Inhibition of collagen biosynthesis and increases in low molecular weight IGF-I binding proteins in the skin of fasted rats

Insulin-like growth factor-I (IGF-I) is an important stimulator of collagen biosynthesis and prolidase activity in connective tissue cells. The disturbances in skin collagen metabolism (reflected by significant decrease in skin collagen content, collagen biosynthesis and prolidase activity) in fasted rats were accompanied by decrease in serum IGF-I level. Fasted rat serum was found to contain about 58% of IGF-I (101.6 +/- 15.4 ng/ml) as compared to control rat serum (175.7 +/- 19.8 ng/ml), while the skin of control and fasted rats contained similar concentrations of IGF-I (about 77 ng/g tissue). The insulin-like growth factor binding proteins (IGFBPs) of sera and tissue extracts (known to regulate IGF-I activity) were analysed by ligand blotting. In the serum of control rats one IGFBP band of about 46 kDa (corresponding to the acid-dissociated IGFBP-3) was detected. In the serum of fasted rats the 46 kDa IGFBP was not observed, however, an other IGFBP of about 30 kDa (corresponding to low molecular weight IGFBPs, e.g. IGFBP-1 or IGFBP-2) was found. The intensity of IGF-I binding to the 30 kDa IGFBP was much higher than that of IGFBP-3, found in control rat serum. Control and fasted rat skin contained similar IGFBPs, however their IGF-I binding abilities were much lower, compared to their serum counterparts. It was found that 46 kDa and 30 kDa proteins, observed in ligand blotting represent IGFBP-3 and IGFBP-1 or IGFBP-2. respectively as demonstrated by western immunoblot analysis. An increase in IGF-binding to 30 kDa IGFBP-1 and/or IGFBP-2 (known as an inhibitors of IGF-dependent functions) in the skin of fasted rats may explain the mechanism of reduced collagen biosynthesis and deposition in tissues during fasting.     

IGF binding proteins and their functions

The insulin-like growth factor (IGF) binding proteins (IGFBPs) have several functions, including transporting the IGFs in the circulation, mediating IGF transport out of the vascular compartment, localizing the IGFs to specific cell types, and modulating both IGF binding to receptors and growth-promoting actions. The functions of IGFBPs appear to be altered by posttranslational modifications. IGFBP-3, -4, -5, and -6 have been shown to be glycosylated. Likewise all the IGFBPs have a complex disulfide bond structure that is required for maintenance of normal IGF binding. IGFBP-2, -3, -4, and -5 are proteolytically cleaved, and specific proteases have been characterized for IGFBP-3, -4, and -5. Interestingly, attachment of IGF-I or II to IGFBP-4 results in enhancement of proteolysis, whereas attachment of either growth factor to IGFBP-5 results in inhibition of proteolytic cleavage. Cleavage of IGFBP-3 results in the appearance of a 31 kDa fragment that is 50-fold reduced in its affinity for the IGF-I or IGF-II. In spite of the reduction in its affinity, this fragment is capable of potentiating the effect of IGF-I on cell growth responses; therefore, proteolysis may be a specific mechanism that alters IGFBP modulation of IGF actions. Other processes that result in a reduction in IGF binding protein affinity are associated with potentiation of cellular responses to IGF-I and -II. Specifically, the binding of IGFBP-3 to cell surfaces is associated with its ability to enhance IGF action and with a ten- to 12-fold reduction in its affinity for IGF-I and IGF-II. Likewise, binding of IGFBP-5 to extracellular matrix (ECM) results in an eightfold reduction in its affinity and a 60% increase in cell growth in response to IGF-I. Another post-translational modification that modifies IGFBP activity is phosphorylation. IGFBP-1, -2, -3, and -5 have been shown to be phosphorylated. Phosphorylation of IGFBP-1 results in a sixfold enhancement in its affinity for IGF-I and -II. Following this enhancement of IGFBP-1 affinity, this binding protein loses its capacity to potentiate IGF-I growth-promoting activity. Future studies using site-directed mutagenesis to modify these proteins should enable us to determine the effect of these posttranslational modifications on the ability of IGFBPs to modulate IGF biologic activity. © 1993 Wiley-Liss, Inc.     

