Transient activation of Jun N-terminal kinases and protection from apoptosis by the insulin-like growth factor I receptor can be suppressed by dicumarol.

The insulin-like growth factor I receptor (IGF-IR) activated by its ligands insulin-like growth factor (IGF)-I or IGF-II mediates suppression of apoptosis and contributes to tumorigenesis and cell growth. Here we investigated the activation of the stress-activated protein kinases including Jun N-terminal Kinases and p38 MAPK by IGF-I in interleukin-3-dependent FL5.12 lymphocytic cells that overexpress the IGF-IR (FL5.12/WT). We have shown previously that IGF-I protects these cells from apoptosis induced by interleukin-3 withdrawal but does not promote proliferation. IGF-I induced a rapid and transient activation of JNK that peaked at 40 min that was paralleled by a transient and robust phosphorylation of c-Jun. p38 was constitutively phosphorylated in FL5.12/WT cells. Activation of the JNK pathway by IGF-I occurred in the presence of phosphatidylinositol 3-kinase inhibitors and could be enhanced by anisomycin. Analysis of a series of FL5.12 cells expressing mutated IGF-IRs and analysis of 32D/IGF-IR cells showed that neither the C terminus of the receptor nor IRS-1 and IRS-2 were required for JNK activation, although tyrosine 950 was essential for full activation. The JNK inhibitor dicumarol suppressed IGF-I-mediated activation of JNK and phosphorylation of c-Jun but did not affect p38 and IkappaB phosphorylation or activation of AKT. IGF-I-mediated protection from apoptosis in FL5.12/WT cells was completely suppressed by dicumarol and partially suppressed by a p38 inhibitor. In the breast carcinoma cell line MCF-7, treatment with dicumarol also induced apoptosis. These data indicate that transient activation of JNK by IGF-I is mediated by signals that are distinct from those leading to phosphatidylinositol 3-kinase and AKT activation. The data further suggest that the SAPK pathways contribute to suppression of apoptosis by the IGF-IR.