Insulin‐induced S6 kinase activation in HeLa cells and its reversal by hyperthermic stress

Insulin treatment of HeLa S3 cells activates an S6‐phosphorylating protein kinase. Although this enzyme has chromatographic properties resembling those of described proteolytic fragments of other protein kinases, namely protein kinase C, protease‐activated kinase II and histone‐4 protein kinase, and although insulin has been proposed by others to cause S6 phosphorylation via proteolytic protein kinase activation, the insulin‐induced increase in S6‐kinase activity described here is probably not due to proteolysis. Rather, the activity indicates the existence, in HeLa cells, of an interconvertible S6 kinase, since the insulin‐induced activity increase was rapidly reversed under hyperthermic stress, and since this effect of hyperthermia was itself reversible. The S6‐kinase activities from serum‐ and from insulin‐stimulated HeLa cells resemble each other closely and are likely to represent the same enzyme. The enzyme may therefore mediate both signals delivered by mitogens and the insulin signal. Analysed at an in vitro transfer of 1 mol phosphate/mol S6, this S6 kinase activity does not phosphorylate the (principal) S6 site recognized by the cAMP‐dependent protein kinase.