ABSTRACT
Cell proliferation is a highly regulated process, and its induction is dependent on extracellular signals which are mediated by specific growth factors. The interaction of the growth factor with its receptor leads to the initiation of several early responses on the membrane as well as in the cytosol. The phosphorylation of S6 is thought to cause conformational changes in the 40S and 60S ribosomal subunits that may lead to alterations in their functions. The data above demonstrate that S6 phosphorylation is differentially regulated by insulin and epidermal growth factor (EGF). One simple explanation would be that insulin does not activate the S6 kinase to the same extent as does EGF. The activity of the type 1 phosphatase remained close to basal levels after 2 h in EGF-treated cells, but there was a 1.5-fold stimulation of the type 2A enzyme.
