Receptor kinase activity is necessary to mediate production of NO through the insulin receptor. Both PI3K and Akt contribute importantly to this process, whereas the contribution of Ras is small.

Stimulation of glucose transport is among the most important metabolic actions of insulin. Studies in adipose cells have demonstrated that insulin stimulates its receptor to phosphorylate tyrosine residues in IRS-1, leading to activation of phosphatidylinositol 3-kinase, which plays a necessary role in mediating the translocation of the insulin-responsive glucose transporter GLUT4 to the cell surface. Akt is a serine-threonine kinase recently identified as a direct downstream target of phosphatidylinositol 3-kinase. A previous study in 3T3-L1 cells showed that overexpression of a constitutively active mutant of Akt is sufficient to recruit GLUT4 to the cell surface. Since effects of overexpression of signaling molecules in tissue culture models do not always reflect physiological function, we have overexpressed a dominant inhibitory mutant of Akt in rat adipose cells to investigate the effects of inhibiting endogenous Akt in a physiologically relevant insulin target cell. Cells were transfected with either wild type (Akt-WT), constitutively active (Akt-myr), or dominant inhibitory (Akt-K179A) forms of Akt, and effects of overexpression of these constructs on insulin-stimulated translocation of a cotransfected epitope-tagged GLUT4 were studied. Overexpression of Akt-WT resulted in significant translocation of GLUT4 to the cell surface even in the absence of insulin. Interestingly, overexpression of Akt-myr resulted in an even larger effect that was independent of insulin. More importantly, overexpression of Akt-K179A (kinase-inactive mutant) significantly inhibited insulin-stimulated translocation of GLUT4. Taken together, our data suggest that Akt is not only capable of stimulating the translocation of GLUT4 but that endogenous Akt is likely to play a significant physiological role in insulin-stimulated glucose uptake in insulin targets such as muscle and adipose tissue.

Caveolae may function as microdomains for signaling that help to determine specific biological actions mediated by the insulin receptor (IR). Caveolin-1, a major component of caveolae, contains a scaffolding domain (SD) that binds to a caveolin-1 binding motif in the kinase domain of the IR in vitro. To investigate the potential role of caveolin-1 in insulin signaling we overexpressed wild-type (Cav-WT) or mutant (Cav-Mut; F92A/V94A in SD) caveolin-1 in either Cos-7 cells cotransfected with IR or rat adipose cells (low and high levels of endogenous caveolin-1, respectively). Cav-WT coimmunoprecipitated with the IR to a much greater extent than Cav-Mut, suggesting that the SD is important for interactions between caveolin-1 and the IR in intact cells. We also constructed several IR mutants with a disrupted caveolin-1 binding motif and found that these mutants were poorly expressed and did not undergo autophosphorylation. Interestingly, overexpression of Cav-WT in Cos-7 cells significantly enhanced insulin-stimulated phosphorylation of Elk-1 (a mitogen-activated protein kinase-dependent pathway) while overexpression of Cav-Mut was without effect. In contrast, in adipose cells, overexpression of either Cav-WT or Cav-Mut did not affect insulin-stimulated phosphorylation of a cotransfected ERK2 (but did significantly inhibit basal phosphorylation of ERK2). Furthermore, we also observed a small inhibition of insulin-stimulated translocation of GLUT4 when either Cav-WT or Cav-Mut was overexpressed in adipose cells. Thus, interaction of caveolin-1 with IRs may differentially modulate insulin signaling to enhance insulin action in Cos-7 cells but inhibit insulin's effects in adipose cells.

Wild-type (F/B), constitutively active (F/B*), and three kinase-inactive (F/Ba−, F/Bb−, F/Bc−) forms of Akt/protein kinase B (PKB) were permanently overexpressed in FDCP2 cells. In the absence of insulin-like growth factor-1 (IGF-1), activities of PKB, cyclic nucleotide phosphodiesterase 3B (PDE3B), and PDE4 were similar in nontransfected FDCP2 cells, mock-transfected (F/V) cells, and F/B and F/B− cells. In F/V cells, IGF-1 increased PKB, PDE3B, and PDE4 activities ∼2-fold. In F/B cells, IGF-1, in a wortmannin-sensitive manner, increased PKB activity ∼10-fold and PDE3B phosphorylation and activity (∼4-fold), but increased PDE4 to the same extent as in F/V cells. In F/B* cells, in the absence of IGF-1, PKB activity was markedly increased (∼10-fold) and PDE3B was phosphorylated and activated (3- to 4-fold); wortmannin inhibited these effects. In F/B* cells, IGF-1 had little further effect on PKB and activation/phosphorylation of PDE3B. In F/B− cells, IGF-1 activated PDE4, not PDE3B, suggesting that kinase-inactive PKB behaved as a dominant negative with respect to PDE3B activation. Thymidine incorporation was greater in F/B* cells than in F/V cells and was inhibited to a greater extent by PDE3 inhibitors than by rolipram, a PDE4 inhibitor. In F/B cells, IGF-1-induced phosphorylation of the apoptotic protein BAD was inhibited by the PDE3 inhibitor cilostamide. Activated PKB phosphorylated and activated rPDE3B in vitro. These results suggest that PDE3B, not PDE4, is a target of PKB and that activated PDE3B may regulate cAMP pools that modulate effects of PKB on thymidine incorporation and BAD phosphorylation in FDCP2 cells.

