Differential IGF-independent effects of insulin-like growth factor bindingproteins (1-6) on apoptosis of breast epithelial cells

We have demonstrated previously that insulin-like growth factor binding pro tein (IGFBP)-3 alone has little growth inhibitory effect on Hs578T human br east cancer cells, but that it can dramatically accentuate the apoptotic re sponse to the physiological trigger, ceramide, in an IGF-independent manner . We have now studied the potential of other IGFBPs (1-6) to interact with apoptotic signalling pathways. Hs578T cells were preincubated with a bindin g protein (100 ng/ml) for 24 h, followed by co-incubation of the binding pr otein with an apoptotic dose of ceramide or RGD-containing peptide for a fu rther 24 h. Apoptosis was assessed using flow cytometry, MTT (3-[4,5-Dimeth ylthiazol-2-yl]-2,5-diphenyltetrazolium bromide; thiazolyl blue) assay and morphological assessment. Binding protein profiles were determined using li gand and immunoblotting techniques. Each of the ICFBPs(1-6)alone had no sig nificant (P > 0.05) growth inhibitory effects relative to control cells. In contrast to IGFBP-3, which significantly (P < 0.05) accentuated C2-induced apoptosis, IGFBP-1, -2, and -6 had no effect, whereas ICFBP-4 and -5 each caused marked (P < 0.01) inhibition of ceramide-induced programmed cell dea th. Apoptosis induced by RGD was also significantly [P < 0.05) reduced by I GFBP-5, whereas IGFBP-3 had no effect. These data provide evidence to sugge st that individual ICFBPs have specific IGF-independent effects and act dif ferentially on apoptotic signalling pathways. (C) 1999 Wiley-Liss, Inc.