Lithium activates the Wnt and phosphatidylinositol 3-kinase Akt signaling pathways to promote cell survival in the absence of soluble survival factors

Mouse proximal tubular cells (BUMPT), when cultured in the absence of growth factors, activate a default apoptotic pathway. Although Wnt signaling antagonizes the effect of proapoptotic triggers, its role in regulating the default pathway of apoptosis is less well defined. The present study examines the hypothesis that lithium ([Li.sup.+]) and (2'Z,3'E)-6-bromoindirubin-3'-oxime (BIO), two glycogen synthase kinase-3[beta] (GSK3[beta]) inhibitors, promote survival of growth factor-deprived renal epithelial cells by activating the Wnt pathway. These studies demonstrate that [Li.sup.+] and BIO activate Wnt signaling as indicated by the following changes: phosphorylation (inhibition) of GSK3[beta]; decreased phosphorylation of [beta]-catenin (a GSK3[beta] substrate); nuclear translocation of [beta]-catenin; specific transcriptional activation of Tcf/catenin-responsive pTopflash constructs; and an increase in the expression of cyclin D1 (indicative of a promitogenic cell response). In addition, [Li.sup.+] or BIO significantly increases the phosphorylation (activation) of Akt, an anti-apoptotic protein, and inhibits apoptosis (decreases both annexin-V staining and caspase-3 activation), during serum deprivation. Inhibition of phosphatidylinositol 3-kinase (responsible for Akt activation) either by wortmanin or LY-294002 prevented [Li.sup.+]-or BIO-induced Akt phosphorylation and reduces cell survival without altering the phosphorylation state of GSK3[beta]. [Li.sup.+] or BIO also increases the expression of insulin-like growth factor-II (IGF-II), a potent proliferative signaling protein. [Li.sup.+] or BIO-free conditioned medium harvested from [Li.sup.+] or BIO-exposed cells also induced Akt phosphorylafion, mimicking the protective effect of the two GSK3[beta] inhibitors on serum-starved cells. Furthermore, the effect of conditioned medium on Akt phosphorylation could be inhibited by either LY-294002 or IGF-binding protein. BIO, a specific GSK3[beta] inhibitor, replicated the protective effect of [Li.sup.+] on cell viability, suggesting that GSK3[beta] activation is important for initiating the apoptotic pathway. Taken together, these data suggest that [Li.sup.+] or BIO promotes renal epithelial cell survival by inhibiting apoptosis through GSK3[beta]-dependent activation of the Wnt pathway and subsequent release of IGF-II. Extracellular IGF-II serves as an autocrine survival factor that is responsible, in part, for activating the anti-apoptotic phosphatidylinositol-3-kinase-Akt pathway during serum deprivation. serum deprivation; renal epithelial cells; insulin-like growth factors; [beta]-catenin; glycogen synthase kinase-3[beta]; apoptosis