Signaling through the Smad pathway by insulin-like growth factor-binding protein-3 in breast cancer cells. Relationship to transforming growth factor-beta 1 signaling

Nonsecreted insulin-like growth factor binding protein-3 (IGFBP-3) can induce apoptosis in human prostate cancer cells by IGF-independent mechanisms without being concentrated in the nucleus

Insulin-like growth factor binding protein-3 (IGFBP-3), a secreted protein, has the intrinsic ability to induce apoptosis directly without binding insulin-like growth factors. Previous studies suggested that IGFBP-3 must be secreted to exert its biological functions. IGFBP-3 contains a nuclear localization signal (NLS), and exogenous IGFBP-3 is translocated into the nucleus, suggesting that both secretion and nuclear localization may play important roles in IGFBP-3 action. To address these questions, we fused yellow fluorescent protein (YFP) to mature IGFBP-3 lacking its signal peptide so that it would remain intracellular and mutated the C-terminal NLS of IGFBP-3, (228)KGRKR(232), to MDGEA. Following transfection of PC-3 human prostate cancer cells with these constructs, Western blots indicated that YFP-IGFBP-3 lacking a signal peptide was cell-associated and not present in the extracellular media. Moreover, the fusion protein was not N-glycosylated, indicating that it had not entered the secretory pathway. Confocal imaging showed that intracellular YFP-MDGEA-IGFBP-3 was predominantly cytoplasmic. Transient transfection of nonsecreted YFP-wild-type IGFBP-3 decreased cell viability, as assessed by staining with annexin V followed by flow cytometry. Induction of cell death was caspase-dependent, indicative of apoptosis. Apoptosis also was induced by the nonsecreted NLS mutant (YFP-MDGEA-IGFBP-3) alone and when the IGF-binding site also had been mutated. These results indicate that IGFBP-3 can induce apoptosis in an IGF-independent manner without being secreted or concentrated in the nucleus.

IGF-Binding Protein-3 Modulates TGF-β/BMP-Signaling in Glomerular Podocytes

Podocyte apoptosis initiates progressive glomerulosclerosis in TGF-beta1 transgenic and CD2AP-knockout (CD2AP-/-) mice. It was previously shown that in both mouse models, activation of the TGF-beta pathway is the key event during development of podocyte apoptosis. Furthermore, CD2AP is an important modifier of TGF-beta-induced survival signaling via activation of the phosphoinositol 3-kinase/AKT signaling pathway. This article presents IGF-binding protein-3 (IGFBP-3) as a new modulator of apoptosis and survival signaling in glomerular podocytes. High expression of IGFBP-3 protein in the urine of diseased CD2AP-/- mice was discovered, and IGFBP-3 expression in glomerular podocytes and parietal cells was detected. IGFBP-3 can induce changes in podocyte actin cytoskeleton, leads to apoptosis in cultured murine podocytes, and can enhance TGF-beta1-induced apoptosis in vitro. For studying this process on a molecular level, proapoptotic p38 mitogen-activated protein kinase pathways and antiapoptotic phosphoinositol 3-kinase/AKT pathways were examined in cultured murine podocytes. It was found that IGFBP-3 increments the level of TGF-beta1-induced phosphorylated p38 mitogen-activated protein kinase and decreases the phosphorylation of antiapoptotic AKT. This effect is specific for the co-stimulation of IGFBP-3 with TGF-beta1 because a combination of IGFBP-3 with bone morphogenic protein-7 (BMP-7), another member of the TGF-beta superfamily, results in apoptosis opposing signaling effects with a strong increase of phosphorylated AKT and subsequent functional effects. These results demonstrate that the IGF/IGFBP axis plays an important role in the development of podocyte apoptosis by modulation of TGF-beta and BMP-7-induced pro- and antiapoptotic signals.

Potential of proteomics towards the investigation of the IGF-independent actions of IGFBP-3

Early investigations into the insulin-like growth factor (IGF)-independent actions of insulin-like growth factor-binding protein (IGFBP)-3 have implicated a large array of signaling proteins with links to cell cycle control and apoptosis. However, the actual mechanism of IGFBP-3 action is still unclear. In an effort to clearly understand the mechanism of IGF-independent IGFBP-3 actions, a proteomic approach to identify the actual proteins involved in interaction with IGFBP-3 from different cell compartments, the phosphorylation status of IGFBP-3 under different physiologic conditions and the proteins upregulated by IGFBP-3 are briefly reviewed. The IGF system is a well-recognized key player in diseases such as cancer, diabetes and malnutrition. It is only after the signaling pathways of the IGF-independent actions of IGFBP-3 are clearly understood that the system can be manipulated to affect these disorders.

Insulin-like growth factor binding protein (IGFBP)-3 and IGFBP-5 mediate TGF-beta- and myostatin-induced suppression of proliferation in porcine embryonic myogenic cell cultures

