Characterization of the Promoter Region and 3′ End of the Human Insulin Receptor Gene

The insulin receptor is an essential protein present on the surface of virtually all cells. Little is known about the control of the level of this protein on cellular surfaces, but it has been found that the level of insulin receptor protein correlates roughly with the level of insulin receptor (IR) gene transcripts within cells. Although the protein-encoding region is only about 4000 base pairs (bps), there are multiple species of IR mRNA ranging in size from 5400 to 9400 bps. We have found that the variation in size of these transcripts is due to multiple 3′ ends, presumably reflecting alternative polyadenylation, so that the final IR exon ranges in size from 1400 to 5400 bps. The IR gene promoter is like other housekeeping promoters in that it has no TATA or CAAT boxes, is extremely GC-rich, and has multiple transcriptional initiation sites primarily within a 300-bp GC-rich region. Reporter gene analysis using IR promoter-chloramphenicol acetyltransferase (HIRcat) fusion plasmids established regions responsible for promoter activity and verified the localization of the major IR gene transcriptional initiation sites. However, transfection with HIRcat plasmids containing regions from −153 to −1818 resulted in increased utilization of the most 5′ IR gene mRNA initiation sites in transfected relative to untransfected cells. Reporter gene analysis also established that a region of the IR promoter and first exon containing all of the transcriptional initiation sites is more active in HepG2 than CV1 cells. Because the steady-state level of expression of the IR gene is much higher in HepG2 than CV1 cells, the results of the reporter gene analysis may reflect tissue-specific differences in IR gene transcription. Such tissue-specific transcriptional regulation would be a novel finding in a housekeeping promoter.