[1, 2] This process relies not only on proliferative cascades, in which hepatocytes switch from a quiescent to a proliferative phenotype,[1, 2] but also on metabolic pathways that help maintain cellular homeostasis after liver injury.[3] Growth factors are particularly important for this process, and insulin specifically regulates both metabolism and proliferation in the liver.[4, 5] However, insulin’s effects on liver regeneration are less well understood than those of other growth factors, such as epidermal growth factor (EGF) and hepatocyte growth factor (HGF).[6, 7] Insulin acts through the insulin receptor (IR), a heterotetrameric receptor tyrosine

kinase (RTK) composed of two extracellular alpha subunits, which have ligand-binding activity, and two transmembrane beta subunits that possess tyrosine kinase activity.[8] Once insulin binds to the IR, protein Y-27632 tyrosine kinase is activated, resulting in phosphorylation of the tyrosine residues within the beta subunit. This, in turn, leads to the recruitment of several adaptor proteins, including src-homology 2 domain (SH2)-containing proteins such as phosphatidylinositol 3-kinase (PI3K) and phospholipase C (PLC).[9] PLC hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP2), resulting

in the formation of diacylglycerol (DAG), which activates protein kinase C (PKC), and inositol-1,4,5-trisphosphate (InsP3), which promotes Ca2+ release from intracellular stores upon binding to the InsP3 receptor (InsP3R).[10] Several RTKs, including the IR and the receptors for EGF, HGF, and fibroblast growth factor (FGF), have been found learn more in the nucleus.[11-15] Evidence suggests that the IR, like

the HGF receptor, c-met,[14] translocates to the nucleus upon ligand medchemexpress stimulation to selectively hydrolyze nuclear PIP2 and locally generate InsP3-dependent Ca2+ signals there.[11] Additionally, nucleoplasmic, rather than cytosolic, Ca2+ is important for cellular proliferation and is necessary in particular for progression through early prophase.[16] However, metabolic effects of insulin result from cytosolic, rather than intranuclear, events, typified by activation of protein kinase B/Akt (PKB).[17] Therefore, we examined whether the cytosolic and nuclear effects of IR are mediated separately and whether the subpopulation of IRs reaching the nucleus upon insulin stimulation locally induces InsP3-dependent Ca2+ signals to regulate the proliferative effects of insulin. The liver cancer cell line, SkHep-1, was obtained from the American Type Culture Collection (Manassas, VA). Cells were cultured at 37°C in 5% CO2 in Dulbecco’s modified Eagle’s medium (Gibco, Grand Island, NY), supplemented with 10% fetal bovine serum (FBS), 1 mM of sodium pyruvate, 50 units/mL of penicillin, and 50 g/mL of streptomycin (Gibco, Grand Island, NY).