Intestinotrophic effects of exogenous IGF-I are not diminished in IGF binding protein-5 knockout mice

IGF binding protein-5 (IGFBP-5) modulates the availability of IGF-I to its receptor and potentiates the intestinotrophic action of IGF-I. Our aim was to test the hypothesis that stimulation of intestinal growth due to coinfusion of IGF-I with total parenteral nutrition (TPN) solution is dependent on increased expression of IGFBP-5 through conducting our studies in IGFBP-5 knockout (KO) mice. IGFBP-5 KO, heterozygote (HT) and wild type (WT) male and female mice were maintained with TPN or TPN plus coinfusion of IGF-I [recombinant human (rh)IGF-I; 2.5 mg x kg(-1) x day(-1)] for 5 days. The concentration of IGF-I in serum was 73% greater (P < 0.0001) in mice given TPN + IGF-I infusion compared with TPN alone. IGF-I attenuated the 2-3 g loss of body weight associated with TPN in WT mice, whereas KO and HT mice did not show improvement in body weight with IGF-I treatment. KO and HT mice had significantly greater levels of circulating IGF-I binding proteins (IGFBPs) compared with WT mice. Intestinal growth due to IGF-I was observed in all groups treated with IGF-I based on greater concentrations of protein and DNA in small intestine and colon and significantly greater crypt depth and muscularis thickness in jejunum. Jejunal expression of IGFBP-5 mRNA was greater in WT mice, whereas IGFBP-3 mRNA was greater in KO mice treated with IGF-I. In summary, the absence of the IGFBP-5 gene did not block the ability of IGF-I to stimulate intestinal growth, possibly because greater jejunal expression of IGFBP-3 compensates for the absence of IGFBP-5.     

Enteral nutrients potentiate glucagon-like peptide-2 action and reduce dependence on parenteral nutrition in a rat model of human intestinal failure

Glucagon-like peptide-2 (GLP-2) is a nutrient-dependent, proglucagon-derived gut hormone that shows promise for the treatment of short bowel syndrome (SBS). Our objective was to investigate how combination GLP-2 + enteral nutrients (EN) affects intestinal adaption in a rat model that mimics severe human SBS and requires parenteral nutrition (PN). Male Sprague-Dawley rats were assigned to one of five groups and maintained with PN for 18 days: total parenteral nutrition (TPN) alone, TPN + GLP-2 (100 μg·kg(-1)·day(-1)), PN + EN + GLP-2(7 days), PN + EN + GLP-2(18 days), and a nonsurgical oral reference group. Animals underwent massive distal bowel resection followed by jejunocolic anastomosis and placement of jugular catheters. Starting on postoperative day 4, rats in the EN groups were allowed ad libitum access to EN. Groups provided PN + EN + GLP-2 had their rate of PN reduced by 0.25 ml/day starting on postoperative day 6. Groups provided PN + EN + GLP-2 demonstrated significantly greater body weight gain with similar energy intake and a safe 80% reduction in PN compared with TPN ± GLP-2. Groups provided PN + EN + GLP-2 for 7 or 18 days showed similar body weight gain, residual jejunal length, and digestive capacity. Groups provided PN + EN + GLP-2 showed increased jejunal GLP-2 receptor (GLP-2R), insulin-like growth factor-I (IGF-I), and IGF-binding protein-5 (IGFBP-5) expression. Treatment with TPN + GLP-2 demonstrated increased jejunal expression of epidermal growth factor. Cessation of GLP-2 after 7 days with continued EN sustained the majority of intestinal adaption and significantly increased expression of colonic proglucagon compared with PN + EN + GLP-2 for 18 days, and increased plasma GLP-2 concentrations compared with TPN alone. In summary, EN potentiate the intestinotrophic actions of GLP-2 by improving body weight gain allowing for a safe 80% reduction in PN with increased jejunal expression of GLP-2R, IGF-I, and IGFBP-5 following distal bowel resection in the rat.     