We have previously shown that cultured porcine embryonic myogenic cells (PEMC) produce both insulin-like growth factor binding protein (IGFBP)-3 and IGFBP-5 and secrete these proteins into their media. Exogenously added recombinant porcine (rp) IGFBP-3 and rpIGFBP-5 act via IGF-dependent and IGF-independent mechanisms to suppress proliferation of PEMC cultures. Furthermore, immunoneutralization of endogenous IGFBP-3 and IGFBP-5 in the PEMC culture medium results in increased DNA synthesis rate suggesting that endogenous IGFBP-3 and IGFBP-5 suppress PEMC proliferation. TGF-beta superfamily members myostatin and TGF-beta1 have also been shown to suppress proliferation of myogenic cells, and treatment of cultured PEMC with either TGF-beta1 or myostatin significantly (P < 0.01) increases levels of IGFBP-3 and -5 mRNA. We have previously shown that immunoneutralization of IGFBP-3 decreases the proliferation-suppressing activity of TGF-beta1 and myostatin. Here, we show that immunoneutralization of IGFBP-5 also significantly (P < 0.05) decreases the DNA synthesis-suppressing activity of these molecules. Simultaneous immunoneutralization of both IGFBP-3 and IGFBP-5 in TGF-beta1 or myostatin-treated PEMC cultures restores Long-R3-IGF-I-stimulated DNA synthesis rates to 90% of the levels observed in control cultures receiving no TGF-beta1 or myostatin treatment (P < 0.05). Even though immunoneutralization of IGFBP-3 and -5 increased DNA synthesis rates in TGF-beta1 or myostatin-treated PEMC cultures, phosphosmad2 levels in these cultures were not affected. These findings strongly suggest that IGFBP-3 and IGFBP-5 affect processes downstream from receptor-mediated Smad phosphorylation that facilitate the ability of TGF-beta and myostatin to suppress proliferation of PEMC.

Long-term maintenance of hematopoietic stem cells does not require contact with embryo-derived stromal cells in cocultures

We recently established that two midgestation-derived stromal clones--UG26-1B6, urogenital ridge-derived, and EL08-1D2, embryonic liver-derived--support the maintenance of murine adult bone marrow and human cord blood hematopoietic repopulating stem cells (HSCs). In this study, we investigate whether direct HSC-stroma contact is required for this stem cell maintenance. Adult bone marrow ckit+ Ly-6C- side population (K6-SP) cells and stromal cells were cocultured under contact or noncontact conditions. These experiments showed that HSCs were maintained for at least 4 weeks in culture and that direct contact between HSCs and stromal cells was not required. To find out which factors might be involved in HSC maintenance, we compared the gene expression profile of EL08-1D2 and UG26-1B6 with four HSC-nonsupportive clones. We found that EL08-1D2 and UG26-1B6 both expressed 21 genes at a higher level, including the putative secreted factors fibroblast growth factor-7, insulin-like growth factor-binding proteins 3 and 4, pleiotrophin, pentaxin-related, and thrombospondin 2, whereas 11 genes, including GPX-3 and HSP27, were expressed at a lower level. In summary, we show for the first time long-term maintenance of adult bone marrow HSCs in stroma noncontact cultures and identify some secreted molecules that may be involved in this support.

Roles of insulin-like growth factor (IGF) binding proteins in regulating IGF actions

The insulin-like growth factor (IGF) system is an evolutionarily conserved signaling pathway that is composed of two IGF ligands, two IGF receptors, and six IGF binding proteins. Studies in a variety of species suggest that the IGF signaling system plays a fundamental role in regulating embryonic growth and differentiation as well as in maintaining homeostasis in the adults. In extracellular fluids, IGFs are present in a complex with an IGF-binding protein (IGFBP). These IGFBPs are traditionally thought to function as carrier proteins and regulate circulating IGF turnover, transport, and distribution. Locally expressed IGFBPs can also inhibit and/or potentiate IGF activities. Recent studies have shown that some IGFBPs, in particular IGFBP-3 and -5, possess intrinsic biological activities and can act through IGF-independent mechanisms. In this article, we provide a brief overview of our current understanding of the IGF signaling system with particular reference to IGFBPs.

Insulin-like Growth Factor-binding Protein-3 Plays an Important Role in Regulating Pharyngeal Skeleton and Inner Ear Formation and Differentiation

Insulin-like growth factor-binding protein (IGFBP)-3 is the major insulin-like growth factor (IGF) carrier protein in the bloodstream. IGFBP-3 prolongs the half-life of circulating IGFs and prevents their potential hypoglycemic effect. IGFBP-3 is also expressed in many peripheral tissues in fetal and adult stages. In vitro, IGFBP-3 can inhibit or potentiate IGF actions and even possesses IGF-independent activities, suggesting that local IGFBP-3 may also have paracrine/autocrine function(s). The in vivo function of IGFBP-3, however, is unclear. In this study, we elucidate the developmental role of IGFBP-3 using the zebrafish model. IGFBP-3 mRNA expression is first detected in the migrating cranial neural crest cells and subsequently in pharyngeal arches in zebrafish embryos. IGFBP-3 mRNA is also persistently expressed in the developing inner ears. To determine the role of IGFBP-3 in these tissues, we ablated the IGFBP-3 gene product using morpholino-modified antisense oligonucleotides (MOs). The IGFBP-3 knocked down embryos had delayed pharyngeal skeleton morphogenesis and greatly reduced pharyngeal cartilage differentiation. Knockdown of IGFBP-3 also significantly decreased inner ear size and disrupted hair cell differentiation and semicircular canal formation. Furthermore, reintroduction of a MO-resistant form of IGFBP-3 "rescued" the MO-induced defects. These findings suggest that IGFBP-3 plays an important role in regulating pharyngeal cartilage and inner ear development and growth in zebrafish.

Functional promoter upstream p53 regulatory sequence of IGFBP3 that is silenced by tumor specific methylation

BACKGROUND

Insulin-like growth factor binding protein (IGFBP)-3 functions as a carrier of insulin-like growth factors (IGFs) in circulation and a mediator of the growth suppression signal in cells. There are two reported p53 regulatory regions in the IGFBP3 gene; one upstream of the promoter and one intronic. We previously reported a hot spot of promoter hypermethylation of IGFBP-3 in human hepatocellular carcinomas and derivative cell lines. As the hot spot locates at the putative upstream p53 consensus sequences, these p53 consensus sequences are really functional is a question to be answered.

