The influence of insulin on the downregulation of its receptor was studied in AR42J cultured pancreatic acinar cells, a cell line that has been demonstrated to be metabolically responsive to insulin. Downregulation induced by insulin was time and dose dependent. After a 20-h incubation with 1 microM insulin, Scatchard analysis revealed approximately 80% loss of insulin receptors. Studies of receptor half-life indicated that treatment with insulin accelerated the degradation of both the alpha- and beta-subunits of the insulin receptor by 30-60%. In addition, biosynthetic-labeling studies indicated that insulin inhibited the biosynthesis of the insulin-receptor precursor by greater than 30%. This decreased biosynthesis of the precursor was associated with decreased production of mature receptor subunits. Poly(A)+ RNA was extracted from control cells and cells treated for 24 h with 100 nM insulin. Slot blots and Northern transfers revealed that insulin induced an approximately 50% decrease in insulin-receptor mRNA levels. Therefore, these studies indicate that insulin may diminish the concentration of its receptors in target cells by at least two mechanisms: acceleration of receptor degradation and inhibition of receptor biosynthesis at the level of mRNA.

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The promoter region of the human insulin-receptor (HINSR) gene was isolated from a human chromosome 19 bacteriophage library. With S1 nuclease mapping and primer-extension analysis, we identified multiple transcription-initiation sites. Dexamethasone, a known inducer of HINSR transcription, enhanced transcription of all major transcription-initiation sites. DNA sequence analysis indicated that the HINSR promoter has neither a TATA box nor a CAAT box. The HINSR promoter region contains six GGGCGG sequences that may be binding sites for the transcription factor Sp1. In addition, there were three TCCC sequences that were putative promoter regulatory regions. The HINSR gene promoter has structural similarity to the epidermal growth factor receptor gene promoter and has some features of the promoter of the meglutol (hydroxymethylglutaryl, HMG) CoA reductase gene and the early promoter of simian virus 40.