Antibody-induced down-regulation of a mutated insulin receptor lacking an intact cytoplasmic domain

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Abstract
Insulin receptor down-regulation was studied in various Chinese hamster ovary (CHO) cell lines expressing transfected human insulin receptor cDNAs. In addition to a cell line expressing the normal receptor (CHO .cntdot. T line), three lines expressing mutated receptors were studied: the CHO .cntdot. T-t line, which expresses a receptor with a degraded cytoplasmic domain due to the removal of the C-terminal 112 amino acids, and the CHO .cntdot. YF1 and CHO .cntdot. YF3 lines, in which important autophosphorylation sites of the receptor kinase (tyrosines-1162 and -1163) have been replaced by phenylalanine. A monoclonal anti-receptor antibody, but not insulin itself, was found to down-regulate cell surface receptor levels in all four cell lines by 60-80% after 18-h treatment at 37.degree.C. Down-regulation of the CHO .cntdot. T-t receptors occurred at similar antibody concentrations and with a similar time course, although the maximum level of CHO .cntdot. T-t down-regulation (60%) was generally lower than the level of CHO .cntdot. T-t down-regulation (80%). Pulse-chase labeling of these two cell types with [35S]methionine revealed that antibody treatment of both CHO .cntdot. T and CHO .cntdot. T-t cells resulted in a similar increase in the rate of degradation of mature receptor subunits. These results indicate that antibody-induced down-regulation of the insulin receptor in these cells can occur in the absence of various autophosphorylation sites of the receptor and that the mechanism of antibody-induced down-regulation is different from that for insulin.

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