The role and regulation of IGFBP-1 phosphorylation in fetal growth restriction

Fetal growth restriction (FGR) increases the risk of perinatal complications and predisposes the infant to developing metabolic, cardiovascular, and neurological diseases in childhood and adulthood. The pathophysiology underlying FGR remains poorly understood and there is no specific treatment available. Biomarkers for early detection are also lacking. The insulin-like growth factor (IGF) system is an important regulator of fetal growth. IGF-I is the primary regulator of fetal growth, and fetal circulating levels of IGF-I are decreased in FGR. IGF-I activity is influenced by a family of IGF binding proteins (IGFBPs), which bind to IGF-I and decrease its bioavailability. During fetal development the predominant IGF-I binding protein in fetal circulation is IGFBP-1, which is primarily secreted by the fetal liver. IGFBP-1 binds IGF-I and thereby inhibits its bioactivity. Fetal circulating levels of IGF-I are decreased and concentrations of IGFBP-1 are increased in FGR. Phosphorylation of human IGFBP-1 at specific sites markedly increases its binding affinity for IGF-I, further limiting IGF-I bioactivity. Recent experimental evidence suggests that IGFBP-1 phosphorylation is markedly increased in the circulation of FGR fetuses suggesting an important role of IGFBP-1 phosphorylation in the regulation of fetal growth. Understanding of the significance of site-specific IGFBP-1 phosphorylation and how it is regulated to contribute to fetal growth will be an important step in designing strategies for preventing, managing, and/or treating FGR. Furthermore, IGFBP-1 hyperphosphorylation at unique sites may serve as a valuable biomarker for FGR.     

Insulin-like growth factor (IGF)-I binding to a cell membrane associated IGF binding protein-3 acid-labile subunit complex in human anterior pituitary gland

The binding characteristics of [(125) I]insulin-like growth factor (IGF)-I were studied in human brain and pituitary gland. Competition binding studies with DES(1-3)IGF-I and R(3) -IGF-I, which display high affinity for the IGF-I receptor and low affinity for IGF binding proteins (IGFBPs), were performed to distinguish [(125) I]IGF-I binding to IGF-I receptors and IGFBPs. Specific [(125) I]IGF-I binding in brain regions and the posterior pituitary was completely displaced by DES(1-3)IGF-I and R(3) -IGF-I, indicating binding to IGF-I receptors. In contrast, [(125) I]IGF-I binding in the anterior pituitary was not displaced by DES(1-3)IGF-I and R(3) -IGF-I, suggesting binding to an IGF-binding site that is different from the IGF-I receptor. Binding affinity of IGF-I to this site was about 10-fold lower than for the IGF-I receptor. Using western immunoblotting we were also unable to detect IGF-I receptors in human anterior pituitary. Instead, western immunoblotting and immunoprecipitation experiments showed a 150-kDa IGFBP-3-acid labile subunit (ALS) complex in the anterior pituitary and not in the posterior pituitary and other brain regions. RT-PCR experiments showed the expression of ALS mRNA in human anterior pituitary indicating that the anterior pituitary synthesizes ALS. In the brain regions and posterior pituitary, IGFBP-3 was easily washed away during pre-incubation procedures as used in the [(125) I]IGF-I binding experiments. In contrast, the IGFBP-3 complex in the anterior pituitary could not be removed by these washing procedures. Our results indicate that the human anterior pituitary contains a not previously described tightly cell membrane-bound 150-kDa IGFBP-3-ALS complex that is absent in brain and posterior pituitary.     

Insulin-like growth factor (IGF)-binding proteins: interactions with IGFs and intrinsic bioactivities

The insulin-like growth factor (IGF)-binding proteins (IGFBPs) are a family of six homologous proteins with high binding affinity for IGF-I and IGF-II. Information from NMR and mutagenesis studies is advancing knowledge of the key residues involved in these interactions. IGF binding may be modulated by IGFBP modifications, such as phosphorylation and proteolysis, and by cell or matrix association of the IGFBPs. All six IGFBPs have been shown to inhibit IGF action, but stimulatory effects have also been established for IGFBP-1, -3, and -5. These generally involve a decrease in IGFBP affinity and may require cell association of the IGFBP, but precise mechanisms are unknown. The same three IGFBPs have well established effects that are independent of type I IGF receptor signaling. IGFBP-1 exerts these effects by signaling through alpha(5)beta(1)-integrin, whereas IGFBP-3 and -5 may have specific cell-surface receptors with serine kinase activity. The regulation of cell sensitivity to inhibitory IGFBP signaling may play a role in the growth control of malignant cells.     

