Lipid rafts/caveolae are essential for insulin-like growth factor-1 receptor signaling during 3T3-L1 preadipocyte differentiation induction

(alpha)v(beta)3 integrins and Pyk2 mediate insulin-like growth factor I activation of Src and mitogen-activated protein kinase in 3T3-L1 cells

IGF-I stimulates cell growth through interaction of the IGF receptor with multiprotein signaling complexes. However, the mechanisms of IGF-I receptor-mediated signaling are not completely understood. We have previously shown that IGF-I-stimulated 3T3-L1 cell proliferation is dependent on Src activation of the ERK-1/2 MAPK pathway. We hypothesized that IGF-I activation of the MAPK pathway is mediated through integrin activation of Src-containing signaling complexes. The disintegrin echistatin decreased IGF-I phosphorylation of Src and MAPK, and blocking antibodies to (alpha)v and beta3 integrin subunits inhibited IGF-I activation of MAPK, suggesting that (alpha)v(beta)3 integrins mediate IGF-I mitogenic signaling. IGF-I increased ligand binding to (alpha)v(beta)3 as detected by immunofluorescent staining of ligand-induced binding site antibody and stimulated phosphorylation of the beta3 subunit, consistent with inside-out activation of (alpha)v(beta)3 integrins. IGF-I increased tyrosine phosphorylation of the focal adhesion kinase (FAK) Pyk2 (calcium-dependent proline-rich tyrosine kinase-2) to a much greater extent than FAK, and increased association of Src with Pyk2 but not FAK. The intracellular calcium chelator BAPTA prevented IGF-I phosphorylation of Pyk2, Src, and MAPK, suggesting that IGF-I activation of Pyk2 is calcium dependent. Transient transfection with a dominant-negative Pyk2, which lacks the autophosphorylation and Src binding site, decreased IGF-I activation of MAPK, but no inhibition was seen with transfected wild-type Pyk2. These results indicate that IGF-I signaling to MAPK is dependent on inside-out activation of (alpha)v(beta)3 integrins and integrin-facilitated multiprotein complex formation involving Pyk2 activation and association with Src.

Insulin-like growth factor-1 receptor signaling in 3T3-L1 adipocyte differentiation requires lipid rafts but not caveolae

Previously, we have found that lipid rafts/caveolae were essential for insulin-like growth factor-1 (IGF-1) receptor signaling during 3T3-L1 preadipocytes differentiation induction. However, it was not identified as to which of the membrane lipid-ordered microdomains mediates the receptor signal. Using small double-stranded RNA-mediated interference (RNAi), we successfully suppressed the caveolin-1 protein expression. In cells stably transfected with vector expressing small interfering RNA (siRNA) fragment, no caveolin-1 protein or caveola was detected. On the other hand, removal of caveolin-1 did not affect the caveolinless lipid rafts or the localization of IGF-1 receptor in lipid rafts on plasma membrane. IGF-1 receptor signal transduction and induced cellular differentiation were normal in RNAi cells with only lipid rafts. Furthermore, these IGF-1 receptor signaling events were still sensitive to the cholesterol-binding reagents. Thus, our results suggest that lipid rafts are sufficient for IGF-1 receptor signaling and the recruitment of signal molecules by caveolin-1 is not essential for IGF-1 receptor signaling.

1?,25-dihydroxyvitamin D3 stimulates steroid sulphatase activity in HL60 and NB4 acute myeloid leukaemia cell lines by different receptor-mediated mechanisms

Steroid sulphatase is a key enzyme in the biosynthesis of bioactive estrogens and androgens from highly abundant inactive circulating sulphated steroid precursors. Little is known about how the expression/activity of this enzyme is regulated. In this article, we show that of 1alpha,25(OH)2D3 stimulates an increase steroid sulphatase activity in the HL60 myeloid leukaemic cell line that is inhibited by a specific nuclear VDR (VDRnuc) antagonist and unaffected by plasma membrane-associated vitamin D receptor (VDRmem) agonists and antagonists. 1alpha,25(OH)2D3-mediated up-regulation of steroid sulphatase activity in HL60 cells was augmented by RXR agonists, blocked by RXR-specific antagonists, and RAR specific agonists and antagonists had no effect. In contrast, the 1alpha,25(OH)2D3-mediated up-regulation of steroid sulphatase activity in the NB4 myeloid leukaemic cell line was unaffected by the specific VDRnuc and RXR antagonists, but was blocked by a VDRmem-specific antagonist and was increased by VDRmem-specific agonists. The findings reveal that VDRnuc-RXR-heterodimers play a key role in the 1alpha,25(OH)2D3-mediated up-regulation of steroid sulphatase activity in HL60 cells. However, in NB4 cells, VDRnuc-derived signals do not play an obligatory role, and non-genomic VDRmem-derived signals are important.

