Differential insulin-like growth factor (IGF)-independent interactions of IGF binding protein-3 and IGF binding protein-5 on apoptosis in human breast cancer cells. Involvement of the mitochondria.

Abstract

We have demonstrated previously in Hs578T cells that insulin-like growth factor binding protein (IGFBP)-3 can significantly accentuate ceramide (C2)-induced apoptosis, but has no effect on cell death induced by integrin detachment [using an arginine-glycine-aspartic acid (RGD)-containing peptide]. In contrast we found that IGFBP-5 could inhibit apoptosis induced by either C2 or integrin detachment. It is now clear that the mitochondria not only provide the energy required for cell viability, but can also play an important role during the commitment phase to apoptosis. We used a mitochondrial respiratory chain inhibitor, antimycin A, at both apoptotic and nonapoptotic doses to further investigate the IGF-independent actions of IGFBP-3 and IGFBP-5 on C2 and RGD-induced apoptosis in the Hs578T cells. Hs578T cells had one of three treatments. 1: They were incubated with increasing doses of antimycin A for 24 h. 2: They were coincubated with an apoptotic dose of either C2 or RGD together with a nonapoptotic dose of antimycin A for 24 h. 3: They were incubated with a binding protein (100 ng/ml) for 24 h followed by coincubation of the binding protein with an apoptotic dose of antimycin A for a further 24 h. Cell viability was assessed by trypan blue dye exclusion and MTT assay, and apoptosis was confirmed and measured by morphologic assessment and flow cytometry. We found that antimycin A initiated apoptosis at 10 micromol/L and above. We also demonstrated that a nonapoptotic dose of antimycin A (0.1 micromol/L) significantly inhibited C2-induced apoptosis, whereas it significantly accentuated RGD-induced cell death. In addition, we found that cell death induced by antimycin A can be accentuated by IGFBP-3 but is not affected by IGFBP-5. These data indicate that IGFBP-3 can directly enhance apoptosis triggered via the mitochondria; either directly by a mitochondrial inhibitor or by C2 (which we demonstrate to act via effects on the mitochondria in this model). IGFBP-5, however, appears to confer survival effects via a distinct pathway not involving the mitochondria.

Insulin-like growth factor binding protein 5 and apoptosis in mammary epithelial cells

“…Recent studies have indicated that IGFBPs may act independently of IGFs to induce apoptosis in other cell systems (Nickerson et al, 1997;Perks et al, 2000); however, when MECs were incubated with 10 nM IGFBP-5 or -3 alone apoptosis was not significantly increased compared to serumstarved cells. A modest survival effect was seen although this was not statistically significant.…”

IGF-binding protein-5: flexible player in the IGF system and effector on its own

“…Similarly, IGFBP-5 improves the survival of breast cancer cells. This function appears to be IGF-independent and to rely on an apoptosis-inhibiting effect triggered via a pathway which does not involve the mitochondria (Perks et al 1999(Perks et al , 2000. However, breast carcinoma cells apparently are also susceptible to growth inhibitory signals of IGFBP-5 at certain stages if triggered by appropriate signals such as vitamin D (Rozen & Pollak 1999), anti-oestrogens (Parisot et al 1999) or androgen deprivation (Nickerson et al 1999).…”

Transcriptional Effects of Chronic Akt Activation in the Heart

“…IGFBP-5 may have direct and/or indirect anti-apoptotic activity (31)(32)(33)(34). Therefore, IGFBP-5 up-regulation, in an Akt-dependent manner, may be of particular importance to the cardioprotective effects of Akt.…”