METHODS

In this study, we examined the p53 consensus sequences upstream of the IGFBP-3 promoter for the p53 induced expression of IGFBP-3. Deletion, mutagenesis, and methylation constructs of IGFBP-3 promoter were assessed in the human hepatoblastoma cell line HepG2 for promoter activity.

RESULTS

Deletions and mutations of these sequences completely abolished the expression of IGFBP-3 in the presence of p53 overexpression. In vitro methylation of these p53 consensus sequences also suppressed IGFBP-3 expression. In contrast, the expression of IGFBP-3 was not affected in the absence of p53 overexpression. Further, we observed by electrophoresis mobility shift assay that p53 binding to the promoter region was diminished when methylated.

CONCLUSION

From these observations, we conclude that four out of eleven p53 consensus sequences upstream of the IGFBP-3 promoter are essential for the p53 induced expression of IGFBP-3, and hypermethylation of these sequences selectively suppresses p53 induced IGFBP-3 expression in HepG2 cells.

Lysophosphatidic Acid Interacts with Transforming Growth Factor-β Signaling to Mediate Keratinocyte Growth Arrest and Chemotaxis

Lysophosphatidic acid (LPA, 1-acyl-glycerol-3-phosphate) plays an important role in diverse biological responses including cell proliferation, differentiation, survival, migration, and tumor cell invasion. The most prominent source of LPA is platelets from which it is released after thrombin activation and is assumed to be an essential function of this lysophospholipid in cutaneous wound closure. Therefore, we examined the role of LPA on biological responses of keratinocytes. Although LPA potently enhances keratinocyte migration, it strongly induces growth arrest of proliferating epidermal cells. Thus, LPA possesses analogous actions to transforming growth factor-beta (TGF-beta), which is also released from degranulating platelets at wounded sites. In contrast to LPA, the intracellular signaling events of TGF-beta have been clearly identified and indicate that Smad3 is involved in chemotaxis and cell growth arrest of keratinocytes induced by this cytokine. Here we show that LPA, although it does not alter TGF-beta release is capable to activate Smad3 and results in a heteromerization with Smad4 and binding of the complex to its specific DNA-promoter elements. LPA completely fails to induce chemotaxis in Smad3-deficient cells, whereas growth inhibition is at least in part reduced. These findings indicate an essential role of Smad3 in diverse biological properties of LPA-stimulated keratinocytes.

IGFBP-3 activates TGF-beta receptors and directly inhibits growth in human intestinal smooth muscle cells

We have shown that human intestinal smooth muscle cells produce IGF-I and IGF binding protein-3 (IGFBP-3). Endogenous IGF-I acts in autocrine fashion to stimulate growth of these cells. IGFBP-3 inhibits the binding of IGF-I to its receptor and thereby inhibits IGF-I-stimulated growth. In several carcinoma cell lines and some normal cells, IGFBP-3 regulates growth independently of IGF-I. Two mechanisms for this effect have been identified: IGFBP-3 can directly activate transforming growth factor-beta (TGF-beta) receptors or it can undergo direct nuclear translocation. The aim of the present study was to determine whether IGFBP-3 acts independently of IGF-I and to characterize the mechanisms mediating this effect in human intestinal smooth muscle cells. The direct effects of IGFBP-3 were determined in the presence of an IGF-I receptor antagonist to eliminate its IGF-I-dependent effects. Affinity labeling of TGF-beta receptors (TGF-betaRI, TGF-betaRII, and TGF-betaRV) with 125I-labeled TGF-beta1 showed that IGFBP-3 displaced binding to TGF-betaRII and TGF-betaRV in a concentration-dependent fashion. IGFBP-3 stimulated TGF-betaRII-dependent serine phosphorylation (activation) of both TGF-betaRI and of its primary substrate, Smad2(Ser465/467). IGFBP-3 also caused IGF-I-independent inhibition of basal [3H]thymidine incorporation. The effects of IGFBP-3 on Smad2 phosphorylation and on smooth muscle cell proliferation were independent of TGF-beta1 and were abolished by transfection of Smad2 siRNA. Immunoneutralization of IGFBP-3 increased basal [3H]thymidine incorporation, implying that endogenous IGFBP-3 inhibits proliferation. We conclude that endogenous IGFBP-3 directly inhibits proliferation of human intestinal smooth muscle cells by activation of TGF-betaRI and Smad2, an effect that is independent of its effect on IGF-I-stimulated growth.

IGF-binding proteins mediate TGF-beta 1-induced apoptosis in bovine mammary epithelial BME-UV1 cells

TGF-beta 1 is an antiproliferative and apoptogenic factor for mammary epithelial cells (MEC) acting in an auto/paracrine manner and thus considered an important local regulator of mammary tissue involution. However, the apoptogenic signaling pathway induced by this cytokine in bovine MEC remains obscure. The present study was focused on identification of molecules involved in apoptogenic signaling of transforming growth factor-beta 1 (TGF-beta 1) in the model of bovine mammary epithelial cell line (BME-UV1). Laser scanning cytometry (LSC), Western blot and electrophoretic mobility shift assay (EMSA) were used for analysis of expression and activity of TGF-beta 1-related signaling molecules. The earliest response occurring within 1-2 h after TGF-beta 1 administration was an induction and activation of R-Smads (Smad2 and Smad3) and Co-Smad (Smad4). An evident formation of Smad-DNA complexes began from 2nd hour after MEC exposure to TGF-beta 1. Similarly to Smads, proteins of AP1 complex: phosphorylated c-Jun and JunD appeared to be early reactive molecules; however, an increase in their expression was detected only in cytosolic fraction. In the next step, an increase of IGF binding protein-3 (IGFBP-3) and IGFBP-4 expression was observed from 6th hour followed by a decrease in the activity of protein kinase B (PKB/Akt), which occurred after 24 h of MEC exposure to TGF-beta 1. The decrease in PKB/Akt activity coincided in time with the decline of phosphorylated Bad expression (inactive form). Present study supported additional evidence that stimulation of insulin-like growth factor I (IGF-I) was associated with complete abrogation of TGF-beta 1-induced activation of Bad and Bax and in the consequence protection against apoptosis. In conclusion, apoptotic effect of TGF-beta 1 in bovine MEC is mediated by IGFBPs and occurs through IGF-I sequestration, resulting in inhibition of PKB/Akt-dependent survival pathway.