Enteral nutrients potentiate the intestinotrophic action of glucagon-like peptide-2 in association with increased insulin-like growth factor-I responses in rats

Glucagon-like peptide-2 (GLP-2) is a nutrient-dependent, intestinotrophic hormone derived from posttranslational processing of proglucagon in the distal bowel. GLP-2 is thought to act through indirect mediators, such as IGF-I. We investigated whether intestinal expression of GLP-2 and IGF-I system components are increased with the mucosal growth induced by enteral nutrient (EN) and/or a low dose of GLP-2 in parenterally fed rats. Rats were randomized to four treatment groups using a 2 x 2 design and maintained with parenteral nutrition (PN) for 7 days: PN alone, EN, GLP-2, and EN+GLP-2; n = 7-9. The two main treatment effects are +/-GLP-2 (100 microg.kg body wt(-1).day(-1)) and +/-EN (43% of energy needs, days 4-6). Combination treatment with EN+GLP-2 induced synergistic intestinal growth in ileum, resulting in greater mucosal cellularity, sucrase segmental activity, and gain of body weight (ENxGLP-2, P < 0.04). In addition, EN+GLP-2 induced a significant 28% increase in plasma concentration of bioactive GLP-2, a significant 102% increase in ileal proglucagon mRNA with no change in ileal dipeptidyl peptidase-IV (DPP-IV) specific activity, and significantly reduced plasma DPP-IV activity compared with GLP-2. This indicates that EN potentiates the intestinotrophic action of GLP-2. Proliferation of enterocytes due to GLP-2 infusion was associated with greater expression of ileal proglucagon, GLP-2 receptor, IGF-I, IGF binding protein-3 mRNAs, and greater IGF-I peptide concentration in ileum (P < 0.032). Ileal IGF-I mRNA was positively correlated with expression of proglucagon, GLP-2R, and IGFBP-5 mRNAs (R2 = 0.43-0.56, P < 0.0001). Our findings support the hypothesis that IGF-I is one of the downstream mediators of GLP-2 action in a physiological model of intestinal growth.     

Suppressor of cytokine signaling-2 limits intestinal growth and enterotrophic actions of IGF-I in vivo

Suppressors of cytokine signaling (SOCS) typically limit cytokine receptor signaling via the JAK-STAT pathway. Considerable evidence demonstrates that SOCS2 limits growth hormone (GH) action on body and organ growth. Biochemical evidence that SOCS2 binds to the IGF-I receptor (IGF-IR) supports the novel possibility that SOCS2 limits IGF-I action. The current study tested the hypothesis that SOCS2 normally limits basal or IGF-I-induced intestinal growth and limits IGF-IR signaling in intestinal epithelial cells. Intestinal growth was assessed in mice homozygous for SOCS2 gene deletion (SOCS2 null) and wild-type (WT) littermates at different ages and in response to infused IGF-I or vehicle or EGF and vehicle. The effects of SOCS2 on IGF-IR signaling were examined in ex vivo cultures of SOCS2 null and WT intestine and Caco-2 cells. Compared with WT, SOCS2 null mice showed significantly enhanced small intestine and colon growth, mucosal mass, and crypt cell proliferation and decreases in radiation-induced crypt apoptosis in jejunum. SOCS2 null mice showed significantly greater growth responses to IGF-I in small intestine and colon. IGF-I-stimulated activation of IGF-IR and downstream signaling intermediates were enhanced in the intestine of SOCS2 null mice and were decreased by SOCS2 overexpression in Caco-2 cells. SOCS2 bound directly to the endogenous IGF-IR in Caco-2 cells. The intestine of SOCS2 null mice also showed enhanced growth responses to infused EGF. We conclude that SOCS2 normally limits basal and IGF-I- and EGF-induced intestinal growth in vivo and has novel inhibitory effects on the IGF-IR tyrosine kinase pathway in intestinal epithelial cells.     

Guanylyl Cyclase-G Modulates Jejunal Apoptosis and Inflammation in Mice with Intestinal Ischemia and Reperfusion

Background

Membrane bound guanylyl cyclase-G (mGC-G), a novel form of GC mediates ischemia and reperfusion (IR)-induced renal injury. We investigated the roles of mGC-G in intestinal IR-induced jejunal damage, inflammation, and apoptosis.

