A potential mechanism for short time exposure to hypoxia-induced DNA synthesis in primary cultured chicken hepatocytes: Correlation between Ca ²⁺/PKC/ MAPKs and PI3K/Akt/mTOR

Less information is available concerning the molecular mechanisms of cell survival after hypoxia in hepatocytes. Therefore, this study examined the effect of hypoxia on DNA synthesis and its related signal cascades in primary cultured chicken hepatocytes. Hypoxia increased [³H] thymidine incorporation, which was increased significantly after 0-24 h of hypoxic exposure. Indeed, the percentage of cell population in the S phase was increased in hypoxia condition. However, the release of LDH indicating cellular injury was not changed under hypoxic conditions. Hypoxia increased Ca²⁺ uptake and PKC translocation from the cytosol to the membrane fraction. Among the PKC isoforms, hypoxia stimulated the translocation of PKC α and ε. Hypoxia also phosphorylated the p38 and p44/42 mitogen-activated protein kinases (MAPKs), which were blocked by the inhibition of PKC. On the other hand, hypoxia increased Akt and mTOR phosphorylation, which was blocked in the absence of intra/extracellular Ca²⁺. The inhibition of PKC/MAPKs or PI3K/Akt pathway blocked the hypoxia-induced [³H] thymidine incorporation. However, hypoxia-induced Ca²⁺ uptake and PKC translocation was not influenced by LY 294002 or Akt inhibitor and hypoxia-induced MAPKs phosphorylation was not changed by rapamycin. In addition, LY 294002 or Akt inhibitor has no effect on the phosphorylation of MAPKs. It suggests that there is no direct interaction between the two pathways, which cooperatively mediated cell cycle progression to hypoxia in chicken hepatocytes. Hypoxia also increased the level of the cell cycle regulatory proteins [cyclin D₁, cyclin E, cyclin-dependent kinase (CDK) 2, and CDK 4] and p-RB protein but decreased the p21 and p27 expression levels, which were blocked by inhibitors of upstream signal molecules. In conclusion, short time exposure to hypoxia increases DNA synthesis in primary cultured chicken hepatocytes through cooperation of Ca ²⁺/PKC, p38 MAPK, p44/42 MAPKs, and PI3K/Akt pathways.