Differential effect of fasting on IGF-BPs in serum of young and adult rats and its implication to impaired skin GAG content

During fasting or aging of animals there is a decreased content of skin glycosaminoglycans (GAGs). It has been found that the skin of adult rats contains about 60% of GAGs found in the skin of young animals. Fasting of both groups of animals (young and adult) resulted in decrease of GAG content. However, GAG content in the skin of fasted young rats decreased by 30% and in fasted adult rats by 15% only, compared to fed animals, respectively. The mechanism for the phenomena is not known. We considered insulin-like growth factor-I (IGF-I) as a potential candidate involved in regulation of GAG biosynthesis in both experimental models of animals. Adult rat sera were found to contain about 75% of IGF-I recovered from young rat sera. Fasting of both groups of animals resulted in dramatic decrease in serum IGF-I levels to about 50% of initial values. Since IGF-I activity and IGF-I serum half-life depends on the level of specific IGF-binding proteins (IGFBPs) we determined (i) relationship between main groups of IGFBPs, namely high molecular weight binding proteins (HMWBPs) and low molecular weight binding proteins (LMWBPs) and (ii) the amounts of IGF-I bound to respective proteins in the sera of all experimental animals. Control young rat serum was found to contain about 90% of HMWBPs and about 10% of LMWBPs as determined by ligand binding assay. In contrast, control adult rat serum contained about 60% of HMWBPs and about 40% of LMWBPs. Fasting of both groups of animals resulted in significant increase in serum levels of LMWBPs. Control young rat serum was found to contain about 8% IGF-I bound to LMWBPs while serum of control adult rats contained 18% IGF-I bound to these proteins. In sera of fasted young animals however, about 75% of the bound IGF-I was recovered from LMWBPs (about 60% of total serum IGF-I) while in sera of fasted adult animals only about 56% of the bound IGF-I was recovered from LMWBPs (about 50% of total serum IGF-I). Evidence was provided that during fasting of both groups of animals there is a significant decrease in serum BP-3 and dramatic increase in serum BP-1 concentrations, compared to respective controls. However, the concentration of BP-1 in serum of fasted young rats was increased by about 60 fold while in serum of fasted adult rats only by about 10 fold, compared to respective control animals. Negative correlation between skin GAG content and LMWBPs derived IGF-I during fasting of young (r = - 0.943, p < 0.001) and adult ( r = - 0.571, p < 0.01) rats was found.The data presented suggest that the effects of aging and fasting on decreased skin GAG content may be due to induction of LMWBPs that are known to (i) inhibit IGF-I dependent function and (ii) increase clearance of IGF-I from circulation. However, the effects of fasting are distinct in respect to young and adult rats suggesting that mechanisms involved in regulation of IGF-I bioactivity during aging are more complex that during fasting.     

Potential role of pyrroline 5-carboxylate in regulation of collagen biosynthesis in cultured human skin fibroblasts

Although insulin-like growth factor-I (IGF-I) is known as an important stimulator of collagen biosynthesis in collagen-producing cells, the mechanism and endpoints by which it regulate the process remain largely unknown. Serum of acutely fasted rats contained reduced amount of IGF-I (72+/-16 ng/ml) and showed about 75% reduced ability to stimulate collagen and DNA synthesis in confluent human skin fibroblasts in comparison to the effect of control rat serum (IGF-I, 168+/-19 ng/ml). An addition of IGF-I (at least 40 ng/ml) to fasted rat serum restored its mitogenic activity but could not restore its ability to stimulate collagen biosynthesis to control values during 24 h of incubation. However, when the cells were incubated in fasted rat serum supplemented with 40 ng/ml of IGF-I for 48 h, collagen biosynthesis was restored to control values. It suggests that the stimulatory role of IGF-I in collagen biosynthesis undergo indirectly. We considered pyrroline-5-carboxylate (P5C) as a candidate to play a direct role in this process. Since IGF-I and P5C are known to be decreased in serum of fasted rats it seems that the action of IGF-I on collagen biosynthesis may involve participation of P5C. We have found that serum of fasted rats (showing low level of P5C) supplemented with 1 mmol/l P5C induced collagen biosynthesis in confluent human skin fibroblasts during 24 h to control values. Supporting evidence comes from the experiment showing stimulatory action of P5C on collagen biosynthesis in fibroblasts cultured in serum-free medium. Our results postulate potential role of P5C in regulation of collagen biosynthesis and indicate participation of this molecule in the pathway of IGF-I action in this process.     