Materials and methods

Male C57BL/6 wild-type (WT) and mGC-G gene knockout (KO) mice were treated with a sham operation or 45 min of superior mesenteric arterial obstruction followed by 3, 6, 12, or 24 h of reperfusion.

Results

Sham-operated KO mice had significantly lower plasma nitrate and nitrite (NOx) levels and jejunal villus height, crypt depth, and protein expression of phosphorylated-nuclear factor-kappa-B (NF-κB), phosphorylated-c-Jun N-terminal kinases (JNK) 2/3, phosphorylated-p38, and B-cell lymphoma-2 (Bcl-2). They had significantly greater jejunal interleukin-6 mRNA, cytochrome c protein, and apoptotic index compared with sham-operated WT mice. Intestinal IR significantly decreased plasma NOx in WT mice and increased plasma NOx in KO mice. The jejunal apoptotic index and caspase 3 activities were significantly increased, and nuclear phosphorylated-NF-κB and phosphorylated-p38 protein were significantly decreased in WT, but not KO mice with intestinal IR. After reperfusion, KO mice had an earlier decrease in jejunal cyclic GMP, and WT mice had an earlier increase in jejunal proliferation and a later increase in cytosol inhibitor of kappa-B-alpha. Intestinal IR induced greater increases in plasma and jejunal interleukin-6 protein in WT mice and a greater increase in jejunal interleukin-6 mRNA in KO mice.

Conclusions

mGC-G is involved in the maintenance of jejunal integrity and intestinal IR-induced inflammation and apoptosis. These results suggest that targeting cGMP pathway might be a potential strategy to alleviate IR-induced jejunal damages.     

Diminished growth and enhanced glucose metabolism in triple knockout mice containing mutations of insulin-like growth factor binding protein-3, -4, and -5

IGF-I and IGF-II are essential regulators of mammalian growth, development and metabolism, whose actions are modified by six high-affinity IGF binding proteins (IGFBPs). New lines of knockout (KO) mice lacking either IGFBP-3, -4, or -5 had no apparent deficiencies in growth or metabolism beyond a modest growth impairment (approximately 85-90% of wild type) when IGFBP-4 was eliminated. To continue to address the roles of these proteins in whole animal physiology, we generated combinational IGFBP KO mice. Mice homozygous for targeted defects in IGFBP-3, -4, and -5 remain viable and at birth were the same size as IGFBP-4 KO mice. Unlike IGFBP-4 KO mice, however, the triple KO mice became significantly smaller by adulthood (78% wild type) and had significant reductions in fat pad accumulation (P < 0.05), circulating levels of total IGF-I (45% of wild type; P < 0.05) and IGF-I bioactivity (37% of wild type; P < 0.05). Metabolically, triple KO mice showed normal insulin tolerance, but a 37% expansion (P < 0.05) of beta-cell number and significantly increased insulin secretion after glucose challenge, which leads to enhanced glucose disposal. Finally, triple KO mice demonstrated a tissue-specific decline in activation of the Erk signaling pathway as well as weight of the quadriceps muscle. Taken together, these data provide direct evidence for combinatorial effects of IGFBP-3, -4, and -5 in both metabolism and at least some soft tissues and strongly suggest overlapping roles for IGFBP-3 and -5 in maintaining IGF-I-mediated postnatal growth in mice.     

