Although buffering nuclear Ca2+ inhibits cell proliferation,[16] inhibiting InsP3-mediated Ca2+ CHIR-99021 mw signals in the nucleus is more specific than solely buffering nuclear Ca2+. Of note, because nuclear InsP3 was buffered in our PH studies, this likely attenuated the proliferative effects not only of IR, but also of c-met and EGF receptor (EGFR)

as well. Therefore, these findings serve to provide the first in vivo evidence for the general importance of RTK-induced nuclear InsP3 formation in liver regeneration. Upon activation, the IR is internalized by endosomes. The acidic environment (pH, ∼6) within endosomes promotes ligand-receptor dissociation, as well as inactivation of the IR. Insulin is degraded within the endosome, and the IR is either sent to lysosomes for degradation or recycled to the plasma membrane for another round of binding, activation, and internalization.[33] These trafficking steps are crucial for insulin signal transduction, because phosphorylated internalized receptors can bind to intracellular substrates to achieve insulin’s biological actions.[33] After insulin binding, the IR undergoes endocytosis through the classic clathrin-dependent

pathway, as do other RTKs.[28] It has been observed that a subpopulation of IR on the PM is associated with caveolin-enriched membrane domains,[29, 34] although the functional significance of this selleck kinase inhibitor has not been clear. Our results suggest that these endocytic pathways act together to mediate the translocation of the IR to the nucleus. This is consistent with the observation that clathrin is involved in EGFR nuclear translocation in SkHep-1 cells.[12] Movement of plasma membrane receptors to the nucleus has also been described for other RTKs, for some G-protein-coupled receptors, and in various cell types.[13, 15,

35, 36] The steps that follow endocytosis as the IR moves to the nucleus have not yet been elucidated. However, nuclear translocation of FGF is mediated by importin β,[15] and movement of c-met to the nucleus depends on importin β and the chaperone, GRB2-associated binding protein 上海皓元 1 (Gab1),[14] which can bind to other RTKs as well.[37] However, whether these proteins also participate in IR translocation to the nucleus remains to be determined. The metabolic actions of insulin in the liver are largely mediated by the PI3K-PKB pathway. Akt is activated at the plasma membrane upon IR-mediated phosphorylation of PI3K.[17] We found that Akt activation is not inhibited by blocking IR movement to the nucleus, suggesting that insulin’s effects on cell proliferation are mediated by nuclear IR and are independent of IR in the cytosol. Interestingly, insulin’s metabolic effects are enhanced in the liver-specific Gab1 knockout mouse.