| Popis: |
The downstream targets of insulin/IGF-1 that lead to adipocyte differentiation appear to include the PI 3-kinase/p70 S6 kinase pathway. Recently several laboratories have shown that protein kinase B (PKB), an insulin-responsive serine/threonine kinase, is a direct target of PI 3-kinase and can activate p70 S6 kinase. The present study examines the potential role of PKB in insulin/IGF-1-dependent adipocyte differentiation. We have shown that PKB is expressed in 3T3-L1 preadipocytes, and that it is stimulated by insulin. The involvement of PI 3-kinase and p70 S6 kinase in adipocyte differentiation suggests that PKB may link PI 3-kinase to p70 S6 kinase signals. We have tested whether PKB is sufficient for 3T3-L1 preadipocyte differentiation. A version of PKB that is constitutively activated by linkage to the viral gag gene (Gag-PKB) was expressed in 3T3-L1 preadipocytes and shown to induce spontaneous differentiation in the absence of added insulin/IGF-1. The cells assumed a spherical shape and they acquired characteristic lipid droplets that stained positively for the neutral lipid stain, Oil Red O. Northern blot analysis demonstrated upregulation of LPL and aP2 mRNA, specific indicators of adipocyte differentiation. A plasmid vector, in which Gag-PKB was placed under the control of the glucocorticoid-responsive MMTV promoter, was then expressed in 3T3-L1 preadipocytes. When Gag-PKB expression was induced by dexamethasone, spontaneous differentiation followed, despite the absence of insulin/IGF-1. These results suggest that the genetic program of adipocyte differentiation is subject to regulation by PKB. Inducible expression of Gag-PKB will permit the generation of stable cell lines to further explore the role of PKB in adipocyte differentiation. |
