IL-4 PROTECTS CELLS FROM APOPTOSIS VIA THE INSULIN-RECEPTOR SUBSTRATEPATHWAY AND A 2ND INDEPENDENT SIGNALING PATHWAY

Although it has been known for some time that IL-4 protects cells from death via apoptosis, very little is known about the mechanism by whic h IL-4 exerts this effect, In this report, we have studied the molecul ar mechanisms of the IL-4-induced prevention of apoptosis in the 32D a nd B cell systems. IL-3 withdrawal has been shown to induce G1-arrest in 32D cells and subsequent death by apoptosis, We found that overexpr ession of IRS-1 protected 32D cells from death induced by IL-3 depriva tion, IL-4 was able to protect: 32D cells from apoptosis in the presen ce or absence of IRS-1. However, the protection from apoptosis of cell s cultured in IL-4 was greater in the presence of IRS-1 and it reached levels comparable to those of cells cultured in IL-3. The IRS-1-depen dent prevention of apoptosis is linked to the activation of PI 3'-kina se since wortmannin and LY294002, two inhibitors of PI3'-K, partially inhibited the prevention of apoptasis mediated by IL-4 in 32D-IRS-1 ce lls after IL-3 withdrawal but not in 32D cells lacking IRS-1 expressio n, In addition, we found that the IRS/PI 3'-K pathway is, at least in part, responsible for the prevention of apoptosis by IL-4 in normal sp lenic B cell cultures. In spite of the ability of murine IL-4 to parti ally protect 32D cells lacking IRS-I from apoptosis, human IL-4 was no t able to prevent cell death in 32D-lRS-1 cells transfected with the h uman IL-4 receptor mutated in the insulin-IL-4 receptor motif (14R-mot if) at amino acid 497 (Y497F), This mutation has previously been shown to abrogate the tyrosine phosphorylation of IRS-1 by human IL-4. Thes e results demonstrate that IL-4 protects 32D cells from apoptosis by t wo different pathways, one of them mediated by IRS-I, In addition, the se results suggest that the 14R-motif of the IL-4R is linked to both p athways.