Effect of recombinant porcine IGFBP-3 on IGF-I and long-R3-IGF-I-stimulated proliferation and differentiation of L6 myogenic cells

Insulin-like growth factor (IGF)-I stimulates both proliferation and differentiation of myogenic precursor cells. In vivo, IGFs are bound to one of the members of a family of six high-affinity IGF binding proteins (IGFBP 1-6) that regulate their biological activity. One of these binding proteins, IGFBP-3, affects cell proliferation via both IGF-dependent and IGF-independent mechanisms and it has generally been shown to suppress proliferation of cultured cells; however, it also may stimulate proliferation depending upon the cell type and the assay conditions. Cultured porcine embryonic myogenic cells (PEMCs) produce IGFBP-3 and its level drops significantly immediately prior to differentiation. Additionally, IGFBP-3 suppresses both IGF-I and Long-R3-IGF-I-stimulated proliferation of embryonic porcine myogenic cells. In this study, we have examined the effects of recombinant porcine IGFBP-3 (rpIGFBP-3) on IGF-I- and Long-R3-IGF-I-stimulated proliferation and differentiation of the L6 myogenic cell line. L6 cells potentially provide a good model for studying the actions of IGFBP-3 on muscle because they contain no non-muscle cells and they do not produce detectable levels of IGFBP-3. RpIGFBP-3 suppresses both IGF-I and Long-R3-IGF-I-stimulated proliferation of L6 cells, indicating that it suppresses proliferation via both IGF-dependent and IGF-independent mechanisms. Our data also show that rpIGFBP-3 causes IGF-independent suppression of proliferation without increasing the level of phosphosmad-2 in L6 cultures. Additionally, rpIGFBP-3 suppresses IGF-I-stimulated differentiation of L6 cells. In contrast, however, rpIGFBP-3 does not suppress Long-R3-IGF-I-stimulated differentiation. This suggests that rpIGFBP-3 does not have IGF-independent effects on L6 cell differentiation.

Involvement of Smad signaling in sphingosine 1-phosphate-mediated biological responses of keratinocytes

The lysophospholipid sphingosine 1-phosphate and the cytokine-transforming growth factor beta are both released from degranulating platelets at wound sites, suggesting a broad spectrum of effects involved in wound healing. Interestingly, both of these molecules have been previously shown to induce chemotaxis but to strongly inhibit the growth of keratinocytes, while stimulating the proliferation of fibroblasts. In contrast to sphingosine 1-phosphate, the signaling cascade of the growth factor has been extensively examined. Specifically, Smad3 has been shown to be an essential mediator of transforming growth factor beta-dependent chemotaxis of keratinocytes and mediates, in part, its growth-inhibitory effect. Here we show that sphingosine 1-phosphate, independently of transforming growth factor beta secretion, induces a rapid phosphorylation of Smad3 on its C-terminal serine motif and induces its partnering with Smad4 and the translocation of the complex into the nucleus. Moreover, sphingosine 1-phosphate fails to induce chemotaxis or inhibit the growth of Smad3-deficient keratinocytes, suggesting that Smad3 plays an unexpected functional role as a new target in sphingosine 1-phosphate signaling. Both sphingosine 1-phosphate receptors and the transforming growth factor beta-type I receptor serine/threonine kinase are essential for activation of Smad3 by this lysophospholipid and the dependent biological responses, indicating a novel cross-talk between serine/threonine kinase receptors and G-protein coupled receptors.

1alpha,25-dihydroxyvitamin D3 induced growth inhibition of PC-3 prostate cancer cells requires an active transforming growth factor beta signaling pathway

BACKGROUND

Prostate cancer growth inhibition by 1alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3) is best characterized in the androgen dependent LNCaP cell line, where treatment with this hormone causes cell cycle arrest and apoptosis. 1,25(OH)2D3 also inhibits the growth of PC-3 prostate cancer cells, but not through the induction of G1 arrest or apoptosis. In this study, we have sought to elucidate the mechanism/s involved in PC-3 cell growth inhibition by 1,25(OH)2D3.

EXPERIMENTAL METHODS

We determined the effect of transforming growth factor beta (TGFbeta) blocking antibodies on 1,25(OH)2D3 mediated growth inhibition of PC-3 cells. In addition, we also studied the effects of 1,25(OH)2D3 on TGFbeta signaling and receptor expression. Finally, we assessed the role of TGFbeta signaling in the induction of the growth inhibitory protein, insulin like growth factor binding protein 3 (IGFBP-3), by 1,25(OH)2D3.

RESULTS

We find that 1,25(OH)2D3 action in PC-3 cells is mediated through at least two distinct pathways, the TGFbeta pathway and the IGFBP-3 pathway. We show that 1,25(OH)2D3 treatment elevates TGFbeta production and signaling, as well as receptor levels, in PC-3 cells. Further, using a blocking antibody against TGFbeta substantially reduces 1,25(OH)2D3 mediated growth inhibition without affecting IGFBP-3 induction, suggesting that IGFBP-3, alone, is insufficient to inhibit the growth of PC-3 cells.