Expression of insulin-like growth factor binding protein-4, insulin-like growth factor-I receptor, and insulin-like growth factor-I in the mouse uterus throughout the estrous cycle

Recent evidence suggests that a regulated insulin-like growth factor (IGF) system mediates the effects of estrogen, promoting the proliferation and differentiation of specific uterine cell types throughout the estrous cycle and during gestation in the rodent. Previous studies have shown that IGFs are differentially expressed in the mouse uterus during the periimplantation period. In the current study, we examined the expression of IGF binding protein-4 (IGFBP-4), IGF-I receptor (IGF-IR), and IGF-I in the mouse uterus throughout the estrous cycle. Ligand blot analysis was conducted on uterine homogenates using [125I]IGF-I. IGFBP-4 was detected in all uterine homogenates, varying in intensity throughout the estrous cycle. In situ hybridization studies at metestrus and diestrus demonstrated an intense IGFBP-4 mRNA signal in antimesometrial stromal cells between the luminal epithelium and the myometrium, but at proestrus and estrus, no IGFBP-4 signal was detected. No IGF-I mRNA was detected at any stage of the estrous cycle by in situ hybridization. However, by RT-PCR analysis, IGF-I mRNA was detected at all stages of the estrous cycle. RT-PCR analysis also showed IGF-IR mRNA throughout the estrous cycle. Using immunohistochemistry, IGF-IR immunostaining was detected throughout the estrous cycle and on days 2-7 of gestation, but was restricted to the glandular epithelium. These results suggest that uterine IGFBP-4 expression may not be dependent on uterine IGF-I expression. They also suggest that IGFBP-4 may play a role in uterine physiology independent of the inhibition of IGF-I action, and that IGF-IR is constitutively expressed in the mouse uterus.     

Insulin-like growth factor (IGF)-I, IGF-I receptor, and IGF binding protein-3 messenger ribonucleic acids and protein in corpora lutea from prostaglandin F(2alpha)-treated gilts

Insulin-like growth factor-I (IGF-I) is produced within the porcine corpus luteum (CL) and is thought to play an autocrine/paracrine role in CL development/function during the early luteal phase. This study examines the hypotheses that the luteolytic actions of prostaglandin F(2alpha) (PGF(2alpha)) during the early luteal phase may involve either a decrease in IGF-I or IGF receptor (IGF-IR), or an increase in IGF binding protein (IGFBP)-3, expression, any of which could interfere with the luteotropic actions of IGF-I in this tissue. Cycling gilts were treated twice daily with PGF(2alpha) (or saline) on Days 5-9 of the cycle to induce premature luteolysis. CL were collected on Days 6-9, and RNA, protein, or progesterone was extracted. By slot blot analysis, steady-state levels of IGF-I and IGFBP-3 mRNA were not different in PGF(2alpha)-treated vs. control animals; however, IGF-IR mRNA was increased in treated animals on Day 9. No changes in IGF-I content (ng/CL measured by RIA) were observed with respect to treatment. According to ligand blot analysis, the levels of IGFBP-3 increased on Day 6 and decreased on Days 8-9, while IGFBP-2 was higher on Days 6-7 and decreased on Day 9 in treated animals. IGF-IR levels, determined from Western blots, were higher on Day 7 (P < 0.05) and lower on Day 9 in PGF(2alpha)-treated animals vs. control animals (P < 0.05). In conclusion, PGF(2alpha)-induced premature luteolysis was associated with an increase in steady-state levels of IGF-IR mRNA, but it did not appear to be linked to changes in mRNA levels for IGF-I or IGFBP-3. However, since IGFBP-2 and -3 protein levels increased early in the treatment period (Days 6-7), it is possible that they may mediate the luteolytic actions of PGF(2alpha) by sequestering IGF-I and preventing its interaction with the IGF-IR.     

Integrin expression and integrin-mediated adhesion in vitro of human multipotent stromal cells (MSCs) to endothelial cells from various blood vessels

