Cloning and characterization of a naturally occurring soluble form of TGF-beta type I receptor

IL12Rβ1ΔTM is a secreted product of il12rb1 that promotes control of extrapulmonary tuberculosis

IL12RB1 is a human gene that is important for resistance to Mycobacterium tuberculosis infection. IL12RB1 is expressed by multiple leukocyte lineages, and encodes a type I transmembrane protein (IL12Rβ1) that associates with IL12p40 and promotes the development of host-protective T(H)1 cells. Recently, we observed that il12rb1—the mouse homolog of IL12RB1—is alternatively spliced by leukocytes to produce a second isoform (IL12Rβ1ΔTM) that has biological properties distinct from IL12Rβ1. Although the expression of IL12Rβ1ΔTM is elicited by M. tuberculosis in vivo, and its overexpression enhances IL12p40 responsiveness in vitro, the contribution of IL12Rβ1ΔTM to controlling M. tuberculosis infection has not been tested. Here, we demonstrate that IL12Rβ1ΔTM represents a secreted product of il12rb1 that, when absent from mice, compromises their ability to control M. tuberculosis infection in extrapulmonary organs. Furthermore, elevated M. tuberculosis burdens in IL12Rβ1ΔTM-deficient animals are associated with decreased lymph node cellularity and a decline in TH1 development. Collectively, these data support a model wherein IL12Rβ1ΔTM is a secreted product of il12rb1 that promotes resistance to M. tuberculosis infection by potentiating T(H) cells response to IL-12.

Ectodomain shedding of the receptor for advanced glycation end products: a novel therapeutic target for Alzheimer's disease

Receptor for advanced glycation end products (RAGE) mediates diverse physiological and pathological effects and is involved in the pathogenesis of Alzheimer's disease (AD). RAGE is a receptor for amyloid beta peptides (Ab), mediates Abeta neurotoxicity and also promotes Abeta influx into the brain and contributes to Abeta aggregation. Soluble RAGE (sRAGE), a secreted RAGE isoform, acts as a decoy receptor to antagonize RAGE-mediated damages. Accumulating evidence has suggested that sRAGE represents a promising pharmaceutic against RAGE-mediated disorders. Recent studies revealed proteolysis of RAGE as a previously unappreciated means of sRAGE production. In this review we summarize these findings on the proteolytic cleavage of RAGE and discuss the underlying regulatory mechanisms of RAGE shedding. Furthermore, we propose a model in which proteolysis of RAGE could restrain AD development by reducing Abeta transport intothe brain and Abeta production via BACE. Thus, the modulation of RAGE proteolysis provides a novel intervention strategy for AD.

Transforming growth factor-beta (TGF-beta1) activates TAK1 via TAB1-mediated autophosphorylation, independent of TGF-beta receptor kinase activity in mesangial cells

Transforming growth factor-beta1 (TGF-beta1) is a multifunctional cytokine that signals through the interaction of type I (TbetaRI) and type II (TbetaRII) receptors to activate distinct intracellular pathways. TAK1 is a serine/threonine kinase that is rapidly activated by TGF-beta1. However, the molecular mechanism of TAK1 activation is incompletely understood. Here, we propose a mechanism whereby TAK1 is activated by TGF-beta1 in primary mouse mesangial cells. Under unstimulated conditions, endogenous TAK1 is stably associated with TbetaRI. TGF-beta1 stimulation causes rapid dissociation from the receptor and induces TAK1 phosphorylation. Deletion mutant analysis indicates that the juxtamembrane region including the GS domain of TbetaRI is crucial for its interaction with TAK1. Both TbetaRI-mediated TAK1 phosphorylation and TGF-beta1-induced TAK1 phosphorylation do not require kinase activity of TbetaRI. Moreover, TbetaRI-mediated TAK1 phosphorylation correlates with the degree of its association with TbetaRI and requires kinase activity of TAK1. TAB1 does not interact with TGF-beta receptors, but TAB1 is indispensable for TGF-beta1-induced TAK1 activation. We also show that TRAF6 and TAB2 are required for the interaction of TAK1 with TbetaRI and TGF-beta1-induced TAK1 activation in mouse mesangial cells. Taken together, our data indicate that TGF-beta1-induced interaction of TbetaRI and TbetaRII triggers dissociation of TAK1 from TbetaRI, and subsequently TAK1 is phosphorylated through TAB1-mediated autophosphorylation and not by the receptor kinase activity of TbetaRI.

