Differentiation-dependent suppression of platelet-derived growth factor signaling in cultured adipocytes

A critical component of vertebrate cellular differentiation is the acquisit ion of sensitivity to a restricted subset of peptide hormones and growth fa ctors. This accounts for the unique capability of insulin (and possibly ins ulin-like growth factor-1), but not other growth factors, to stimulate gluc ose uptake and anabolic metabolism in heart, skeletal muscle, and adipose t issue. This selectivity is faithfully recapitulated in the cultured adipocy te Line, 3T3-L1, which responds to insulin, but not platelet derived growth factor (PDGF), with increased hexose uptake. The serine/threonine protein kinases Akt1 and Akt2, which have been implicated as mediators of insulin-s timulated glucose uptake, as well as glycogen, lipid, and protein synthesis , were shown to mirror this selectivity in this tissue culture system. This was particularly apparent in 3T3-L1 adipocytes overexpressing an epitope-t agged form of Akt2 in which insulin activated Akt2 10-fold better than PDGF . Similarly, in 3T3-L1 adipocytes, only insulin stimulated phosphorylation of Akt's endogenous substrate, GSK-3 beta. Other signaling molecules, inclu ding phosphatidylinositol 3-kinase, pp70 SG-kinase, mitogen-activated prote in kinase, and PHAS-1/4EBP-1, did not demonstrate this selective responsive ness to insulin but were instead activated comparably by both insulin and P DGF. Moreover, concurrent treatment with PDGF and insulin did not diminish activation of phosphatidylinositol 3-kinase, Akt, or glucose transport, ind icating that PDGF did not simultaneously activate an inhibitory mechanism. Interestingly, PDGF and insulin comparably stimulated both Akt isoforms, as well as numerous other signaling molecules, in undifferentiated 3T3-L1 pre adipocytes. Collectively, these data suggest that differential activation o f Akt in adipocytes may contribute to insulin's exclusive mediation of the metabolic events involved in glucose metabolism. Moreover, they suggest a n ovel mechanism by which differentiation-dependent hormone selectivity is co nferred through the suppression of specific signaling pathways operational in undifferentiated cell types.