Estradiol (E2) stimulates not only secretion of prolactin (PRL) and proliferation of PRL-producing cells (PRL cells) in the anterior pituitary, but also the expression of growth factors. In insulin-like growth factor-I (IGF-I) knockout (KO) mice, the number of PRL cells is decreased and administration of IGF-I does not increase either the number of PRL cells or plasma PRL levels, indicating that IGF-I plays a pivotal role in PRL cells. The effect of E2 on PRL cells in KO mice was investigated by immunohistochemistry and real-time RT-PCR. The number of PRL cells in KO mice was significantly lower than in the wild-type (WT) control mice. E2 increased the PRL mRNA in WT and KO mice; however, an increase of PRL mRNA in KO was less than that in WT. In addition, no vasoactive intestinal peptide (VIP)-immunoreactive cells were found in KO mice, therefore IGF-I is essential for VIP expression. To investigate the roles of IGF-I on PRL cells in the postnatal development, double-immunostaining with PRL and BrdU was performed in WT and KO mice from days 5–20. The percentages of PRL cells and BrdU-labeled cells in the anterior pituitary of KO mice were lower than in WT mice. Thus, IGF-I may be responsible for proliferation and differentiation of PRL cells in this postnatal period. Differentiation and the proliferation of PRL cells are controlled by IGF-I during the postnatal development, and IGF may be a mediator of E2 action through VIP induction in PRL cells of adults.     

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.     

Reduction of spontaneous and irradiation-induced apoptosis in small intestine of IGF-I transgenic mice

Insulin-like growth factor I (IGF-I) may promote survival of putative stem cells in the small intestinal epithelium. Mitosis and apoptosis were quantified in crypts of nonirradiated and irradiated IGF-I transgenic (TG) and wild-type (WT) littermates. The mean apoptotic index was significantly greater in WT vs. TG littermates. After irradiation, apoptotic indexes increased, and WT mice showed a more dramatic increase in apoptosis than TG mice at the location of putative stem cells. After irradiation, no mitotic figures were observed in WT crypts, whereas mitosis was maintained within the jejunal epithelium of TG mice. The abundance and localization of Bax mRNA did not differ between nonirradiated littermates. However, there was more Bax mRNA in TG vs. WT mice after irradiation. Bax mRNA was located along the entire length of the irradiated crypt epithelium, but there was less Bax protein observed in the bottom third of TG mouse crypts compared with WT littermates. IGF-I regulates cell number by stimulating crypt cell proliferation and decreasing apoptosis preferentially within the stem cell compartment.     

Involvement of ligand occupancy in Insulin-like growth factor-I (IGF-I) induced cell growth in osteoblast like MC3T3-E1 cells

Growth factors and matrix proteins regulate the proliferation and differentiation of osteoblasts. The insulin-like growth factor (IGF) system comprises IGF-I, IGF-II, and six high-affinity IGF-binding proteins (IGFBPs). IGFs stimulate cell growth in many types of tissue; IGF-binding proteins regulate cellular actions and can affect cell growth. IGF-I is involved in differentiation, proliferation, and matrix formation in osteoblasts; IGFBP-5 is associated with the extracellular matrix (ECM) and can potentiate the actions of IGF-I. We investigated the effect of ECM proteins on the responses of MC3T3-E1 osteoblast cells to IGF-I and IGFBP-5. In addition, because extracellular signal-regulated kinases 1 and 2 (Erk 1/2) affect cell growth, we evaluated the effects of IGFBP-5 on Erk 1/2 phosphorylation in MC3T3-E1 cells. IGF-I caused an increase in IGFBP-5 expression in cultured MC3T3-E1 cells, and IGF-I plus IGFBP-5 significantly increased cell growth. Likewise, the addition of IGF-I and IGFBP-5 to cultured MC3T3-E1 cells increased the synthesis of the ECM proteins osteopontin (OPN) and thrombospondin-1 (TSP-1), which can bind to alphaVbeta3 integrin receptors on the cell surface. By contrast, the addition of an antibody against ECM proteins inhibited the effects of OPN and TSP-1 on IGFBP-5 expression. The stimulatory effect of IGFBP-5 was mediated via Erk 1/2 activation. These data suggest that IGFBP-5 regulates Erk 1/2 phosphorylation in cultured MC3T3-E1 cells via ECM proteins that may ultimately stimulate the growth of osteoblasts. We determined whether occupation of the alphaVbeta3 integrin receptor affects IGF-I receptor (IGF-IR)-mediated signaling and function in MC3T3-E1 osteoblast cells. Occupation of the alphaVbeta3 integrin receptor with ECM proteins induced IGF-I-stimulated IGF-IR phosphorylation. Conversely, in the presence of the alphaVbeta3-specific disintegrin echistatin, IGF-I-stimulated IGF-IR activation was inhibited. IGF-I-stimulated IGF-IR phosphorylation was accompanied by IRS-1 phosphorylation and MAPK activation. However, these effects were attenuated by echistatin. Thus, occupancy of the alphaVbeta3 disintegrin receptor modulates IGF-I-induced IGF-IR activation and IGF-IR-mediated function in MC 3T3-E1 osteoblasts.     