The roles of IGF-I and IGFBP-3 in the regulation of proximal tubule, and renal cell carcinoma cell proliferation

BACKGROUND

Insulin-like growth factor I (IGF-I), a potent proximal tubule cell (PTC) mitogen, has been implicated in the progression of many human cancers. Our previous work on human renal tissues has suggested that IGF-I and several of its binding proteins (IGFBP-3 and -6) are up-regulated in clear cell renal cell carcinoma (RCC).

METHODS

To further elucidate the role of IGF-I and IGFBPs in RCC growth, immunohistochemistry, thymidine incorporation, and Western analysis were performed in primary cultures of normal PTC (priPTC) and clear-cell RCC (priRCC), as well as in SN12K1 cells (a cell line derived from metastatic RCC).

RESULTS

By immunohistochemistry, IGFBP-3 and IGF-I were prominently expressed in SN12K1 cells, and weakly expressed in priPTC and priRCC. Incubation with 100 ng/mL IGF-I significantly augmented DNA synthesis by priPTC (mean +/- SD 120.7%+/- 19.7% of controls, P < 0.05), priRCC (238.7%+/- 279.9% of controls, P < 0.01), and SN12K1(120.0%+/- 22.9% of controls, P < 0.05). Neutralizing antibodies to IGF-I and IGF-I receptor significantly suppressed SN12K1 growth (81.9%+/- 13.5% of control, P < 0.01 and 87.4%+/- 16.2% of control, P < 0.05, respectively). Removal of endogenous IGFBP-3 by an anti-IGFBP-3 increased SN12K1 DNA synthesis (243.9%+/- 35.3% of control, P < 0.001), which was partially abrogated by coincubation with exogenous IGFBP-3 (135.97%+/- 5.9% of controls, P < 0.001). Using Western analysis, IGFBP-3 expression was enhanced in IGF-I-stimulated SN12K1 cells exposed to exogenous IGF-I. Coincubation with anti-IGFBP-3 further enhanced IGF-I-induced DNA synthesis.

CONCLUSION

RCC cells express IGF-I and IGFBP-3, and are responsive to exogenous IGF-I stimulation. Moreover, in SN12K1 cells (derived from metastatic RCC), autocrine IGF-I and IGFBP-3 actions, respectively, stimulated and inhibited growth. These results suggest that IGF-I and IGFBP-3 may be potential candidates for therapeutic manipulation in patients with advanced RCC.

Insulin-like growth factor binding protein-3 stimulates phosphatidylinositol 3-kinase in MCF-7 breast carcinoma cells

Insulin-like growth factor binding protein-3 (IGFBP-3) is the most abundant IGFBP in serum and other biological fluids. Apart from its capacity for specific and high-affinity binding to IGFs, it also has so-called "IGF-independent" activities that modulate cell proliferation and survival/apoptosis. However, the molecular elements of the IGFBP-3 signalling pathway remain obscure. In this study, we investigated the possible implication of phosphatidylinositol 3-kinase (PI 3-kinase) activity in MCF-7 breast carcinoma cells. In cells incubated with IGFBP-3, both total and insulin receptor substrate-1 (IRS-1)-associated PI 3-kinase activities were rapidly stimulated, with maximal effects after 3 and 10min of incubation, respectively. IGFBP-3-induced PI 3-kinase activity was unaffected by the state of IRS-1 tyrosine phosphorylation. Since IGFBP-3 failed to stimulate PI 3-kinase activity in MDA-MB 231 breast carcinoma cells, its effects in MCF-7 cells could be considered as cell-type-specific. Pertussis toxin abolished IGFBP-3-stimulation of PI 3-kinase activity, suggesting that this IGFBP-3 signalling pathway depends upon a pertussis toxin-sensitive G protein. Our results provide further evidence that IGFBP-3 directly triggers a specific intracellular signal in MCF-7 cells.

Role of insulin-like growth factor binding proteins (IGFBPs) in breast cancer proliferation and metastasis

Cancers of the breast, prostate, and lung commonly metastasize to the bone resulting in osteolysis, pathologic fracture, pain and significant clinical morbidity. To date, the reason for such selectivity in the site of metastasis remains largely unknown. The bone is a rich source of many chemokines and growth factors, including: insulin-like growth factor (IGF) I and II, transforming growth factor-beta (TGF-beta), interleukins, and tumour necrosis factor-alpha (TNF-alpha). We propose that exposure of breast cancer cells to the bone microenvironment results in alterations in gene expression that favour the growth and proliferation of tumour cells in the bone. To investigate this hypothesis, MDA-MB-231 breast carcinoma cells were exposed to bone-derived conditioned media (BDCM) generated by culturing fetal rat calvaria for 24 h under serum free conditions. Using cDNA microarray technology, we have identified the insulin-like growth factor family of binding proteins (IGFBPs) as genes whose expression profiles are consistently and significantly altered with exposure to this simulated bone environment in vitro, when compared to untreated controls. Our data suggests that the upregulation of IGFBP-3 seen with exposure to the bone microenvironment is directly linked to an increase in TGF-beta mediated cell proliferation. Furthermore, this process appears to be functioning through an IGF-independent mechanism.

