Jane B. Clarke

CCAAT box binding protein NF-Y facilitates in vivo recruitment of upstream DNA binding transcription factors.

NF‐Y binds a CCAAT motif found in many eukaryotic polymerase II‐dependent promoters. In the HLA‐DRA promoter it has been demonstrated that stereo‐specific alignment between this motif and the upstream elements X1 and X2 is required for activation. To study the underlying mechanism for this requirement, a panel of transfected cell lines that maintained integrated, wild‐type and mutant promoters were analyzed by in vivo genomic footprinting. Cell lines harboring a mutated CCAAT element exhibited a loss of interactions at the CCAAT site, as expected, and no transcriptional activity. Most importantly, mutation of the CCAAT sequence nearly abolished in vivo binding at the X1 and X2 sites, while mutations of X1 and X2 had little effect on CCAAT box binding. However, X1 and X2 binding was interdependent. In vitro, X1 binding activities are known to be stabilized by NF‐Y binding. Interaction between NF‐Y and X box binding proteins was demonstrated by reciprocal co‐immunoprecipitation in the absence of DNA and co‐affinity purification in the presence of DNA. Collectively, these studies indicate that occupancy of the CCAAT element represents an early event affecting other protein‐DNA interactions and suggest that NF‐Y stabilizes and interacts with X box factors to mediate this function. These findings may represent a common theme among promoters containing a CCAAT element.

Protease-resistant form of insulin-like growth factor-binding protein 5 is an inhibitor of insulin-like growth factor-I actions on porcine smooth muscle cells in culture.

IGFs are pleiotrophic mitogens for porcine smooth muscle cells (pSMC) in culture. The effects of IGFs on cells are modulated by various insulin-like growth factor-binding proteins (IGFBP). IGFBP-5 is synthesized by pSMC and binds to the extracellular matrix. However, IGFBP-5 is also secreted into conditioned medium of cultured cells and is cleaved into fragments by a concomitantly produced protease. These fragments have reduced affinity for the IGFs and cleavage makes it difficult to assess the role of intact IGFBP-5. To study the consequence of accumulation of intact IGFBP-5 in medium, we determined the cleavage site in IGFBP-5 and prepared a protease resistant mutant. Amino acid sequencing of purified IGFBP-5 fragments suggested Arg138-Arg139 as the primary cleavage site. Arg138-Arg139-->Asn138-Asn139 mutations were introduced to create protease-resistant IGFBP-5, which has the same affinity for IGF-I as the native protein. This mutant IGFBP-5 remained intact even after 24 h of incubation and it inhibited several IGF-I actions when added to pSMC culture medium. The mutant IGFBP-5 (500 ng/ml) decreased IGF-I stimulated cellular DNA synthesis by 84%, protein synthesis by 77%, and it inhibited IGF-I stimulated migration of pSMC by 77%. It also inhibited IGF-I stimulation of IRS-1 phosphorylation. In contrast, the same amount of native IGFBP-5 did not inhibit IGF-I actions. The significance of inhibitory effects of the protease resistant IGFBP-5 was further demonstrated in pSMC transfected with mutant or native IGFBP-5 cDNAs. The mutant IGFBP-5 accumulated in culture medium of transfected cells, while native IGFBP-5 was degraded into fragments, PSMC overexpressing the mutant IGFBP-5 also responded poorly to IGF-I compared with mock transfected cells. IGF-I (5 ng/ml) increased [35S]methionine incorporation into control cells by 36% above the basal level, but it did not significantly change (4%) in pSMC cultures that were producing the mutant IGFBP-5. In conclusion, the accumulation of protease-resistant IGFBP-5 in the medium was inhibitory to IGF-I actions on pSMC. This suggests that proteolysis can prevent IGFBP-5 from acting as an inhibitor of IGF-I-stimulated effects and that it serves as an important mechanism for regulating cellular responsiveness to IGF-I.

