Activation of integrin and ceramide signalling pathways can inhibit the mitogenic effect of insulin-like growth factor I (IGF-I) in human breast cancer cell lines

Cell counting, cell cycle analysis and Western immunoblotting were used to examine the effects of non-apoptotic doses of a ceramide analogue, C2, and a synthetic arginine-glycine-aspartic acid (RGD)-containing peptide, RGD, i n MCF-7 and T47D cells to determine whether activation of these signalling pathways could alter the mitogenic potential of insulin-like growth factor I (IGF-I). IGF-I alone increased total cell number in both cell lines, asso ciated with a rise in the percentage of cells in the S-phase of the cell cy cle and a co-incident increase in cyclin A production. Treatments alone had no effects on cell number or cyclin A production relative to controls. C2 inhibited IGF-I-induced mitogenesis in both lines, whereas RGD was only eff ective in the T47D line. Despite inhibition of cell proliferation, IGF-I st imulation of cells in S-phase and of cyclin A levels were unaffected; howev er, an IGF-I-induced increase in cyclin B1 levels was inhibited by 30%. Low -dose induction of integrin and ceramide signalling pathways causes cells t o be blocked in S-phase, thereby inhibiting the normal cycle of events asso ciated with the IGF-I-induced mitotic signal. Activating these pathways may not only restrict tumour growth by induction of apoptosis but they may als o directly inhibit IGF-l-induced cell proliferation.