Epidermal homeostasis: the role of the growth hormone and insulin-like growth factor systems

GH and IGF-I and -II were first identified by their endocrine activity. Specifically, IGF-I was found to mediate the linear growth-promoting actions of GH. It is now evident that these two growth factor systems also exert widespread activity throughout the body and that their actions are not always interconnected. The literature highlights the importance of the GH and IGF systems in normal skin homeostasis, including dermal/epidermal cross-talk. GH activity, sometimes mediated via IGF-I, is primarily evident in the dermis, particularly affecting collagen synthesis. In contrast, IGF action is an important feature of the dermal and epidermal compartments, predominantly enhancing cell proliferation, survival, and migration. The locally expressed IGF binding proteins play significant and complex roles, primarily via modulation of IGF actions. Disturbances in GH and IGF signaling pathways are implicated in the pathophysiology of several skin perturbations, particularly those exhibiting epidermal hyperplasia (e.g., psoriasis, carcinomas). Additionally, many studies emphasize the potential use of both growth factors in the treatment of skin wounds; for example, burn patients. This overview concerns the role and mechanisms of action of the GH and IGF systems in skin and maintenance of epidermal integrity in both health and disease.

Role of insulin-like growth factor binding protein (IGFBP)-3 in TGF-beta- and GDF-8 (myostatin)-induced suppression of proliferation in porcine embryonic myogenic cell cultures

Both transforming growth factor (TGF-beta) and growth and development factor (GDF)-8 (myostatin) affect muscle differentiation by suppressing proliferation and differentiation of myogenic cells. In contrast, insulin-like growth factors (IGFs) stimulate both proliferation and differentiation of myogenic cells. In vivo, IGFs are found in association with a family of high-affinity insulin-like growth factor binding proteins (IGFBP 1-6) that affect their biological activity. Treatment of porcine embryonic myogenic cell (PEMC) cultures with either TGF-beta(1) or GDF-8 suppressed proliferation and increased production of IGFBP-3 protein and mRNA (P < 0.005). An anti-IGFBP-3 antibody that neutralizes the biological activity of IGFBP-3 reduced the ability of either TGF-beta(1) or GDF-8 to suppress PEMC proliferation (P < 0.005). However, this antibody did not affect proliferation rate in the presence of both TGF-beta(1) and GDF-8. These data show that IGFBP-3 plays a role in mediating the activity of either TGF-beta(1) or GDF-8 alone but not when both TGF-beta(1) and GDF-8 are present. In contrast to findings in T47D breast cancer cells, treatment of PEMC cultures with IGFBP-3 did not result in increased levels of phosphosmad-2. Since TGF-beta and GDF-8 are believed to play a significant role in regulating proliferation and differentiation of myogenic cells, our current data showing that IGFBP-3 plays a role in mediating the activity of these growth factors in muscle cell cultures strongly suggest that IGFBP-3 also may be involved in regulating these processes in myogenic cells.

Interaction of insulin-like growth factor binding protein-3 with latent transforming growth factor-beta binding protein-1

Insulin-like growth factor binding protein-3 (IGFBP-3) inhibits the replication and promotes apoptosis in various cell lines in an IGF-independent manner. We utilized a yeast two-hybrid system to identify binding partners for IGFBP-3 in a mouse embryo cDNA library. A partial cDNA encoding mouse latent transforming growth factor beta (TGF-beta) binding protein-1 (LTBP-1) was identified. This cDNA encoded a mouse LTBP-1 mRNA fragment corresponding to amino acid residues 1160-1712. Analysis of C-terminal deleted mutants indicated that the IGFBP-3 interacting domain resides in the 552 residue C-terminal fragment encoding amino acids 831-1383. The interaction of IGFBP-3 with recombinant human LTBP-1 immobilized on nitrocellulose was also demonstrated. Neither binding of IGF-1 to IGFBP-3 nor binding of latency associated protein (LAP) with LTBP-1 inhibited the interaction of IGFBP-3 with LTBP-1. Furthermore the large latent complex, 125I-TGF-beta/LAP/LTBP-1 was able to bind to immobilized IGFBP-3. These data demonstrate that IGFBP-3 can bind to LTBP-1 and provide a potential mechanism whereby IGFBP-3 can interact with the TGF-beta system.

Expression and regulation of intracellular SMAD signaling in scleroderma skin fibroblasts

OBJECTIVE

Scleroderma is characterized by excessive synthesis and accumulation of matrix proteins in lesional tissues. Transforming growth factor beta (TGFbeta) plays a central role in the pathogenesis of fibrosis by inducing and sustaining activation of fibroblasts; however, the underlying mechanisms are poorly understood. We undertook this study to examine the expression and function of SMADs, recently characterized intracellular effectors of TGFbeta signaling, in scleroderma fibroblasts.

METHODS

Primary dermal fibroblasts obtained from 14 patients with scleroderma and from 4 healthy adult volunteers were studied. Northern analysis was used to determine the expression of endogenous SMAD messenger RNA (mRNA), and Western analysis was used to determine SMAD protein expression. Intracellular compartmentalization of cellular SMAD proteins in the presence and absence of TGFbeta was studied by antibody-mediated immunofluorescence confocal microscopy. The effect of TGFbeta blockade on SMAD subcellular distribution was determined using anti-TGFbeta antibodies as well as a dominant-negative TGFbeta receptor type II (TGFbetaRII) vector to disrupt TGFbeta responses. SMAD-regulated luciferase reporter expression was examined to investigate the potential functional significance of activation and nuclear accumulation of endogenous SMADs in scleroderma fibroblasts.

RESULTS

Protein and mRNA levels of SMAD3, but not of SMAD4 or SMAD7, were variably elevated in scleroderma fibroblasts compared with those from healthy controls. In sharp contrast to control fibroblasts, which displayed predominantly cytoplasmic localization of SMAD3/4 in the absence of exogenous TGFbeta, in scleroderma fibroblasts SMAD3 and SMAD4 consistently showed elevated nuclear localization. Furthermore, phosphorylated SMAD2/3 levels were elevated and nuclear localization of phosphorylated SMAD2/3 was increased, suggesting activation of the SMAD pathway in scleroderma fibroblasts. Blockade of autocrine TGFbeta signaling with antibodies or by expression of dominant-negative TGFbetaRII failed to normalize SMAD subcellular distribution, suggesting that elevated nuclear SMAD import was due to alterations downstream of the TGFbeta receptors. The activity of a SMAD-responsive minimal promoter-reporter construct was enhanced in transiently transfected scleroderma fibroblasts.

