INSULIN-LIKE GROWTH-FACTOR-I PROMOTES MULTIDRUG-RESISTANCE IN MCLM COLON-CANCER CELLS

Insulin-like growth factor-I (IGF-I) is known as a potent mitogen for a variety of cell types, including colon cancer cell lines. The object ive of this study was to determine the effect of IGF-I on cell death i nduced by cytotoxic agents actinomycin D (Act-D), lovastatin (LOV), an d doxorubicin (DOX) in the MCLM mouse colon cancer cell line, and the mechanisms involved. Subconfluent monolayer MCLM cells were treated wi th IGF-I (25 ng/ml) for 12 h in serum-free media. Various concentratio ns of cytotoxic agents then were added to the cells that were incubate d continually at 37 degrees C for 24 h. Cell survival was determined w ith the MTT 4-5-dimenthylthiazol-2-yl]-2,5-diphenyltetrazolium bromide ) assay, which assesses mitochondrial function in living cells. The mR NA expression for multidrug resistance gene-I (mdr-I, c-H-ras, and man ganese superoxide dismutase (MnSOD) in cells treated with IGF-I was ex amined by Northern blot or RNase protection assays. The levels of p-gl ycoprotein, a drug efflux pump encoded by the mdr-l gene, were assesse d by Western immunoblotting. Results demonstrated that 1) IGF-I signif icantly inhibited the cell death and apoptosis of MCLM cells treated w ith Act-D, LOV, or DOX; 2) IGF-I increased mRNA expression for mdr-I, c-H-ras, and MnSOD; 3) the p-glycoproteins in cells treated with IGF-I or stably transfected with c-H-ras were elevated when compared with c ontrol. These results suggest that IGF-I protects MCLM cells against d eath induced by cytotoxic agents; this acquired drug resistance may be mediated by multiple mechanisms, including promoting expression of md r-l, c-H-ras, and MnSOD; whereas, the p-glycoprotein level stimulated by IGF-I may result partly from the increase of c-H-ras in the cells. (C) 1998 Wiley-Liss, Inc.