Cloning and sequence determination of bovine insulin-like growth factor binding protein-2 (IGFBP-2): comparison of its structural and functional properties with IGFBP-1.

Insulin-like growth factor binding proteins (IGFBPs) are secreted by several cell types and can modify IGF actions. Mandin-Darby Bovine Kidney (MDBK) cells have been shown to secrete a 34,000 Da form of IGF binding protein whose N-terminal sequence is similar to a form of IGFBP purified from rat BRL-3A cells that has recently been named IGFBP-2. These studies report the complete amino acid sequence of bovine IGFBP-2 and compare its functional properties with human IGFBP-1. The protein is 81% identical to rat IGFBP-2. When compared with both rat IGFBP-2 and human IGFBP-1, the positions of all 18 cysteine residues are conserved. Similarly an RGD sequence is present near the carboxyl terminus in both proteins. IGFBP-2 has a higher affinity for IGF-II than for IGF-I and its affinity for both forms of IGF is greater than for human IGFBP-1. Like IGFBP-1 the protein can enhance the DNA synthesis response of porcine aortic smooth muscle cells to IGF-I; however, IGFBP-2 was much less potent. The maximum potentiation of the IGF-mediated mitogenic response that could be achieved was approximately 42% that of IGFBP-1. This potentiation is dependent upon a factor contained in platelet poor plasma and if this factor is omitted from the incubation medium, IGFBP-2 inhibits DNA synthesis. The purification of IGFBP-2 will allow more detailed comparisons to be made between it and other forms of IGFBPs in physiologic test systems.     

Matrix metalloproteinase-7 degrades all insulin-like growth factor binding proteins and facilitates insulin-like growth factor bioavailability

Proteolytic modification of insulin-like growth factor binding proteins (IGFBPs) plays an important physiological role in regulating insulin-like growth factor (IGF) bioavailability. Recently, we demonstrated that matrix metalloproteinase-7 (MMP-7)/Matrilysin produced by various cancer cells catalyzes the proteolysis of IGFBP-3 in vitro and regulates IGF bioavailability, resulting in an anti-apoptotic effect against anchorage-independent culture. In the present study, we investigated whether MMP-7 contributes to proteolysis of the other five IGFBPs, IGFBP-1, IGFBP-2, IGFBP-4, IGFBP-5, and IGFBP-6, and whether this results in phosphorylation of the IGF type 1 receptor (IGF-1R). MMP-7 cleaved all six IGFBPs, resulting in IGF-mediated IGF-1R phosphorylation, which was inhibited by EDTA treatment. These results suggest that MMP-7 derived from cancer cells can regulate IGF bioavailability in the microenvironment surrounding the tumor, where various kinds of IGF/IGFBP complexes are found, thereby favoring cancer cell growth and survival during the processes of invasion and metastasis.     

Insulin-like growth factor (IGF) binding protein-5 blocks skeletal muscle differentiation by inhibiting IGF actions

