Effect of insulin-like growth factor binding proteins on the response of proximal tubular cells to insulin-like growth factor-I.

The insulin-like growth factor binding proteins (IGFBP) are major modulators of insulin-like growth factor-I (IGF-I) action, but relatively little is known about their production by kidney tubular cells or about their modulating effects on the action of IGF-I on these cells. In this study we demonstrated that rabbit proximal tubular cells express the genes for IGFBP-2, -4 and -5 and secrete 24 and 32 kDa size binding proteins. The rate of IGFBP production by these cells was regulated by several growth factors including hydrocortisone, which was potently stimulatory, and EGF, which was inhibitory. The overall effect of these kidney cell-secreted IGFBPs was to inhibit the mitogenic activity of IGF-I. Similarly, recombinant IGFBP-3, the major circulating IGFBP that in kidney is produced close to the proximal tubules, also inhibited IGF-I stimulated DNA synthesis in cultured rabbit proximal tubular cells and in cultured opossum kidney (OK) cells. IGFBP-3 also inhibited basal DNA synthesis in OK cells in the absence of added IGF-I, suggesting that this IGFBP may have an IGF-I independent action. These findings highlight the important effect that IGFBPs have on the action of IGF-I on kidney cells and support the notion that the changes in IGFBPs observed in various renal diseases may contribute to the pathophysiology of these diseases.