FUNCTIONAL GIP RECEPTORS ARE PRESENT ON ADIPOCYTES

In addition to its important role in maintaining glucose homeostasis, it has recently become apparent that glucose-dependent insulinotropic polypeptide (GIP) is also involved in different steps of lipid metabol ism. GIP has been shown to stimulate the release of lipoprotein lipase from fat, as well as increase the rate of fat incorporation into adip ose tissue. Moreover, GIP has been shown to increase the clearance rat e of chylomicrons in the circulation and to inhibit the action of gluc agon. Despite evidence for GIP effects on fat tissue, GIP receptors ha ve not been identified in fat cells or tissues. The present study was undertaken to identify GIP receptors in isolated adipocytes, as well a s to identify GD? receptors in the established fat cell line, differen tiated 3T3-L1. RNAse protection analysis demonstrated the presence of GIP receptor transcripts in rat adipocytes. A polyclonal GIP receptor antiserum directed at the N-terminus of the receptor detected the pres ence of GIP receptors in both rat fat and differentiated 3T3-L1 cells by Western blot analysis. Moreover, [I-125] GIP binding assays reveale d both specific and displaceable GIP binding sites in differentiated 3 T3-L1 cells (IC50 = 10(-9) M). When undifferentiated 3T3-L1 cells, whi ch appear to express relatively few GIP receptors, were incubated in t he presence of GIP, no effect on intracellular cAMP accumulation was d etected. In contrast, the inclusion of 10 nM GIP in the incubation med ium increased cAMP accumulation in rat fat cells and differentiated 3T 3-L1 cells. This increase in cAMP accumulation was abolished with the specific GIP receptor antagonist GIP(7-30)NH2. The results of these st udies indicate that GTP receptors are present in fat cells and are up- regulated when 3T3-L1 cells undergo differentiation to become adipocyt es. Furthermore, the increase in intracellular cAMP accumulation detec ted upon ligand binding indicates that these receptors are functional.