CHARACTERIZATION OF INSULIN-LIKE GROWTH-FACTOR BINDING-PROTEINS AND REGULATION OF IGFBP3 IN HUMAN MESANGIAL CELLS

IGF-I regulates renal growth and development. Insulin-like growth fact or binding proteins (IGFBPs) are synthesized by the kidney and may mod ulate the local autocrine and/or paracrine actions of IGF-I. We have p reviously demonstrated that mesangial cells (MC) release IGF-I and IGF -binding activity; however, the specific IGFBPs produced by these cell s and the factors involved in their regulation are unknown. We examine d MC for expression of IGFBP-1 to -6 mRNAs and proteins. RNase protect ion assays using total RNA demonstrated that MC express all of the IGF BPs. [I-125]IGF-I Western ligand blot of conditioned medium demonstrat ed that MC release IGFBPs of 24, 29, 32 kDa, and a doublet at 46 kDa, consistent with IGFBP-4 -5, -2 and -3, respectively. IGFBP species of 28 and 34 kDa were also detected. Since IGF-I and TGF-beta are implica ted in glomerular hypertrophy and matrix expansion, we tested their ef fect on IGFBPs released by MC. IGF-I (100 ng/ml), TGF-beta (2 ng/ml) a nd forskolin (10(-5) M) differentially regulated the abundance of IGFB Ps released in the conditioned medium in a time-dependent manner. IGF- I and TGF-beta were potent inducers of the release of IGFBP3 protein; however, TGF-beta, but not IGF-I, increased IGFBP3 mRNA levels. Recomb inant IGFBP3 was tested for its effect on IGF-I-induced mitogenesis. I GFBP3 inhibited IGF-I-stimulated DNA synthesis in a dose-dependent man ner with a peak effect observed at 50 nM IGFBP3. Although TGF-beta is a potent inhibitor of IGF-I-stimulated DNA synthesis, this effect is n ot mediated via IGFBPs. Expression of IGFBP-1 to -6 by MC suggests tha t these proteins may modulate IGF-I bioavailability in the glomerulus. IGF-I itself, TGF-beta and cAMP agonists may indirectly modulate the effects of IGF-I via the release of IGFBPs by MC.