Identification of the Extracellular Matrix Binding Sites for Insulin-like Growth Factor-binding Protein 5

Fibroblast extracellular matrix (ECM) contains two forms of insulin-like growth factor-binding proteins (IGFBPs), IGFBP-3 and IGFBP-5. These studies were undertaken to identify the regions within IGFBP-5 that mediate its binding to fibroblast ECM. Synthetic peptides were prepared that were homologous with two regions of basic amino acids within IGFBP-5 (Arg201-Arg218 and Ala131-Thr141). Increasing concentrations of both peptides competed with IGFBP-5 for binding to ECM but the Arg201-Arg218 peptide was more potent. Mutagenesis was used to define the effect of substituting for these basic residues on ECM binding. Substitution for two peptide B residues K134A and R136A reduced binding by 40%. Substitution of a single basic residue within the peptide A region (K211N) reduced binding to ECM by 49%. Substitution for K211N, K134A, and R136A reduced binding by 52%. More extensive substitutions in the peptide A region, e.g. K211N,R214A,K217A,R218N, resulted in a greater (e.g. 88%) decrease. The positional location of basic residues appeared to be more important than the total number of substitutions since the mutant K202N,K206A,R207A had a 79% reduction in ECM binding. Two basic regions of IGFBP-5 contribute to its binding to ECM, but the region containing amino acids 201-218 has a greater contribution. ECM binding is mediated by charged residues and acts to stabilize IGFBP-5 by protecting it from proteolysis.

Role of insulin-like growth factor binding proteins in the control of IGF actions.

The insulin-like growth factor binding proteins have been shown to modify IGF actions. IGFBP-5 binds to extracellular matrix (ECM) and its ability to potentiate IGF activity is dependent upon the amount that is ECM associated. To determine the specific regions of IGFBP-5 that are required for ECM association, site directed mutagenesis has been used to prepare several forms of IGFBP-5. Mutants that have had the amino acids between positions 201 and 218 altered have been useful. Mutation of the lysine 211 resulted in no change in the affinity of IGFBP-5 for ECM or heparin Sepharose; however, it resulted in a major reduction in affinity for IGF-I following heparin binding. Other mutations which disrupted heparin binding also resulted in loss of this affinity shift. Most distruptive were mutations of amino acids 211, 214, 217 and 218 and 202, 206 and 207. Mutation of residues 201 plus 202 had some effect, but substitution for 207, 211, 217 and 218 had no effect. When binding to intact ECM was analyzed, similar results were obtained. This suggests that amino acids 202, 206 and 214 are definitely involved in heparin and ECM binding. When binding to proteoglycans such as tenascin and heparin sulfate proteoglycan was analyzed, similar results were obtained. IGFBP-5 also binds to other proteins in ECM, including type IV collagen and plasminogen activator inhibitor-I. Specific antisera for plasminogen activator inhibitor-1 can coprecipitate IGFBP-5. IGFBPs are degraded by specific proteases. Three proteases that degrade IGFBP-2, -4 and -5 have been characterized. They are serine proteases that cleave these proteins at basic residues. Although several well characterized serine proteases cleave IGFBP-4 or -5, the proteases in cell conditioned media appear to be distinct.

Tissue Transglutaminase Facilitates the Polymerization of Insulin-like Growth Factor-binding Protein-1 (IGFBP-1) and Leads to Loss of IGFBP-1's Ability to Inhibit Insulin-like Growth Factor-I-stimulated Protein Synthesis