Signaling through the IGF-I receptor by locally produced IGF-I or -II is critical for normal skeletal muscle development and repair after injury. In most tissues, IGF action is modulated by IGF binding proteins (IGFBPs). IGFBP-5 is produced by muscle cells, and previous studies have suggested that when overexpressed it may either facilitate or inhibit IGF actions, and thus potentially enhance or diminish IGF-mediated myoblast differentiation or survival. To resolve these contradictory observations and discern the mechanisms of action of IGFBP-5, we studied its effects in cultured muscle cells. Purified wild-type (WT) mouse IGFBP-5 or a variant with diminished extracellular matrix binding (C domain mutant) each prevented differentiation at final concentrations as low as 3.5 nm, whereas analogs with reduced IGF binding (N domain mutant) were ineffective even at 100 nm. None of the IGFBP-5 variants altered cell number. An IGF-I analog (R(3)IGF-I) with diminished affinity for IGFBPs promoted full muscle differentiation in the presence of IGFBP-5(WT), showing that IGFBP-5 interferes with IGF-dependent signaling pathways in myoblasts. When IGFBP-5(WT) or variants were overexpressed by adenovirus-mediated gene transfer, concentrations in muscle culture medium reached 500 nm, and differentiation was inhibited, even by IGFBP-5(N). As 200 nm of purified IGFBP-5(N) prevented activation of the IGF-I receptor by 10 nm IGF-II as effectively as 2 nm of IGFBP-5(WT), our results not only demonstrate that IGFBP-5 variants with reduced IGF binding affinity impair muscle differentiation by blocking IGF actions, but underscore the need for caution when labeling effects of IGFBPs as IGF independent because even low-affinity analogs may potently inhibit IGF-I or -II if present at high enough concentrations in biological fluids.     

Identification of insulin-like growth factor binding proteins from cultured human epidermal keratinocytes

The role and mechanisms of action of insulin-like growth factors (IGFs) in skin remain unclear. Epidermal keratinocytes possess IGF-I receptors and are responsive to IGF-I, which is primarily derived from underlying dermal fibroblasts. IGF binding proteins (IGFBPs), also synthesized by fibroblasts, may be involved in paracrine targeting of IGF-I to its receptors. We therefore examined whether human keratinocytes synthesize IGFBPs and their mRNAs. Following culture in complete medium (containing bovine pituitary extract and epidermal growth factor) Western ligand blotting (WLB) of cell conditioned medium revealed a major band of 32 kD, a less abundant IGFBP of 24 kD at all passages, and a 37–42 kD IGFBP which increased in abundance in late passage. Immunoprecipitation followed by WLB confirmed that the predominant 32 kD band was IGFBP-2. Radioimmunoassay of IGFBP-1, -3, and -6 revealed detectable levels of IGFBP-3 and significant levels of IGFBP-6, but not IGFBP-1. Northern analysis following culture in complete medium revealed that at early passage IGFBP-1, -2, -4, and -6 mRNAs were detectable. IGFBP-3 and -5 mRNAs were not detectable. Following culture in growth factor-free medium a 37–42 kD band, consistent with IGFBP-3, was predominant and a 24 kD band consistent with IGFBP-4 was also present. These data demonstrate the expression of a distinct pattern of IGFBPs by cultured human keratinocytes dependent on culture conditions. Keratinocyte-derived IGFBPs are likely to play a role in the transport and targeting of IGF-I from dermally derived fibroblasts to the epidermis. © 1995 Wiley-Liss, Inc.     

Cell-specific localization of insulin-like growth factor binding protein mRNAs in rat liver

The liver is a major source of circulating insulin-like growth factor I (IGF-I), and it also synthesizes several classes of IGF binding proteins (IGFBPs). Synthesis of IGF-I and IGFBPs is regulated by hormones, growth factors, and cytokines. They are nutritionally regulated and expressed in developmentally specific patterns. To gain insight into cellular regulatory mechanisms that determine hepatic synthesis of IGF-I and IGFBPs and to identify potential target cells for IGF-I within the liver, we studied the cellular sites of synthesis of IGF-I, IGF receptor, growth hormone (GH) receptor, and IGFBPs in freshly isolated rat hepatocytes, endothelial cells, and Kupffer cells. We also localized cellular sites of IGFBP synthesis by in situ hybridization histochemistry. Western ligand and immunoblot analyses were used to determine IGFBP secretion by isolated cells. Two IGF-I mRNA subtypes with different 5' ends (class 1 and class 2) were detected in all isolated liver cell preparations. Type 1 IGF receptor mRNA was detected in endothelial cells, indicating that these cells are a local target for IGF actions in liver. GH receptor was expressed in all cell preparations, consistent with GH regulation of IGF-I and IGFBP synthesis in multiple liver cell types. The IGFBPs expressed striking cell-specific expression. IGFBP-1 was synthesized only in hepatocytes, and IGFBP-3 was expressed in Kupffer and endothelial cells. IGFBP-4 was expressed at high levels in hepatocytes and at low levels in Kupffer and endothelial cells. Cell-specific expression of distinct IGFBPs in the liver provides the potential for cell-specific regulation of hepatic and endocrine actions of IGF-I.     

