that conventional protein kinase Calpha and beta isoforms were unaffected by heptachlor epoxide. Particulate novel protein kinase Cepsilon, however, was selectively down-regulated by 1, 10, and 20 ppm dietary heptachlor epoxide, whereas the cytosolic isoform was decreased by 1 and 10 ppm heptachlor epoxide for 10 days. The high-dose treatment for 24 hours also decreased particulate novel protein kinase Cepsilon but increased the cytosolic titer. These results demonstrate that this isoform is unique in its sensitivity to heptachlor epoxide. Activator protein-1 DNA binding, a critical factor in tumor promotion, was substantially increased at 3 and 6 hours with 3.7 mg/kg (i.p.) heptachlor epoxide and at 3 and 10 days with 20 ppm dietary heptachlor epoxide. The effects of heptachlor epoxide on protein kinase C and activator protein-1 are similar to those caused by phorbol ester treatments and correlate well to heptachlor levels found to induce tumors in mice. However, heptachlor epoxide did not initially activate protein kinase C with in vivo treatments or with in vitro treatments of a plasma membrane fraction aimed at demonstrating direct activation, as has been shown for phorbol esters. The ability of heptachlor epoxide to down-regulate particulate novel protein kinase Cepsilon correlates to dosages used in in vivo tumor promotion studies. However, this may represent a negative feedback response rather than a causative effect.

Alteration in the expression of protein kinase C isoforms in human hepatocellular carcinoma

This study was designed to investigate the alterations of individual protein kinase C (PKC) isoforms in human liver cancer. Surgical specimens of hepatocellular carcinoma and adjacent normal tissues were extracted into cytosolic and membranous fractions. The level of membrane-bound PKCalpha in the cancer tissue was significantly lower than that in the adjacent normal tissue and consistent with the change in PKC activity. In addition, there was a significant negative correlation between PKCalpha and tumor size. In both cytosolic and membrane fractions, levels of PKCdelta and PKCzeta was significantly higher in the cancer tissue than those in the adjacent normal liver tissue. The alterations in the PKC isoforms signify their roles in the hyperproliferation in liver cancer.

Increased constitutive activity of mitogen-activated protein kinase and renaturable 85 kDa kinase in human-colorectal cancer

Protein kinases play a key role in intracellular signalling, participating at multiple levels along the transduction cascades that trigger mitogenic response. Because protein kinases are involved in mitogenic pathways, they are likely to play a role in the abnormal proliferation of malignant cells. In this study we compared activity of mitogen-activated protein (MAP) kinase and several renaturable kinases in homogenates of 30 surgically resected colorectal cancers and their adjacent normal tissues. Using sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) and membrane autophosphorylation assay on homogenates obtained from normal colon mucosa and adenocarcinoma, we identified at least four renaturable kinases (50, 55, 85, 200 kDa). Compared with adjacent tissue, in most of the cancer samples only the 85-kDa kinase exhibited a higher level of autophosphorylation activity than those in normal matched tissue (P < 0.001). Moreover, the 85-kDa kinase from nearly all cancer homogenates showed faster electrophoretic mobility than the 85-kDa kinase from normal tissue homogenates. Interestingly, the 50-kDa kinase had significantly lower autophosphorylation activity in cancer tissues than those of normal tissue (P< 0.05). To assess p42-p44 MAP kinase activity, proteins were immunoprecipitated from adjacent colon mucosa and adenocarcinoma with anti-extracellular signal-related kinase (ERK) 1/2 antibodies, and MAP kinase activity was measured using MBP as a substrate. These studies revealed that MAP kinase activity in colorectal cancer was significantly higher (P < 0.001) than that in adjacent mucosa. Thus, the constitutive activity of MAP kinase and autophosphorylation activity of 85-kDa kinase are increased, whereas the autophosphorylation activity of another kinase, 50 kDa, is decreased in colorectal adenocarcinoma. However, although signal transduction pathways are markedly altered in this cancer, neither p42/p44 MAP kinase activity nor 85-kDa autokinase activity could be correlated with the established prognostic indicators.