Localization of receptors in lipid rafts can inhibit signal transduction

Processes of cell survival, division, differentiation, and death are guided by the binding of signal molecules to receptors, which activates intracellular signaling networks and ultimately elicits genetic, biochemical, or biomechanical responses within the cell. While intracellular mechanisms for these processes have been well studied, little attention has been given to the role extracellular ligand transport and binding may play in signal initiation. Recent studies have found that the localization of receptors in lipid rafts is critical for the functions of many signaling pathways. By concentrating membrane components, rafts may promote essential interactions for signaling. Lipid rafts can also have negative effects on signaling, but mechanisms remain elusive. We propose that raft-mediated receptor clustering can reduce signaling by prolonging the diffusion of ligands to their receptors. We quantify this effect using a simple diffusion-limited binding model that accounts for the spatial distribution of lipid rafts and receptors on the cell surface. We find that receptor clustering can reduce the apparent rate of receptor binding by up to 80%, consistent with observed increases in epidermal growth factor (EGF) binding by up to 100% following disruption of lipid rafts (Pike and Casey 2002 Biochemistry 41:10315-10322; Roepstorff et al. 2002 J Biol Chem 277:18954-18960). Failure to account for the effects of receptor clustering on rates of ligand binding can skew the interpretation of current methods of cancer diagnosis and treatment. Finally, we discuss how the activation of particular signaling pathways can change over time, depending, in part, on the overall level and spatial distribution of the receptors.

Assembly of alpha-hemolysin on A431 cells leads to clustering of Caveolin-1

Assembly and penetration of 14-strand beta-barrel of staphylococcal alpha-hemolysin (alpha-HL) is an intriguing phenomenon due to its water soluble property. alpha-HL interacts with the Caveolin-1 of A431 cells for its rapid assembly. A nine amino acid, non-hydrophobic peptide derived from alpha-HL has been shown to block the interaction of alpha-HL with the scaffolding domain of Caveolin-1. alpha-HL's presence was also detected in the Caveolin-1 enriched membrane fractions isolated by ultracentrifugation. Moreover, alpha-HL co-precipitates with Caveolin-1 specifically. In a time-dependent process, alpha-HL associates with the Caveolin-1 and co-localizes with Caveolin-1 that results in an extensive clustering of Caveolin-1 at cell-cell contacts. Mutants of alpha-HL devoid of Caveolin-1 binding motif failed to assemble into heptameric oligomers on the surface of A431 cells. Our data suggest that the conformational changes required to form the heptameric assembly might be triggered at the Caveolin-1 binding motif of alpha-HL.

Identification of caveolae and their signature proteins caveolin 1 and 2 in the lens

This study shows that caveolae are present in lens epithelia of rabbit and guinea pig under normal conditions. Caveolae are unique lipid membrane microdomains observed in many cell types. They are believed to play crucial roles in a variety of basic physiological functions including signal transduction, lipid and transcellular transport. Using TEM, immunocytochemistry and immunoblotting we show for the first time the existence of caveolae and the co-localization of their signature marker integral proteins, caveolin-1 and caveolin-2, in the intact lens of rabbit and guinea pig. Thin-section TEM shows that among several species studied, lens epithelia of rabbit and guinea pig exhibited a large number of caveolae. The caveolae were pear shaped, approximately 70 nm in diameter, and were found frequently along the lateral membranes of epithelial cells in the intact lens. In the intact cortical fibers, only a small number of caveolae was seen in the superficial cells. In cultured lens epithelial cells, however, caveolae were observed along all membrane surfaces, but were more abundant at the apical membrane of the cells. Immunofluorescence and immunoblot analyses confirmed the presence of caveolin-1 and caveolin-2 in the lens epithelium. In addition, caveolin-1 and caveolin-2 co-exist in the lens epithelium of both rabbit and guinea pig. HRP tracer study demonstrated that caveolae could carry out endocytosis, suggesting their involvement in molecular transport. Cultured rabbit lens epithelial cells (line N/N1003A) were used to examine the response of caveolae to methyl-beta-cyclodextrin (MBCD), a specific cholesterol-depleting drug. The lens epithelial cells were incubated in freshly prepared MEM medium plus 8% rabbit serum containing 10mm MBCD for 0 (control), 15, 30 or 60 min. Controls for MBCD treatment were cultured in MEM plus 8% rabbit serum. MBCD treatment for 30 min revealed that depletion of cholesterol abolished the majority of caveolae in cultured lens epithelial cells. This result strongly suggests that caveolae are cholesterol-rich lipid rafts that are likely to play important roles in the lens.