We have analyzed an anthracycline biosynthesis gene cluster from Streptomyces nogalater. Based on sequence analysis, a contiguous region of 11 kb is deduced to include genes for the early steps in anthracycline biosynthesis, a regulatory gene (snoA) promoting the expression of the biosynthetic genes, and at least one gene whose product might have a role in modification of the glycoside moiety. The three ORFs encoding a minimal polyketide synthase (PKS) are separated from the regulatory gene (snoA) by a comparatively AT-rich region (GC content 60%). Subfragments of the DNA region were transferred to Streptomyces galilaeus mutants blocked in aclacinomycin biosynthesis, and to a regulatory mutant of S. nogalater. The S. galilaeus mutants carrying the S. nogalater minimal PKS genes produced auramycinone glycosides, demonstrating replacement of the starter unit for polyketide biosynthesis. The product of snoA seems to be needed for expression of at least the genes for the minimal PKS.

We have analyzed an anthracycline biosynthesis gene cluster from Streptomyces nogalater. Based on sequence analysis, a contiguous region of 11 kb is deduced to include genes for the early steps in anthracycline biosynthesis, a regulatory gene (snoA) promoting the expression of the biosynthetic genes, and at least one gene whose product might have a role in modification of the glycoside moiety. The three ORFs encoding a minimal polyketide synthase (PKS) are separated from the regulatory gene (snoA) by a comparatively AT-rich region (GC content 60%). Subfragments of the DNA region were transferred to Streptomyces galilaeus mutants blocked in aclacinomycin biosynthesis, and to a regulatory mutant of S. nogalater. The S. galilaeus mutants carrying the S. nogalater minimal PKS genes produced auramycinone glycosides, demonstrating replacement of the starter unit for polyketide biosynthesis. The product of snoA seems to be needed for expression of at least the genes for the minimal PKS.

Insulin receptor substrates-1 and -2 (IRS-1 and -2) are important substrates of the insulin receptor tyrosine kinase. Previous studies have focused upon the role of IRS-1 in mediating the actions of insulin. In the present study, we demonstrate that IRS-2 can mediate translocation of the insulin responsive glucose transporter GLUT4 in a physiologically relevant target cell for insulin action. Co-immunoprecipitation experiments performed on cell lysates derived from freshly isolated rat adipose cells incubated in the presence or absence of insulin indicated that twice as much phosphatidylinositol 3-kinase was associated with endogenous IRS-1 as with IRS-2 after insulin stimulation. When rat adipose cells in primary culture were transfected with expression vectors for IRS-1 or IRS-2, we observed 40-fold overexpression of human IRS-1 or murine IRS-2. In addition, anti-phosphotyrosine immunoblotting experiments confirmed that the recombinant substrates were phosphorylated in response to insulin stimulation. To examine the role of IRS-2 in insulin-stimulated translocation of GLUT4, we studied the effects of overexpression of IRS-1 and -2 on translocation of a co-transfected epitopetagged GLUT4 (GLUT4-HA). Overexpression of IRS-1 or IRS-2 in adipose cells resulted in a significant increase in the basal level of cell surface GLUT4 (in the absence of insulin). Interestingly, at maximally effective concentrations of insulin (60 nM), the level of cell surface GLUT4 in cells overexpressing IRS-1 or -2 significantly exceeded the maximal recruitment observed in the control cells (160 and 135% of control, respectively; p < 0.003). Our data directly demonstrate that IRS-2, like IRS-1, is capable of participating in insulin signal transduction pathways leading to the recruitment of GLUT4. Thus, IRS-2 may provide an alternative pathway for critical metabolic actions of insulin.

The insulin receptor initiates insulin action by phosphorylating multiple intracellular substrates. Previously, we have demonstrated that insulin receptor substrates (IRS)-1 and -2 can mediate insulin's action to promote translocation of GLUT4 glucose transporters to the cell surface in rat adipose cells. Although IRS-1, -2, and -4 are similar in overall structure, IRS-3 is approximately 50% shorter and differs with respect to sites of tyrosine phosphorylation. Nevertheless, as demonstrated in this study, both IRS-3 and IRS-4 can also stimulate translocation of GLUT4. Rat adipose cells were cotransfected with expression vectors for hemagglutinin (HA) epitope-tagged GLUT4 (GLUT4-HA) and human IRS-1, murine IRS-3, or human IRS-4. Overexpression of IRS-1 led to a 2-fold increase in cell surface GLUT4-HA in cells incubated in the absence of insulin; overexpression of either IRS-3 or IRS-4 elicited a larger increase in cell surface GLUT4-HA. Indeed, the effect of IRS-3 in the absence of insulin was approximately 40% greater than the effect of a maximally stimulating concentration of insulin in cells not overexpressing IRS proteins. Because phosphatidylinositol (PI) 3-kinase is essential for insulin-stimulated translocation of GLUT4, we also studied a mutant IRS-3 molecule (IRS-3-F4) in which Phe was substituted for Tyr in all four YXXM motifs (the phosphorylation sites predicted to bind to and activate PI 3-kinase). Interestingly, overexpression of IRS-3-F4 did not promote translocation of GLUT4-HA, but actually inhibited the ability of insulin to stimulate translocation of GLUT4-HA to the cell surface. Our data suggest that IRS-3 and IRS-4 are capable of mediating PI 3-kinase-dependent metabolic actions of insulin in adipose cells, and that IRS proteins play a physiological role in mediating translocation of GLUT4.