Insulin-like growth factor-binding protein-3 activates a phosphotyrosine phosphatase. Effects on the insulin-like growth factor signaling pathway

The proliferative action of insulin-like growth factors (IGF-I and -II) is mediated via the type I IGF receptor (IGF-IR) and is modulated by their association with high affinity binding proteins, IGFBP-1 to -6. We recently found that, in addition to its ability to bind IGFs, IGFBP-3 also inhibits IGF-IR activation independently of IGF binding and without interacting directly with IGF-IR. Here, we show that IGFBP-3 is capable of blocking the signal triggered by IGFs. Breast carcinoma-derived cells (MCF-7) were stimulated by des(1-3)IGF-I or [Gln(3),Ala(4),Tyr(15),Leu(16)]IGF-I, two IGF analogues with intact affinity for IGF-IR, but with weak or virtually no affinity for IGFBPs, then incubated with IGFBP-3. The activated IGF-IR was desensitized through reversal of its autophosphorylation, following which both phosphatidylinositol 3-kinase and p42(MAPK) activities were depressed. Direct measurement of phosphotyrosine phosphatase activity and reconstitution experiments using tyrosine-phosphorylated insulin receptor substrate-1 (IRS-1) indicated that IGFBP-3 activated a phosphotyrosine phosphatase (PTPase). This action appeared to be peculiar to IGFBP-3 among the IGFBPs, since neither IGFBP-1 nor IGFBP-5 (structurally the closest to IGFBP-3), had any such effect. Several cell lines derived from normal or tumor cells responsive to IGF-I were used to show that IGFBP-3-stimulated PTPase is cell type-specific. Although the precise nature of the phosphatase remains to be determined, the results of this study demonstrate that IGFBP-3 stimulates a phosphotyrosine phosphatase activity that down-regulates the IGF-I signaling pathway, suggesting a major role for IGFBP-3 in regulating cell proliferation.     

Insulin-like growth factor binding protein (IGFBP)-3-bound IGF-I and IGFBP-3-bound IGF-II in growth hormone deficiency

In blood, circulating IGFs are bound to six high-affinity IGFBPs, which modulate IGF delivery to target cells. Serum IGFs and IGFBP-3, the main carrier of IGFs, are upregulated by GH. The functional role of serum IGFBP-3-bound IGFs is not well understood, but they constitute the main reservoir of IGFs in the circulation. We have used an equation derived from the law of mass action to estimate serum IGFBP-3-bound IGF-I and IGFBP-3-bound IGF-II, as well as serum free IGF-I and free IGF-II, in 129 control children and adolescents (48 girls and 81 boys) and in 13 patients with GHD. Levels of serum total IGF-I, total IGF-II, IGFBP-1, IGFBP-2 and IGFBP-3 were determined experimentally, while those of IGFBP-4, IGFBP-5 and IGFPB-6, as well as the 12 affinity constants of association of the two IGFs with the six IGFBPs, were taken from published values. A correction for in vivo proteolysis of serum IGFBP-3 was also considered. In controls, serum total IGF-I, total IGF-II, IGFBP-3, IGFBP-3-bound IGF-I, IGFBP-3-bound IGF-II and free IGF-I increased linearly with age, from less than 1 to 15 years, in the two sexes. The concentrations of serum free IGF-I and free IGF-II were approximately two orders of magnitude below published values, as well as below the affinity constant of association of IGF-I with the type-1 IGF receptor. Therefore, it is unlikely that these levels can interact with the receptor. In the 13 patients with GHD, mean (+/- SD) SDS of serum IGFBP-3-bound IGF-I was -2.89 +/- 0.97. It was significantly lower than serum total IGF-I, free IGF-I or IGFBP-3 SDSs (-2.35 +/- 0.83, -1.12 +/- 0.78 and -2.55 +/- 1.07, respectively, p = 0.0001). The mean SDS of serum total IGF-II, IGFBP-3-bound IGF-II and free IGF-II were -1.25 +/- 0.68, -2.03 +/- 0.87 and 0.59 +/- 1.10, respectively, in GHD. In control subjects, 89.8 +/- 4.47% of serum total IGF-I and 77.3 +/- 9.4% of serum total IGF-II were bound to serum IGFBP-3. In patients with GHD, the mean serum IGFBP-3-bound IGF-I and IGFBP-3-bound IGF-II were 8.63 +/- 8. 53 and 19.1 +/- 14.7% below the respective means of control subjects (p < 0.02). In conclusion, in GHD there was a relative change in the distribution of serum IGFs among IGFBPs, due to the combined effects of the decrease in both total IGF-I and IGFBP-3. As a result, serum IGFBP-3-bound IGF-I and IGFBP-3 bound IGF-II, the main reservoirs of serum IGFs, were severely affected. This suggests that the decrease in serum IGFPB-3-bound IGF-I and IGFBP-3-bound IGF-II might have a negative effect for growth promotion and other biological effects of IGF-I and IGF-II. Finally, the estimation of serum IGFBP-3-bound IGF-I, or the percentage of total IGF-I and IGF-II bound to IGFBP-3, might be useful markers in the diagnosis of GHD.     

