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H Song, J Beattie, IW Campbell, and GJ Allan

Using site-directed mutagenesis, we have undertaken a study of a potential IGF-binding site in the C-terminal domain of rat IGFBP-5, lying close to or within a previously described heparin-binding domain (residues 201-218) in this protein. After analysis of binding activity using three different methods - ligand blotting, solution phase equilibrium binding and biosensor measurement of real-time on- and off-rates - we report that the mutation of two highly conserved residues within this region (glycine 203 and glutamine 209) reduces the affinity of the binding protein for both IGF-I and IGF-II, while having no effect on heparin binding. In addition, we confirm that mutation of basic residues within the heparin-binding domain (R201L, K202E, K206Q and R214A) results in a protein that has attenuated heparin binding but shows only a small reduction in affinity for IGF-I and -II. Previous findings have described the reduction in affinity of IGFBP-5 for IGFs that occurs after complexation of the binding protein with heparin or other components of the extracellular matrix (ECM) and have postulated that such an interaction may result in conformational changes in protein structure, affecting subsequent IGF interaction. Our data suggesting potential overlap of heparin- and IGF-binding domains argue for a more direct effect of ECM modulation of the affinity of IGFBP-5 for ligand by partial occlusion of the IGF-binding site after interaction with ECM.

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Z G Song, X H Zhang, L X Zhu, H C Jiao, and H Lin

Glucocorticoids (GCs) are involved in the muscle wasting caused by trauma, inactivity, and stress. In the present study, three experiments were conducted to investigate the effect of GCs on the expression of genes related to muscle development in chickens. Broilers at 7 or 35 days of age were subjected to dexamethasone (DEX) treatment (2 mg/kg body mass (BM)) for 3 or 7 days. The expression levels of genes such as IGF1, IGF1 receptor, MSTN, WW domain containing E3 ubiquitin (UB) protein ligase 1, myogenic determining factor, and myogenic factor 5 were measured. The results showed that BM gain was significantly suppressed by DEX treatment. The plasma level of insulin was increased (P<0.05) by DEX treatment at feeding, whereas IGF1 was decreased (P<0.05). The expression of genes in the IGF1, myostatin, and UB–proteasome (UBP) pathways were altered by DEX treatment in age- and exposure time-related ways. These results suggest that GCs suppress IGF1 and upregulate myostatin and/or activated myostatin and the UBP pathway, which might be the source of the effect of GCs on muscle development.

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H Song, JH Shand, J Beattie, DJ Flint, and GJ Allan

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.

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S Bramani, H Song, J Beattie, E Tonner, DJ Flint, and GJ Allan

The highly conserved N-and C-terminal domains of IGFBPs are believed to participate in IGF binding, but only recently have some of the critical residues in the IGFBP sequence involved in ligand binding been identified. Here we describe two highly conserved amino acids in the C-terminal domain of rat IGFBP-5 that are involved in binding IGF-I. Site-directed mutagenesis was used to produce two mutants, G203K and Q209A, of rIGFBP-5. Relative to wild-type rIGFBP-5, an 8-fold reduction in affinity for human IGF-I was found for recombinant G203K protein in both IGF-I ligand blots and solution phase ligand binding assays, and a 7-and 6-fold reduction for Q209A respectively. This shows that Gly203 and Gln209 in IGFBP-5 are important determinants in binding IGF-I, and due to their complete conservation in all IGFBP sequences, we suggest that they are likely to be involved in binding IGF-I in all six binding proteins. In addition, these two non-basic residues lie within the ECM binding region (201-218) of IGFBP-5, demonstrating that the C-terminus contains partially overlapping IGF-I and ECM binding sites. We therefore propose that heparin binding to basic amino acids in IGFBP-5 between 201-218 may physically occlude subsequent interaction between IGF-I and Gly203/Gln209, and that this may explain previous work of others showing reduced affinity of ECM bound IGFBP-5 for IGF-I.

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Y-L Zhao, W-D Han, Q Li, Y-M Mu, X-C Lu, L Yu, H-J Song, X Li, J-M Lu, and C-Y Pan

LRP16 gene expression is induced by 17-βestradiol (E2) via estrogen receptor alpha (ERα) in MCF-7 human breast cancer cells. A previous study also demonstrated that ectopic expression of LRP16 gene promoted MCF-7 cell proliferation. To explore the mechanism of hormone-induced LRP16 gene expression, the LRP16 gene promoter region (−2600 to −24 bp upstream of the LRP16 gene translation starting site) was analyzed in the present study by using different 5′-truncated constructs, and a luciferase reporter. The 5′-flanking sequence of −676 to −24 bp (pGL3-S5) was found to be E2-responsive. After exchange of the fragment from −213 to −24 bp with the TK gene proximal promoter region in pGL3-S5, E2 still induced reporter gene activity in MCF-7 and HeLa cells. Sequence analysis showed that the pGL3-S6 (−676 to −214) sequence contains two motifs that may contribute to E2-induced transactivation; namely, an estrogen-responsive element (ERE) half-site/Sp1 at −246 to −227 bp and an E-box site at −225 to −219 bp. Further deletion and mutation analysis of these two motifs indicated that both the 1/2 ERE and Sp1 binding sites were required for E2 action, while E-box deletion did not affect the luciferase activity in MCF-7 and HeLa cells. The results of gel mobility shift and chromatin immunoprecipitation assays confirmed that both ERαand Sp1 were required for hormone-induced transactivation, which involved both ERαand Sp1 directly binding to DNA. Taken together, these findings suggest that ERαand Sp1 play a role in activation of the human LRP16 gene promoter.