The insulin-like growth factors, IGF-I and IGF-II, are polypeptides that stimulate cell growth, survival or differentiation. The IGFs occur in plasma, other extracellular fluids and tissues complexed to members of a family of proteins, the IGF binding proteins (IGFBPs), which determine their bioavailability and modulate their biological activity. The IGFBPs also may regulate cell growth independently of binding IGFs. The properties and regulation of individual IGFBps differ, providing a flexible and versatile regulatory system. In the past year, we have focused on four questions: (i) The regulation of IGFBP-1 promoter activity in rat hepatoma cells by gluccocorticoids and insulin. IGFBP-1 is rapidly regulated in response to metabolic and hormonal changes. Hormonal regulation is predominantly at the level of transcription. Adjacent glucocorticoid and insulin response elements have been identified in the proximal promoter. Glucocorticoid-stimulated promoter activity requires synergistic interactions between the glucocorticoid receptor and the liver-enriched transcription factors, hepatocyte nuclear factor (HNF)-1alpha and -beta. Studies are in progress to identify the signal transduction pathway by which insulin inhibits IGFBP-1 promoter activity, and the transcription factor(s) binding to the insulin response element (IRE) that are responsible for this inhibition. (ii) Growth hormone (GH) induction of acid-labile subunit (ALS) promoter activity in rat hepatoma cells. The major reservoir of IGFs in plasma is a 150-kilodalton complex that contains IGFBP-3 and ALS. Formation of this complex is dependent on GH, which is required for efficient ALS transcription. GH induction of mouse ALS promoter activity is mediated by the binding of members of the Signal Transducers and Activators of Transcription (STA) family, STAT5a and STAT5b, to a single cis-regulatory element that resembles a gamma-interferon activated sequence (GAS). (iii) Formation of the 150 kDa ALS:IGFBP-3 complex in vivo. We previously showed that ALS and IGFBP-3 could form complexes in vitro in the absence of IGFs, contrary to the prevailing hypothesis that ALS associated only with IGF:IGFBP-3 complexes. To determine whether 150-kDa ALS:IGFBP-3 complexes could form in vivo, recombinant human IGFBP-3 was injected intravenously into rats. Although 150-kDa complexes formed in vivo, their abundance was too low to conclusively resolve whether mobilization of IGF-I was required for their formation. (iv) IGF-independent growth-inhibitory actions of IGFBP-3. IGFBP-3 has been reported to inhibit the growth of human breast cancer cells and to stimulate apoptosis in human prostate cancer cells by a direct mechanism that does not involve binding IGF-I. We have attempted to identify a putative growth-inhibitory IGFBP-3 receptor. Although heparin inhibits IGFBP-s binding to cells, we have shown that the putative IGFBP-3 receptor is not a heparan sulfate proteoglycan. Studies are in progress using the yeast two-hybrid system to identify the receptor, to determine the IGFBP-3 domain that is responsible for growth inhibition, and to elucidate the mechanism by which IGFBP-3 mediates the growth inhibitory effects of transforming growth factor beta (TGF-beta).