Multiple hemopoietins, with the exception of interleukin-4, induce modification of Shc and mSos1, but not their translocation.

Shc, grb2, and Son-of-sevenless (mSos1) proteins are potential upstream regulators of p21ras. We show that p52Shc and p46Shc comprise a significant portion of two of the major protein substrates phosphorylated on tyrosine in response to interleukin-2 (IL-2), IL-3, granulocyte-macrophage colony stimulating factor (GM-CSF), Steel factor (SLF), and colony-stimulating factor-1 (CSF-1). Once tyrosine phosphorylated, p52Shc and p46Shc associated with grb2. However, in contrast to published results with epidermal growth factor, treatment with GM-CSF, IL-3, and SLF failed to induce significant biochemically detectable translocation of Shc, grb2, or mSos1 from the cytosol to the plasma membrane. In addition, we did not observe significant epidermal growth factor-induced translocation of Sos1 to the membrane in Rat-1 cells. Treatment with SLF or IL-3 did increase tyrosine phosphorylation of membrane-localized p52Shc, which could then associate with grb2, although the majority of tyrosine-phosphorylated Shc was located in the cytosol. SLF, IL-3, and phorbol ester induced a decrease in the electrophoretic mobility of mSos1. This occurred with slower kinetics than p21ras activation and unlike hemopoietin-induced activation of p21ras was partially inhibited by a specific protein kinase C inhibitor. Thus, growth factor-induced modification of mSos1 may represent a downstream event, subsequent to p21ras activation. Significantly, IL-4, a cytokine that fails to activate p21ras, also failed to induce significant tyrosine phosphorylation of Shc or a shift in mSos1 mobility for the first time correlating these events with the ability of a growth factor to activate p21ras. Together, these data suggest that the current model for regulation of p21ras, which proposes a stable association of Shc-grb2-Sos1 complexes at the plasma membrane, may be an oversimplification.