Lipid rafts-protein association and the regulation of protein activity

Lipid rafts are membrane microdomains enriched in saturated phospholipids, sphingolipids, and cholesterol. They have a varied but distinct protein composition and have been implicated in diverse cellular processes including polarized traffic, signal transduction, endo- and exo-cytoses, entrance of obligate intracellular pathogens, and generation of pathological forms of proteins associated with Alzheimer's and prion diseases. Raft proteins can be permanently or temporarily associated to lipid rafts. Here, we review recent advances on the biochemical and cell biological characterization of rafts, and on the emerging concept of the temporary residency of proteins in rafts as a regulatory mechanism of their biological activity.

Endogenous SHIP2 does not localize in lipid rafts in 3T3-L1 adipocytes

SH2 domain containing inositol polyphosphate 5-phosphatase (SHIP2) dephosphorylates phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3)) into phosphatidylinositol 3,4-bisphosphate (PtdIns(3,4)P(2)). SHIP2 knock-out mice demonstrated that SHIP2 acts as a negative regulator of insulin cascade in vivo. Our two-hybrid study showed that SHIP2 interacts with c-Cbl associated protein (CAP) and c-Cbl, implicated in the insulin signaling. As some proteins implicated in insulin signaling, like insulin receptor, CAP, c-Cbl or TC10, were reported to localize in lipid rafts, we addressed the same question for SHIP2. SHIP2 was detected in the non-raft fraction in CHO-IR, C2C12 myotubes and 3T3-L1 adipocytes except when it is overexpressed in CHO-IR, where we detected SHIP2 in the raft fraction.

Lipid rafts: heterogeneity on the high seas

Lipid rafts are membrane microdomains that are enriched in cholesterol and glycosphingolipids. They have been implicated in processes as diverse as signal transduction, endocytosis and cholesterol trafficking. Recent evidence suggests that this diversity of function is accompanied by a diversity in the composition of lipid rafts. The rafts in cells appear to be heterogeneous both in terms of their protein and their lipid content, and can be localized to different regions of the cell. This review summarizes the data supporting the concept of heterogeneity among lipid rafts and outlines the evidence for cross-talk between raft components. Based on differences in the ways in which proteins interact with rafts, the Induced-Fit Model of Raft Heterogeneity is proposed to explain the establishment and maintenance of heterogeneity within raft populations.

Physical and functional interaction of growth hormone and insulin-like growth factor-I signaling elements

GH and IGF-I are critical regulators of growth and metabolism. GH interacts with the GH receptor (GHR), a cytokine superfamily receptor, to activate the cytoplasmic tyrosine kinase, Janus kinase 2 (JAK2), and initiate intracellular signaling cascades. IGF-I, produced in part in response to GH, binds to the heterotetrameric IGF-I receptor (IGF-IR), which is an intrinsic tyrosine kinase growth factor receptor that triggers proliferation, antiapoptosis, and other biological actions. Previous in vitro and overexpression studies have suggested that JAKs may interact with IGF-IR and that IGF-I stimulation may activate JAKs. In this study, we explore interactions between GHR-JAK2 and IGF-IR signaling pathway elements utilizing the GH and IGF-I-responsive 3T3-F442A and 3T3-L1 preadipocyte cell lines, which endogenously express both the GHR and IGF-IR. We find that GH induces formation of a complex that includes GHR, JAK2, and IGF-IR in these preadipocytes. The assembly of this complex in intact cells is rapid, GH concentration dependent, and can be prevented by a GH antagonist, G120K. However, it is not inhibited by the kinase inhibitor, staurosporine, which markedly inhibits GHR tyrosine phosphorylation. Moreover, complex formation does not appear dependent on GH-induced activation of the ERK or phosphatidylinositol 3-kinase signaling pathways or on the tyrosine phosphorylation of GHR, JAK2, or IGF-IR. These results suggest that GH-induced formation of the GHR-JAK2-IGF-IR complex is governed instead by GH-dependent conformational change(s) in the GHR and/or JAK2. We further demonstrate that GH and IGF-I can synergize in acute aspects of signaling and that IGF-I enhances GH-induced assembly of conformationally active GHRs. These findings suggest the existence of previously unappreciated relationships between these two hormones.