CONCLUSION

This study is the first to demonstrate apparently ligand-independent constitutive activation of the intracellular TGFbeta/SMAD signaling axis in scleroderma fibroblasts. SMAD signaling may be a mechanism contributing to the characteristic phenotype of scleroderma fibroblasts and playing a role in the pathogenesis of fibrosis.

Insulin-like growth factor binding protein-3 is overexpressed in endothelial cells of mouse breast tumor vessels

Angiogenesis is a key process in a variety of human diseases, including cancer. The ability to target selectively the tumor vasculature is potentially useful for the diagnosis and treatment of cancer. Still, little information is available regarding markers that are restricted to the ECs of tumor vessels. cDNA array technology allows simultaneous analysis of relative expression levels of a broad spectrum of genes in 2 related cell populations. We used this technology with the aim of identifying markers specific for TECs. TECs were isolated by CD31-mediated immunomagnetic separation from tumors induced by s.c. injection of NF9006 breast carcinoma cells into syngeneic mice. NECs were isolated from lactating mammary glands. The endothelial nature of isolated cells was confirmed by RT-PCR using CD31-specific primers and by uptake of DiI-Ac-LDL. Macrophage contamination in the EC isolations could be reasonably ruled out by assessing the expression of the macrophage marker c-fms. (32)P-labeled cDNA probes generated by reverse transcription from total RNA were hybridized to mouse-specific gene arrays. Several genes consistently showed differential expression between TECs and NECs. However, expression of only 1 of these genes, IGFBP-3, was restricted exclusively to ECs. Semiquantitative RT-PCR revealed 22- to 33-fold differential expression of IGFBP-3 in the TEC fraction. IGFBP-3 was overexpressed by a factor of 5 in an additional mouse model of breast carcinoma induced by 4T1.2 tumor cells. These results indicate that IGFBP-3 is a potential novel marker of angiogenesis. Elucidation of its role in tumor neovascularization may open the possibility of IGFBP-3 as a therapeutic target for antiangiogenesis.

Insulin-like growth factor-binding protein-3 potentiates epidermal growth factor action in MCF-10A mammary epithelial cells. Involvement of p44/42 and p38 mitogen-activated protein kinases

Insulin-like growth factor-binding protein-3 (IGFBP-3) is inhibitory to the growth of many breast cancer cells in vitro; however, a high level of expression of IGFBP-3 in breast tumors correlates with poor prognosis, suggesting that IGFBP-3 may be associated with growth stimulation in some breast cancers. We have shown previously in MCF-10A breast epithelial cells that chronic activation of Ras-p44/42 mitogen-activated protein (MAP) kinase confers resistance to the growth-inhibitory effects of IGFBP-3 (Martin, J. L., and Baxter, R. C. (1999) J. Biol. Chem. 274, 16407-16411). Here we show that, in the same cell line, IGFBP-3 potentiates DNA synthesis and cell proliferation stimulated by epidermal growth factor (EGF), a potent activator of Ras. A mutant of IGFBP-3, which fails to translocate to the nucleus and has reduced ability to cell-associate, similarly enhanced EGF action in these cells. By contrast, the structurally related IGFBP-5, which shares many functional features with IGFBP-3, was slightly inhibitory to DNA synthesis in the presence of EGF. IGFBP-3 primes MCF-10A cells to respond to EGF because pre-incubation caused a similar degree of EGF potentiation as co-incubation. In IGFBP-3-primed cells, EGF-stimulated EGF receptor phosphorylation at Tyr-1068 was increased relative to unprimed cells, as was phosphorylation and activity of p44/42 and p38 MAP kinases, but not Akt/PKB. Partial blockade of the p44/42 and p38 MAP kinase pathways abolished the potentiation by IGFBP-3 of EGF-stimulated DNA synthesis. Collectively, these findings indicate that IGFBP-3 enhances EGF signaling and proliferative effects in breast epithelial cells via increased EGF receptor phosphorylation and activation of p44/42 and p38 MAP kinase signaling pathways.

Effect of differentiation on levels of insulin-like growth factor binding protein mRNAs in cultured porcine embryonic myogenic cells

Insulin-like growth factor binding proteins (IGFBPs) have been shown to affect proliferation of several cell types via insulin-like growth factor (IGF)-dependent and IGF-independent mechanisms. The goal of this study was to determine if levels of IGFBP-2, -3, -4 and -5 mRNA changed during differentiation of cultured porcine embryonic myogenic cells. Total RNA was isolated from muscle cultures at various stages of differentiation and Northern blots of this RNA were probed with 32P-labeled cDNA probes specific for individual IGFBPs. Fusion, myogenin mRNA, and creatine phosphokinase activity were used as markers of differentiation. The level of IGFBP-3 mRNA in differentiating cultures (120 h in culture) was only one-third of the level in myogenin negative, nonfused cultures (72 h in culture) (P < 0.05, n = 4). In contrast, the level of IGFBP-3 mRNA in extensively fused cultures (144 h in culture) was increased by three-fold as compared to the level in myogenin negative, nonfused cultures (P < 0.05, n = 4) and approximately seven-fold as compared to the 120-h cultures (P < 0.05, n = 4). No significant change in the level of IGFBP-5 mRNA was observed during differentiation of myogenic cultures. IGFBP-2 mRNA levels were not significantly different at 72, 96 and 120 h, but at 144 h IGFBP-2 mRNA level was increased three-fold as compared to nonfused cultures (72 h) (P < 0.05, n = 4). IGFBP-4 mRNA was not detectable on Northern blots of total RNA from porcine myogenic cultures at any stage of differentiation. Changes in IGFBP-3 and IGFBP-2 mRNA levels are associated with differentiation of embryonic porcine myogenic cells in culture and this may indicate that these IGFBPs play a role in differentiation of these cells.

