EXPRESSION OF IGF-II AND IGF-BINDING PROTEINS BY COLON-CANCER CELLS IN RELATION TO GROWTH-RESPONSE TO IGFS

We previously reported that even though virtually all human colon cancers were positive for IGF-I receptors, only 50% responded to growth effects of insulin-like growth factor (IGF)-I (1-100 nM). The present studies were undertaken to determine whether expression and secretion of IGFs (IGF-I, IGF-II) and IGF-binding proteins (BPs; 1-6) were perhaps different in IGF-responsive (COLO 205, COLO 320, Caco-2) and IGF-nonresponsive (HCT 116, HT-29, DLD-1) cells. Several bands (2.0-6.0 kb) of IGF-II mRNA transcripts were detected in all the cell lines; none expressed IGF-I. Significant concentrations of IGF-II (0.2-0.9 ng/10(6) cells) were measured in the conditioned media (CM) of the cells. All cell lines expressed BP2 and/or BP4 mRNA and secreted BP4 (24 kDa) and/or BP2 (32.5 kDa); BP1 was not detected in any cell line. Interestingly, BP3 mRNA was measured only in the responsive cell lines. The relative concentration of total BPs tended to be higher in the CM of nonresponsive cells. Interestingly, a large concentration of 44- to 48-kDa BP (BP3?) was associated with the membranes of only the responsive cell lines. Our present studies thus demonstrate that human colon cancers do not secrete IGF-I and BP1. Of all the IGF-related factors examined, the quantity and the type of BPs expressed by the human colon cancer cell lines (especially BP2, BP4, and BP3) may significantly dictate the growth response of the cells to exogenous IGF-I.