31 Our in vitro find more studies suggest that EGF-induced mitogenic signaling and hepatocyte proliferation were significantly attenuated in eNOS−/− hepatocytes. Moreover, EGF treatment alone was sufficient to induce eNOS phosphorylation at Ser1177 in WT hepatocytes, whereas inhibition of EGFR and PI3K-AKT signaling effectively blocked EGF-induced eNOS phosphorylation and hepatocyte proliferation. Our findings suggest that hepatocyte eNOS is a critical mediator of EGF-induced hepatocyte proliferation, potentially via its influence on the early induction

of Egr-1 and phosphorylation of c-Jun—known mediators of cell-cycle progression. EGF-induced eNOS phosphorylation at Ser1177 is dependent on the phosphorylation and activation of EGFR/PI3K/AKT signaling in hepatocytes. Our in vitro data suggest a modest, but statistically significant, attenuation of EGF-mediated induction of ERK signaling in eNOS−/− hepatocytes, in part as a consequence of elevated basal phospho-ERK level in eNOS−/− hepatocytes, as compared to WT. Interestingly, HGF-induced activation of ERK signaling in hepatocytes was comparable to EGF (Supporting Fig. 5). Nevertheless, our in vitro studies with hepatocytes pretreated with U0126 (a MEK1 inhibitor) suggest an important role for MEK-ERK signaling in EGF-induced mitogenic signaling and cell-cycle progression in hepatocytes. Moreover, our studies

suggest that EGF treatement alone was not sufficient to

induce MMP-9 protein expression in hepatocytes see more in vitro (data not shown). It is likely that endothelial cells and other growth factors, such as HGF, may be responsible for the eNOS-dependent early induction of MMP-9 in regenerating livers. These observations highlight a role for endothelial eNOS, in addition to hepatocyte eNOS, in the early activation MCE公司 of ERK and MMP-9 expression observed in regenerating livers. The complex interactions between hepatocytes and endothelial cells in eliciting an effective regenerative repsonse to PH are well recognized. Hepatocytes are the first cells to undergo proliferation in response to PH. Vascular endothelial growth factor (VEGF) derived from hepatocytes, in turn, activate VEGFR1 in endothelial cells to induce HGF expression.32 Therefore, it is not surprising to find that hepatocyte and endothelial eNOS may have distinct, interdependent redundant roles to ensure effective liver regeneration in response to PH. In conclusion, the present study highlights the functional significance of eNOS in liver regeneration after PH. Hepatocyte cell-cycle progression and proliferation in response to PH is severely impaired in eNOS−/− mice. Impairments in early priming events mediated via the activation MEK/ERK/Egr1 and c-Jun-AP-1 signaling, as well as attenuated induction of MMP-9, may contribute to impaired hepatocyte proliferation in eNOS−/− mice.