Cellular actions of the insulin-like growth factor binding proteins

In addition to their roles in IGF transport, the six IGF-binding proteins (IGFBPs) regulate cell activity in various ways. By sequestering IGFs away from the type I IGF receptor, they may inhibit mitogenesis, differentiation, survival, and other IGF-stimulated events. IGFBP proteolysis can reverse this inhibition or generate IGFBP fragments with novel bioactivity. Alternatively, IGFBP interaction with cell or matrix components may concentrate IGFs near their receptor, enhancing IGF activity. IGF receptor-independent IGFBP actions are also increasingly recognized. IGFBP-1 interacts with alpha(5)beta(1) integrin, influencing cell adhesion and migration. IGFBP-2, -3, -5, and -6 have heparin-binding domains and can bind glycosaminoglycans. IGFBP-3 and -5 have carboxyl-terminal basic motifs incorporating heparin-binding and additional basic residues that interact with the cell surface and matrix, the nuclear transporter importin-beta, and other proteins. Serine/threonine kinase receptors are proposed for IGFBP-3 and -5, but their signaling functions are poorly understood. Other cell surface IGFBP-interacting proteins are uncharacterized as functional receptors. However, IGFBP-3 binds and modulates the retinoid X receptor-alpha, interacts with TGFbeta signaling through Smad proteins, and influences other signaling pathways. These interactions can modulate cell cycle and apoptosis. Because IGFBPs regulate cell functions by diverse mechanisms, manipulation of IGFBP-regulated pathways is speculated to offer therapeutic opportunities in cancer and other diseases.

Type-1 transforming growth factor-beta differentially modulates tumoricidal activity of murine peritoneal macrophages against metastatic variants of the B16 murine melanoma

Transforming growth factor-beta 1 (TGF-beta 1) renders mouse peritoneal macrophages tumoricidal against metastatic variants of the B16 mouse melanoma in vitro. Both direct cytotoxicity and indirect cytotoxicity were observed. A subthreshold concentration (10 U/ml) of recombinant murine interferon-gamma (rMuIFN-gamma) enhanced the direct tumoricidal activity of TGF-beta 1-activated macrophages from 29% to 88% but did not change their indirect tumoricidal profile. Data obtained from macrophages preincubated with either TGF-beta 1 or rMuIFN-gamma showed that TGF-b1 can initiate tumoricidal activity better than rMuIFN-gamma. These effects were plasma-membrane mediated because targeting macrophages with liposomal TGF-beta 1 was ineffective. The order of tumoricidal susceptibility of the B16 melanoma lines to activated macrophages was B16F1 > B16F10 > B16BL6, in inverse order of metastatic potential.

Secretory leukocyte protease inhibitor mediates proliferation of human endometrial epithelial cells by positive and negative regulation of growth-associated genes

Secretory leukocyte protease inhibitor (SLPI) inhibits chymotrypsin, trypsin, elastase, and cathepsin G. This protein also exhibits proliferative effects, although little is known about the molecular mechanisms underlying this activity. We have generated SLPI-ablated epithelial sublines by stably transfecting the Ishikawa human endometrial cell line with an antisense human SLPI RNA expression vector. We demonstrate a positive correlation between cellular SLPI production and proliferation. We further show that Ishikawa sublines expressing low to undetectable SLPI have correspondingly increased and decreased expression, respectively, of transforming growth factor-beta 1 and cyclin D1 genes, relative to parental cells. SLPI selectively increased cyclin D1 gene expression, with the effect occurring in part at the level of promoter activity. Cellular SLPI levels negatively influenced the anti-proliferative and pro-apoptotic insulin-like growth factor-binding protein-3 expression. We also identified lysyl oxidase, a phenotypic inhibitor of the ras oncogenic pathway and a tumor suppressor, as SLPI-repressed gene, whose expression is up-regulated by transforming growth factor-beta1. Our results suggest that SLPI acts at the node(s) of at least three major interacting growth inhibitory pathways. Because expression of SLPI is generally high in epithelial cells exhibiting abnormal proliferation such as in carcinomas, SLPI may define a novel pathway by which cellular growth is modulated.

IGF-binding protein-3-induced growth inhibition and apoptosis do not require cell surface binding and nuclear translocation in human breast cancer cells

IGF-binding protein-3 (IGFBP-3) has both antiproliferative and proapoptotic effects on the growth of human breast cancer cells in vitro. However, the mechanisms governing these effects are not well understood. IGFBP-3 has been shown to associate with the cell surface through carboxyl-terminal residues. This suggests that it may interact with a specific cell surface receptor, although a signaling receptor for IGFBP-3 has not yet been fully characterized. IGFBP-3 also translocates to the nucleus and has been shown to interact with the nuclear RXRalpha, with evidence that this interaction may mediate its growth inhibitory and proapoptotic effects. Here we demonstrate that a mutant form of IGFBP-3 that has reduced cell surface binding and does not translocate to the nucleus is still growth inhibitory, elicits a potent G(1) cell cycle arrest, and induces apoptosis via modulation of Bcl-2 family members in human breast cancer cells. This suggests the existence of multiple pathways by which IGFBP-3 elicits its growth effects.