Epigenetic regulation of insulin-like growth factor binding protein-3 (IGFBP-3) in cancer

Epigenetics refers to heritable changes in gene expression that are independent of alterations in DNA sequence. It is now accepted that disruption of epigenetic mechanisms plays a key role in the pathogenesis of cancer: culminating in altered gene function and malignant cellular transformation. DNA methylation and histone modifications are the most widely studied changes but non-coding RNAs such as miRNAs are also considered part of the epigenetic machinery. The insulin-like growth factor (IGF) axis is composed of two ligands, IGF-I and –II, their receptors and six high affinity IGF binding proteins (IGFBPs). The IGF axis plays a key role in cancer development and progression. As IGFBP genes have consistently been identified among the most common to be aberrantly altered in tumours, this review will focus on epigenetic regulation of IGFBP-3 in cancer for which the majority of evidence has been obtained.     

Regulation of vascular smooth muscle cell responses to insulin-like growth factor (IGF)-I by local IGF-binding proteins

Insulin-like growth factor (IGF)-I is a pleiotropic hormone that regulates vascular smooth muscle cell (VSMC) migration, proliferation, apoptosis, and differentiation. These actions are mediated by the IGF-I receptor. How activation of the same receptor by the same ligand leads to these diverse cellular responses is not well understood. Here we describe a novel mechanism specifying VSMC responses to IGF-I stimulation, distinctive for the pivotal roles of local IGF-binding proteins (IGFBPs). The role of local IGFBPs was indicated by comparing the activities of IGF-I and des-1-3-IGF-I, an IGF-I analog with reduced binding affinity to IGFBPs. Compared with IGF-I, des-1-3-IGF-I was more potent in stimulating DNA synthesis but much less potent in inducing directed migration of VSMCs. When the effects of individual IGFBPs were tested, IGFBP-2 and IGFBP-4 were found to inhibit IGF-I-stimulated DNA synthesis and migration. IGFBP-5 had an inhibitory effect on IGF-I-stimulated DNA synthesis, but it strongly potentiated IGF-I-induced VSMC migration. By using a non-IGF-binding IGFBP-5 mutant and an IGF-I-neutralizing antibody, it was demonstrated that IGFBP-5 also stimulates VSMC migration in an IGF-independent manner. This effect of IGFBP-5 was inhibited by soluble heparin and by treating cells with heparinase. Mutation of the heparin-binding motif of IGFBP-5 reduced its migration promoting activity. These findings suggest that local IGFBPs are important determinants of cellular responses to IGF-I stimulation, and a key player in this paradigm is IGFBP-5. IGFBP-5 not only modulates IGF-I actions, but it also stimulates cell migration by interacting with cell-surface heparan sulfate proteoglycans.