Differential association of phosphatidylinositol 3-kinase with insulin receptor substrate (IRS)-1 and IRS-2 in human thymocytes in response to IL-7.

IL-7 is central to T cell development, inducing proliferation and differentiation, but the signal transduction mechanisms by which it accomplishes these functions are poorly understood. We demonstrate that, in addition to activation of the Jak1 and Jak3 kinases, IL-7 stimulation of human thymocytes results in the rapid tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1) and IRS-2 proteins. We show that the Jak1 and Jak3 kinases are associated with both IRS-1 and IRS-2 in thymocytes. However, the pool of Jak3 kinase associated with IRS-1 appears to be preferentially activated upon IL-7 stimulation. We further demonstrate that the 160- to 185-kDa IRS proteins associate with the p85 regulatory subunit of phosphatidylinositol 3 (PI3)-kinase in an IL-7-dependent manner, although significant differences were evident in the levels of PI3-kinase activity associating with each IRS protein. Although IRS-1 displays a higher degree of basal association, IL-7 induces a much greater increase in the activity associated with IRS-2. Lastly, we show that the PI3-kinase activity associated with the IRS proteins appears to be distinct from that bound to the IL-7R, suggesting the existence of separately regulated pools of PI3-kinase activity. This study suggests that IL-7R signaling diverges at the level of the IRS-1 and IRS-2 proteins, possibly with the two branches regulated by Jak3 and Jak1 activities, respectively.