Caveolar and lipid raft localization of the growth hormone receptor and its signaling elements: impact on growth hormone signaling

The growth hormone receptor (GHR) is a cell surface receptor that mediates the somatogenic and metabolic effects of the growth hormone (GH). GHR signaling is transduced via the receptor-associated cytoplasmic tyrosine kinase called Janus protein kinase 2 (JAK2). The major intracellular signaling systems activated by JAK2 in response to GH include the signal transducer and activator of transcription (STAT) 5 and extracellular signal-regulated kinase (ERK)-1 and -2 pathways. In this report, we investigate the role of cholesterol-rich plasma membrane microdomains (caveolae and lipid rafts) in GH signaling. By subcellular fractionation of the GH-responsive 3T3-F442A murine preadipocyte, we found dramatic enrichment (6.7-fold) of plasma membrane GHR in the caveolae membranes (CM). JAK2 was also represented in the CM fraction, but was less enriched (2.5-fold) than GHR. ERK1/2 and the important ERK pathway upstream small adaptor protein, Grb2 (growth factor receptor-bound protein 2), were also enriched in caveolae (2.3- and 8.3-fold, respectively), but STAT5 was barely detected in the same fraction. Correspondingly, GH-induced tyrosine-phosphorylated GHR, JAK2, and ERK1/2 were highly represented in the CM fraction, whereas tyrosine-phosphorylated STAT5 was enriched in the non-membranous fraction of the post-nuclear supernatant. Additionally, GH induced further accumulation of GHR, Grb2, and SHC proteins in the CM fraction. Interestingly, treatment of the cells with the caveolae-disrupting agent, methyl-beta-cyclodextrin (mbetaCD), selectively inhibited GH-induced ERK1/2 activation but not STAT5 phosphorylation; repletion of cholesterol in mbetaCD-treated cells restored GH-induced ERK activation. Comparison of 3T3-F442A cells with the GHR-expressing human IM-9 lymphoblasts revealed similar enrichment of GHR in the lipid raft fraction of IM-9 as in the CM fraction of 3T3-F442A, but there were dramatic differences in the ERKs and Grb2. The IM-9 cell, in which ERKs are not activated by GH, displayed no enrichment of ERKs and Grb2 in the lipid raft fraction. Our results suggest that localization of GHRs in the CM fraction of the plasma membrane plays important roles in signaling.

IGF-I regulates caveolin 1 and IRS1 interaction in caveolae

Caveolae are hot spots in IGF-I signalling as suggested by the facts that IGF-I receptors localize in caveolae, directly interact with and tyrosine phosphorylate caveolin 1, the major caveolar protein. Also a number of IGF-IR substrates reside in caveolae, supporting a role of these organelles in the regulation of IGF-I action. Recently, we have demonstrated that IGF-I could specifically regulate Shc phosphorylation in caveolae. Here we show that also IRS1 localizes in this region where it is tyrosine phosphorylated in the presence of IGF-I. Moreover, IRS1 co-immunoprecipitates with caveolin 1 and the specific phosphocaveolin 1-IRS1 interaction is increased by IGF-I.

Involvement of cholesterol-rich lipid rafts in interleukin-6-induced neuroendocrine differentiation of LNCaP prostate cancer cells

IL-6 is an inflammatory cytokine that has been linked to aggressive prostate cancer (PCa). Previous studies have demonstrated that IL-6 can enhance the differentiation of PCa cells toward a neuroendocrine (NE) phenotype, a possible indicator of hormone-refractory disease. In this report, we present evidence that the mechanism of IL-6-stimulated NE differentiation employs a detergent-resistant (lipid raft) membrane compartment for signal transduction in LNCaP PCa cells. Signal transducer and activator of transcription (STAT)3, a mediator of IL-6 signaling, was rapidly phosphorylated and translocated to the nucleus in LNCaP cells treated with IL-6. Both processes were inhibited by filipin, a cholesterol-binding compound that disrupts plasma membrane lipid rafts. Isolation of Triton X-100-insoluble raft fractions from LNCaP cells by discontinuous sucrose gradient centrifugation demonstrated that the 80-kDa IL-6 receptor localized almost exclusively to the raft compartment. Although STAT3 was located predominantly in the Triton X-100-soluble subcellular fraction in exponentially growing cells, abundant phosphorylated STAT3 was detected in the raft fraction after stimulation with IL-6. Increases in expression of the NE marker, neuron-specific enolase, and neuron-specific enolase promoter activity after IL-6 treatment were reduced after membrane rafts were disrupted by filipin treatment. LNCaP cells expressed the raft-resident proteins flotillin-2 and G(ialpha2), but notably not caveolins, the predominant structural protein present in caveolar membrane rafts in many tissues and tumor cells. These results are the first to define a role for lipid raft membrane microdomains in signal transduction mechanisms capable of promoting the NE phenotype in PCa cells, and they demonstrate that the raft compartment is capable of mediating such signals in the absence of caveolins. Our results also suggest a mechanistic role for membrane cholesterol in cell signaling events relevant to PCa progression.