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The IGF-binding protein (IGFBP)-5 protein contains consensus heparin binding motifs in both its carboxy (C)-terminal and central domains, although only the C-terminal site has previously been shown to be functional. We have made two chimeric IGFBP proteins by switching domains between rat IGFBP-5 and -2, named BP552 and BP522 to reflect the domains present, and a truncated rat IGFBP-5 mutant (1-168), named BP550. The ability of these proteins and wild-type (wt) IGFBPs-5 and -2 to bind to either IGFs or heparin was determined using biosensor real-time analysis and heparin ligand blotting respectively. We report that the chimeric molecules have IGF binding affinities comparable to those of the native IGFBPs from which they were derived and, as expected, the binding of BP550 to IGFs was greatly compromised. More surprising was the finding that the ability of BP552 and BP550 to bind to heparin was equivalent to that of wtIGFBP-5, whereas wtIGFBP-2 and BP522 failed to bind. These results demonstrate that the active heparin binding site in BP552 and BP550 is contained within the central domain of IGFBP-5, and that this site is active only in the absence of the C-terminal domain. We subsequently mutated two basic amino acids (R136A:R137A) in the central consensus binding sites between residues 132-140. This resulted in the loss of heparin binding for BP550, confirming the importance of these two basic amino acids in the central domain heparin binding activity. In light of these findings, we suggest that C-terminally truncated fragments of IGFBP-5 generated in vivo by proteolysis could retain heparin/extracellular matrix binding properties.

We have used quantitative RT-PCR to analyse the mRNA expression profile of the major components of the IGF axis in different stages of murine mammary gland development, including late pregnancy, lactation and involution. We have shown that all the genes studied, IGF-I, IGF-II, IGF receptor (IGFR) and IGF-binding protein (IGFBP)-1 to -6, were expressed in every stage, albeit at greatly differing levels and displaying unique expression profiles between developmental stages. IGF-I was always expressed at significantly higher levels than either IGF-II or IGFR. This suggests that IGF-I may be the more important IGF during mammary morphogenesis. Overall, IGFBP-3 demonstrated the highest level of expression of any of the IGFBP genes throughout all the developmental stages studied. However, within developmental stages, by far the highest level of expression of any of the IGFBPs was that of IGFBP-5 at day 2 of involution; this was almost an order of magnitude higher than any of the other IGFBP levels recorded. This corroborated our previous findings that the levels of IGFBP-5 protein are highly elevated in the involuting mammary gland, and demonstrated that this up-regulation of IGFBP-5 operates at the level of transcriptional control or message stability. Comparison of the expression profile for these different genes would strongly suggest that they are likely to have differential functions throughout mammary gland development, and also highlights potential interactions and co-regulation between different members of this axis. In addition, our results have identified some similarities and differences in the expression of IGFBPs between the mouse mammary epithelial cell line, HC11, and the normal mammary gland which are worthy of study, most notably the differential regulation of IGFBP-2 and the site of expression of IGFBP-4 and -6. Overall, this study has demonstrated the importance and complexity of the IGF axis during mammary gland development and provides a valuable resource for future research in this area.