Abstract
Insulin‐like growth factor binding protein‐3 (IGFBP‐3) has been proposed to mediate the growth inhibitory effects of transforming growth factor (TGF)‐β in breast and prostate cancer cells. Both TGF‐β and exogenous IGFBP‐3 inhibit DNA synthesis in Mv1 mink lung epithelial cells (CCL64). The present study asks whether IGFBPs synthesized by CCL64 cells mediate growth inhibition by TGF‐β. CCL64 cells synthesize and secrete a single 34‐kDa IGFBP that was identified as IGFBP‐2 by immunoprecipitation and immunodepletion. Recombinant bovine IGFBP‐2 inhibited CCL64 DNA synthesis in serum‐free media in an IGF‐independent manner. Coincubation with Leu^60^‐IGF‐I, an IGF‐I analog that binds to IGFBPs with higher affinity than to IGF‐I receptors, decreased the inhibition by bIGFBP‐2. Leu^60^‐IGF‐I also decreased the inhibition of CCL64 DNA synthesis by TGF‐β by up to 70%, whereas Long‐R3‐IGF‐I, an IGF‐I analog with higher affinity for IGF‐I receptors than for IGFBPs, did not decrease inhibition, suggesting that the effect of Leu^60^‐IGF‐I resulted from its forming complexes with endogenous IGFBPs. Leu^60^‐IGF‐I did not decrease TGF‐β stimulation of a Smad3‐dependent reporter gene. Following incubation of intact CCL64 cells with bIGFBP‐2 at 0°C, bIGFBP‐2 was recovered in membrane fractions; membrane association was abolished by coincubation with Leu^60^‐IGF‐I. If exogenous and secreted IGFBP‐2 must bind to CCL64 cells to inhibit DNA synthesis, Leu^60^‐IGF‐I might reduce the inhibition of DNA synthesis by bIGFBP‐2 or TGF‐β by inhibiting the association of IGFBP‐2 in the media with CCL64 cells. Since TGF‐β does not increase IGFBP‐2 abundance, we propose that TGF‐β sensitizes CCL64 cells to the latent growth inhibitory activity of endogenous IGFBP‐2 by potentiating an intracellular IGFBP‐2 signaling pathway or by promoting the association of secreted IGFBP‐2 with the plasma membrane. J. Cell. Physiol. 190: 63–73, 2002. Published 2002 Wiley‐Liss, Inc.