Differential effect of pp120 on insulin endocytosis by two variant insulin receptor isoforms

The insulin receptor is expressed as two variably spliced isoforms that differ by the absence (isoform A) or presence (isoform B) of a 12-amino acid sequence encoded by exon 11 at the carboxy terminus of the alpha-subunit. Coexpression of the A isoform and pp120, a substrate of the insulin receptor tyrosine kinase, in NIH 3T3 fibroblasts increased receptor A-mediated insulin endocytosis and degradation by two-to threefold compared with cells expressing receptors alone. Because B is the predominant isoform in the liver and binds insulin with lower affinity than A, we have examined the effect of pp120 on receptor B-mediated endocytosis. In contrast to isoform A, the effect of pp120 on isoform B-mediated insulin internalization and degradation in stably transfected NIH 3T3 cells was minimal.