Insulin-like growth factor-binding protein-1 (IGFBP-1) binds to insulin-like growth factors (IGFs) and has been shown to inhibit or stimulate cellular responses to IGF-I in vitro. This capacity of IGFBP-1 to inhibit or stimulate IGF-I actions correlates with its ability to form stable high molecular weight multimers. Since the ability of some proteins to polymerize is dependent upon transglutamination, we determined if tissue transglutaminase could catalyze this reaction and the effect of polymerization of IGFBP-1 upon IGF-I action. Following incubation with pure tissue transglutaminase (Tg), IGFBP-1 formed covalently linked multimers that were stable during SDS-polyacrylamide gel electrophoresis using reducing conditions. Dephosphorylated IGFBP-1 polymerized more rapidly and to a greater extent compared with native (phosphorylated) IGFBP-1. Exposure to IGF-I stimulated transglutamination of IGFBP-1 in vitro. An IGFBP-1 mutant in which Gln66-Gln67 had been altered to Ala66-Ala67 (Q66A/Q67A) was relatively resistant to polymerization by Tg compared with native IGFBP-1. Tg localized in fibroblast membranes was also shown to catalyze the formation of native IGFBP-1 multimers, however, Q66A/Q67A IGFBP-1 failed to polymerize. Although the mutant IGFBP-1 potently inhibited IGF-I stimulated protein synthesis in pSMC cultures, the same concentration of native IGFBP-1 had no inhibitory effect. The addition of higher concentrations of native IGFBP-1 did inhibit the protein synthesis response, and this degree of inhibition correlated with the amount of monomeric IGFBP-1 that was present. In conclusion, IGFBP-1 is a substrate for tissue transglutaminase and Tg leads to the formation of high molecular weight covalently linked multimers. Polymerization is an important post-translational modification of IGFBP-1 that regulates cellular responses to IGF-I.

Insulin-like growth factor binding protein-5 (IGFBP-5) interacts with thrombospondin-1 to induce negative regulatory effects on IGF-I actions

Insulin‐like growth factor binding protein‐5 (IGFBP‐5) and thrombospondin‐1 (TS‐1) are both present in extracellular matrix (ECM). Both proteins have been shown to bind to one another with high affinity. The purpose of these studies was to determine how the interaction between IGFBP‐5 and TS‐1 modulates IGF‐I actions in porcine aortic smooth muscle cells (pSMC) in culture. The addition of increasing concentrations of TS‐1 to pSMC cultures enhanced the protein synthesis and cell migration responses to IGF‐I; whereas the addition of IGFBP‐5 alone resulted in minimal changes. In contrast, the addition of IGFBP‐5 to cultures that were also exposed to IGF‐I and TS‐1 resulted in inhibition of protein synthesis. When the cell migration response was assessed, the response to IGF‐I plus TS‐1 was also significantly inhibited by the addition of IGFBP‐5, whereas 1.0 μg/ml of IGFBP‐5 alone had no effect on the response to IGF‐I.To determine the molecular mechanism by which this inhibition occurred, a mutant form of IGFBP‐5 that does not bind to IGF‐I was tested. This mutant was equipotent compared to native IGFBP‐5 in its ability to inhibit both protein synthesis and cell migration responses to IGF‐I plus TS‐1 thus excluding the possibility that IGFBP‐5 was inhibiting the response to TS‐1 and IGF‐I by inhibiting IGF‐I binding to the IGF‐I receptor. To determine if an interaction between TS‐1 and IGFBP‐5 was the primary determinant of the inhibitory effect of IGFBP‐5, an IGFBP‐5 mutant that bound poorly to TS‐1 was utilized. The addition of 1.0 μg/ml of this mutant did not inhibit the protein synthesis or cell migration responses to IGF‐I plus TS‐1. To determine the mechanism by which IGFBP‐5 binding to TS‐1 inhibited cellular responses to TS‐1 plus IGF‐I, TS‐1 binding to integrin associated protein (IAP) was assessed. The addition of IGFBP‐5 (1.0 μg/ml) inhibited TS‐1–IAP association. In contrast, a mutant form of IGFBP‐5 that bound poorly to TS‐1 had a minimal effect on TS‐1 binding to IAP. Further analysis showed that IGFBP‐5 addition altered the ability of TS‐1 to modulate the SHPS‐1/IAP interaction. When the IGFBP‐5 mutant that did not bind to IGF‐I was incubated with TS‐1 and IGF‐I, it inhibited the capacity of TS‐1 to enhance the IGF‐I receptor phosphorylation and MAP kinase activation in response to IGF‐I. In contrast, the IGFBP‐5 mutant that did not bind to TS‐1 had no effect on IGF‐I stimulated IGF‐I receptor phosphorylation or MAP kinase activation. These results indicate that IGFBP‐5 inhibits the binding of TS‐1 to IAP, and this results in an alteration of the ability of TS‐1 to modulate the disruption of the IAP/SHPS‐1 interaction which leads to attenuation of the ability of TS‐1 to enhance cellular responsiveness to IGF‐I.