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 metabolism. GIP has been shown to stimulate the release of lipoprotein lipase from fat, as well as increase the rate of fat incorporation into adipose tissue. Moreover, GIP has been shown to increase the clearance rate of chylomicrons in the circulation and to inhibit the action of glucagon. Despite evidence for GIP effects on fat tissue, GIP receptors have not been identified in fat cells or tissues. The present study was undertaken to identify GIP receptors in isolated adipocytes, as well as to identify GIP receptors in the established fat cell line, differentiated 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 presence of GIP receptors in both rat fat and differentiated 3T3-L1 cells by Western blot analysis. Moreover, 125I GIP binding assays revealed both specific and displaceable GIP binding sites in differentiated 3T3-L1 cells (IC50 = 10(-9) M). When undifferentiated 3T3-L1 cells, which appear to express relatively few GIP receptors, were incubated in the presence of GIP, no effect on intracellular cAMP accumulation was detected. In contrast, the inclusion of 10 nM GIP in the incubation medium increased cAMP accumulation in rat fat cells and differentiated 3T3-L1 cells. This increase in cAMP accumulation was abolished with the specific GIP receptor antagonist GIP(7-30)NH2. The results of these studies indicate that GIP receptors are present in fat cells and are up-regulated when 3T3-L1 cells undergo differentiation to become adipocytes. Furthermore, the increase in intracellular cAMP accumulation detected upon ligand binding indicates that these receptors are functional.