Role Of Caveolin-1 In Hepatocyte Proliferation And Liver Regeneration

Caveolae are defined as pits of 50–100-nm diameter in the plasma membrane and have a characteristic fask shape. As distinct domains of the plasma membrane, caveolae participate in many cellular processes such as cholesterol trafficking, endocytosis, membrane sorting, signaling and oncogenesis. Caveolin-1(Cav-1) is an essential component of caveolae, and highly expressed in differentiated cells including adipocytes, fibroblasts, endothelial and smooth muscle cells. Caveolin-1 and 2 co-localize and form a stable hetero-oligomeric complex.Liver is the center of human metabolism, in which phosphatidylcholine (PC) is enriched. Except for the well-known CDP-choline pathway (Kennedy pathway), there is another pathway for PC synthesis in mammalian liver, named phosphatidylethanolamine N-methyltransferase (PEMT) pathway, in which phosphatidylethanolamine is methylated into PC. It has been reported that PEMT was downregulated in patients with liver diseases. Our previous work shows when PEMT2 was overexpressed in the rat liver carcinoma cell line CBRH-7919, the cell growth was inhibited and the cells were induced apoptosis, indicating that lipid metabolism is involved in the regulation of hepatocyte proliferation.Caveolin-1 has been previously implicated in the regulation of cellular proliferation and lipogenesis. For one thing, in carcinomas including breast cancer, Cav-1 is down-regulated; Cav-1 interacts with upstream signaling proteins of ERK1/2, and inhibits ERK1/2 activation through the caveolin scaffolding domain (CSD) on its N-terminal region, both in vitro and in vivo. Therefore Cav-1 inhibits cell proliferation and tumor metastasis, and is believed as a tumor suppressed gene. But Cav-1 is up-regulated in carcinoma of bladder, esophageal, thyroid, and prostate, indicating its tumor promoting role.For the other thing, triglycerides provide most of the energy and free fatty acids used for new membrane synthesis during liver regeneration. Current data now suggest that Cav-1 plays an essential role in the regulation of liver regeneration and that it is implicated in the regulation of triglyceride accumulation, which is an essential process that plays a critical role in the regulation of liver regeneration. It appears that Cav-1 can both inhibit cell proliferation and support liver regeneration, but the exact role Cav-1 plays in hepatocyte proliferation and transformation remains unclear.To investigate the role of caveolin-1 in proliferation of human hepatocytes, the expression vectors named psiRNA-CAV3, psiRNA-CAV6, psiRNA-CAV7 were constructed and transfected (psiRNA-CAV3 and psiRNA-CAV6 for transient use) into the human liver cell line CCL-13 (Chang liver, CHL) in vitro. The Cav-1 down-regulated cell clones transfected psiRNA-CAV7 were selected by the antibiotic Zeocin. The proliferation of the cell strain CAV7 was examined by MTT, in which untransfected Chang liver and HepG2 cells were set as control. Expression of Caveolin-1, Akt, Erk1/2, p-Akt and p-Erk1/2 etc. in the transfected and control cells was detected by Western blot. And the results include:(1) After Cav-1 expression down-regulated by RNAi, the Chang liver cells increased faster at first (24h and 72h, P<0.05; 96h, P<0.01), but slower later.(2) P-Akt and p-Erk1/2 expression was down-regulated in CAV7 cells, which indicates Akt and Erk1/2 pathways which were related with growth and proliferation were inhibited after Cav-1 down-regulation.(3) Erk1/2 was activated 120 hours after transient transfection and when Cav-1 was down-regulated in CAV3 and CAV6 cells,which suggests Cav-1 functions as a natural endogenous inhibitor of the p42/44 MAP kinase cascade.(4) SAPK/JNK were inhibited while c-myc was activated in all transfected cells after Cav-1 down-regulation.(5) Cav-1 expression reached its peak 24 hours after partial hepatectomy, and was still higher than sham-operated (SH) rats 96 hours after PH, which indicates Cav-1 may play a role in liver regeneration. In addition, Erk1/2 expression also increased and reached its peak 96h after PH.Linked with other researches, caveolin-1 may play an important role both in hepatocyte proliferation and liver regeneration by regulating signaling through cell membrane and triglyceride metabolism in hepatocytes.