Effects of aging and caloric restriction on IGF-I, IGF-I receptor, IGFBP-3 and IGFBP-4 gene expression in the rat stomach and colon

The purpose of this study was to determine the effects of aging and caloric restriction (CR) on insulin-like growth factor-I (IGF-I), IGF-I receptor (IGF-IR), IGF-binding protein-3 (IGFBP-3) and IGFBP-4 expression in the stomach and colon of male Fischer 344 rats. Stomach and colonic RNA were prepared from ad libitum (AL) fed or long-term CR rats. Stomach IGF-I, IGFBP-3 and IGFBP-4 mRNA levels increased significantly (P相似文献   

Insulin-like growth factor I and glucagon-like peptide-2 responses to fasting followed by controlled or ad libitum refeeding in rats

Luminal nutrients stimulate structural and functional regeneration in the intestine through mechanisms thought to involve insulin-like growth factor I (IGF-I) and glucagon-like peptide-2 (GLP-2). We investigated the relationship between IGF-I and GLP-2 responses and mucosal growth in rats fasted for 48 h and then refed for 2 or 4 days by continuous intravenous or intragastric infusion or ad libitum feeding. Fasting induced significant decreases in body weight, plasma concentrations of IGF-I and bioactive GLP-2, jejunal mucosal cellularity (mass, protein, DNA, and villus height), IGF-I mRNA, and ileal proglucagon mRNA. Plasma IGF-I concentration was restored to fed levels with 2 days of ad libitum refeeding but not with 4 days of intravenous or intragastric refeeding. Administration of an inhibitor of endogenous GLP-2 (rat GLP-2 3-33) during ad libitum refeeding partially attenuated mucosal growth and prevented the increase in plasma IGF-I to fed levels; however, plasma GLP-2 and jejunal IGF-I mRNA were restored to fed levels. Intragastric refeeding restored intestinal cellularity and functional capacity (sucrase activity and sodium-glucose transporter-1 expression) to fed levels, whereas intravenous refeeding had no effect. Intestinal regeneration after 4 days of intragastric or 2 days of ad libitum refeeding was positively associated with increases in plasma concentrations of GLP-2 and jejunal IGF-I mRNA. These data suggest that luminal nutrients stimulate intestinal growth, in part, by increased expression of both GLP-2 and IGF-I.     

IGFBP-5 regulates muscle cell differentiation by binding to IGF-II and switching on the IGF-II auto-regulation loop

IGF-II stimulates both mitogenesis and myogenesis through its binding and activation of the IGF-I receptor (IGF-IR). How this growth factor pathway promotes these two opposite cellular responses is not well understood. We investigate whether local IGF binding protein-5 (IGFBP-5) promotes the myogenic action of IGF-II. IGFBP-5 is induced before the elevation of IGF-II expression during myogenesis. Knockdown of IGFBP-5 impairs myogenesis and suppresses IGF-II gene expression. IGF-II up-regulates its own gene expression via the PI3K-Akt signaling pathway. Adding IGF-II or constitutively activating Akt rescues the IGFBP-5 knockdown-caused defects. However, an IGF analogue that binds to the IGF-IR but not IGFBP has only a limited effect. When added with low concentrations of IGF-II, IGFBP-5 restores IGF-II expression and myogenic differentiation, whereas an IGF binding–deficient IGFBP-5 mutant has no effect. These findings suggest that IGFBP-5 promotes muscle cell differentiation by binding to and switching on the IGF-II auto-regulation loop.