Effect of dexamethasone on the alternative splicing of the insulin receptor mRNA and insulin action in HepG2 hepatoma cells.

We have shown that culturing HepG2 cells in Ham's F-12 medium supplemented with calf serum, dexamethasone, and triiodothyronine causes an increase in the insulin sensitivity and responsiveness for glucose incorporation into glycogen.This correlates with increased expression of the mRNA encoding the B isoform of the insulin receptor.Of all the components in the medium, we found that dexamethasone exerted the greatest effect.Dexamethasone alone could cause both a switch in expression from the A to the B isoform of the insulin receptor and also an increase in insulin sensitivity for both glucose incorporation into glycogen and 2-deoxyglucose transport.In addition, we found that expression of the B isoform of the insulin receptor is developmentally regulated in the 3T3-Ll adipocyte leading to the suggestion that the alternatively spliced B isoform of the insulin receptor may play an important role in signal transduction in insulin target tissues such as liver, fat, and muscle.The insulin receptor (IR)' protein is a heterotetrameric protein composed of two a-subunits that confer the ability to bind insulin and two @-subunits that contain the membrane spanning and the tyrosine kinase domains (for reviews, see Czech, 1985;Kahn, 1985; Olefsky, 1990).The a-subunit is entirely extracellular and is linked by disulfide bonds to the extracellular portion of the @-subunit.Following binding of insulin to the a-subunits, the first observable event is autophosphorylation on the cytoplasmic portion of the @-subunit leading to an increase in the receptor's intrinsic tyrosine kinase activity toward other substrates (Czech, 1985;Kasuga et al., 1982;White and Kahn, 1989).The human IR is encoded by a single gene that is located on chromosome 19 and composed of 22 exons (Seino et al., 1989).The mature IR, however, exists as two isoforms, designated A and B, which result from alternative splicing of the primary transcript (Ebina et al., 1985; Seino e t al., 1989;Ullrich et al.,