Mitogen activation of resting lymphocytes exposes cryptic insulin receptors

Mitogen activation of resting lymphocytes induces expression of high affinity insulin receptors on the plasma membrane. The mechanism underlying this effect on insulin receptor expression was examined by comparing levels of insulin receptor mRNA and protein in resting and mitogen-activated rodent lymphocytes. Analysis of RNA levels indicated that resting and concanavalin A-activated lymphocytes contained equivalent amounts of insulin receptor mRNA with predominant transcripts of 7.9 and 9.5 kilobases. Although little or no insulin binding was detectable on intact resting lymphocytes, detergent solubilization of these cells resulted in the appearance of readily detectable insulin binding activity that could be immunoprecipitated with anti-insulin receptor antibodies. Detergent-solubilized resting and mitogen-activated lymphocytes expressed equivalent amounts of insulin receptors that bound insulin with similar affinity (KD = 90 pM) and migrated on reduced SDS-polyacrylamide gels with apparent masses of approximately 130 and approximately 95 kDa. Insulin receptors from resting lymphocytes appeared to be associated with the plasma membrane since 125I labeling of intact lymphocytes radiolabeled the insulin receptor, insulin binding activity was detected in membrane fractions of hypotonically lysed cells, and trypsin treatment of intact cells destroyed > 90% of the insulin binding activity in detergent extracts. These results suggest that resting lymphocytes express insulin receptor mRNA and protein and that mitogen activation exposes cryptic insulin receptors present in the plasma membrane of resting lymphocytes.