Overexpression of an inhibitory insulin-like growth factor binding protein (IGFBP), IGFBP-4, delays onset of prostate tumor formation

Insulin-like growth factor (IGF) binding proteins (IGFBPs) have been shown to either inhibit or enhance the action of IGF, or act in an IGF-independent manner in the prostate. We have overexpressed the IGF-inhibitory IGFBP-4 in the malignant M12 prostate epithelial cell line to determine the effects on tumor formation and apoptosis. Overexpression was determined by Not-them, Western immunoblot and Western radioligand blot analysis. IGF induced proliferation was reduced in the IGFBP-4 transfected cells compared with control cells (P less than or equal to 0.01). Colony formation in soft agar was significantly inhibited up to 14 days after plating in the IGFBP-4 transfected cells when compared with the M12 controls (P less than or equal to 0.01): however, in the presence of des(1-3)IGF-I, there was no significant difference between the control and IGFBP-4 transfectants in colony formation in soft, agar. Apoptosis in an IGFBP-4 transfected cell Line was significantly increased in response to induction by B-hydroxyurea compared with the control Line. When injected sc into male athymic/nude mice, a marked delay was noted in tumor formation in animals receiving IGFBP-4 transfected cells (P less than or equal to 0.01). Interestingly, IGFBP-2 protein levels were reduced in the conditioned media of all IGFBP-4 transfected cell cultures. These data indicate that an inhibitory IGFBP may significantly delay the growth of malignant prostate epithelial cells and enhance the sensitivity of these cells to apoptosis.