Lack of activation of the IRS-signaling system in THP-1 myelomonocytic cells. (A) Molt-4 or THP-1 cells were incubated for 5 minutes at 37°C in the presence or absence of IFN-α as indicated. Cell lysates were immunoprecipitated with the indicated antibodies and immunoblotted with antiphosphotyrosine. (B) The blot shown in (A) was stripped and reblotted with the αIRS-1^CT antibody. (C) KG1A or THP-1 cells were serum starved for 2 hours, and were subsequently incubated for 10 minutes at 37°C in the presence or absence of IFN-α or insulin as indicated. Cell lysates were immunoprecipitated with either an antibody against IRS-2 or preimmune rabbit serum as indicated, analyzed by SDS-PAGE and immunoblotted with antiphosphotyrosine.