BAT3 interacts with transforming growth factor-beta (TGF-beta) receptors and enhances TGF-beta1-induced type I collagen expression in mesangial cells

Transforming growth factor-beta1 (TGF-beta1) plays essential roles in a wide array of cellular processes, such as in development and the pathogenesis of tissue fibrosis, including that associated with progressive kidney diseases. Tight regulation of its signaling pathways is critical, and proteins that associate with the TGF-beta receptors may exert positive or negative regulatory effects on TGF-beta signaling. In the present study we employed a yeast-based two-hybrid screening system to identify BAT3 (HLA-B-associated transcript 3) as a TGF-beta receptor-interacting protein. Analysis of endogenously expressed BAT3 in various tissues including the kidney reveals the existence of approximately 140-kDa full-length protein as well as truncated forms of BAT3 whose expression is developmentally regulated. Endogenous BAT3 protein interacts with TGF-beta receptors type I and type II in renal mesangial cells. Functional assays show that expression of full-length BAT3 results in enhancement of TGF-beta1-stimulated transcriptional activation of p3TP-Lux reporter, and these effects require the presence of functional TGF-beta signaling receptors as demonstrated in R-1B and DR-26 mutant cells. Moreover, expression of full-length BAT3, but not C-terminal truncated mutant of BAT3, enhanced TGF-beta1-induced type I collagen expression in mesangial cells, whereas knock down of BAT3 protein expression by small interfering RNA suppressed the expression of type I collagen induced by TGF-beta1. Our findings suggest that BAT3, a TGF-beta receptor-interacting protein, is capable of modulating TGF-beta signaling and acts as a positive regulator of TGF-beta1 stimulation of type I collagen expression in mesangial cells.

ALK1 opposes ALK5/Smad3 signaling and expression of extracellular matrix components in human chondrocytes

INTRODUCTION

TGF-beta is a multifunctional regulator of chondrocyte proliferation, differentiation, and extracellular matrix production. Dysregulation of TGF-beta action has been implicated in cartilage diseases such as osteoarthritis. TGF-beta signaling is transduced through a pair of transmembrane serine/threonine kinases, known as the type I (ALK5) and type II receptors. However, recent studies on endothelial cells have identified ALK1 as a second type I TGF-beta receptor and have shown that ALK1 and ALK5 have opposing functions in these cells. Here we examined ALK1 expression and its regulation of TGF-beta signaling and responses in human chondrocytes.

MATERIALS AND METHODS

ALK1 expression in human chondrocytes was examined by RT-PCR and Western blot. The ability of ALK1 to form complexes with other TGF-beta receptors was determined by affinity labeling/immunoprecipitation and by immunoprecipitation followed by Western blot. The effect of ALK1 on TGF-beta1-induced signaling and responses was determined by varying ALK1 expression levels and measuring transcriptional activity using promoter/luciferase assays, Smad1/5 and Smad3 phosphorylation, and expression of type II collagen, PAI-1, and fibronectin.

RESULTS

Our results indicate that ALK1 is expressed in human chondrocytes and that it is a component of the TGF-beta receptor system, associating with ALK5, type II TGF-beta receptor, endoglin, and betaglycan. Furthermore, we show that both ALK1 and ALK5 are needed for TGF-beta-induced phosphorylation of intracellular mediators Smad1/5, whereas only ALK5 is essential for TGF-beta1-induced phosphorylation of Smad3. In addition, our results show that ALK1 inhibits, whereas ALK5 potentiates, TGF-beta-induced Smad3-driven transcriptional activity and the expression of PAI-1, fibronectin, and type II collagen in chondrocytes.

CONCLUSIONS

Our results suggest that ALK1 and ALK5 display opposing functions in human chondrocytes, implicating an essential role for ALK1 in the regulation of TGF-beta signaling and function in these cells.

Characterization of novel transforming growth factor-beta type I receptors found in malignant pleural effusion tumor cells

Background

Tumors expressing a transforming growth factor-beta type I receptor (TβRI) mutant with sequence deletions in a nine-alanine (9A) stretch of the signal peptide are reported to be highly associated with disease progression. Expression of this mutant could interfere with endogenous TGFβ signaling in the cell. However, little is known about the importance of the remaining part of the signal peptide on the cellular function of TβRI.

Results

We cloned and identified four new in-frame deletion variants of TβRI, designated DM1 to DM4, in pleural effusion-derived tumor cells. Intriguingly, DM1 and DM2, with a small region truncated in the putative signal peptide of TβRI, had a serious defect in their protein expression compared with that of the wild-type receptor. Using serial deletion mutagenesis, we characterized a region encoded by nucleotides 16–51 as a key element controlling TβRI protein expression. Consistently, both DM1 and DM2 have this peptide deleted. Experiments using cycloheximde and MG132 further confirmed its indispensable role for the protein stability of TβRI. In contrast, truncation of the 9A-stretch itself or a region downstream to the stretch barely affected TβRI expression. However, variants lacking a region C-terminal to the stretch completely lost their capability to conduct TGFβ-induced transcriptional activation. Intriguingly, expression of DM3 in a cell sensitive to TGFβ made it significantly refractory to TGFβ-mediated growth inhibition. The effect of DM3 was to ablate the apoptotic event induced by TGFβ.

Conclusion

We identified four new transcript variants of TβRI in malignant effusion tumor cells and characterized two key elements controlling its protein stability and transcriptional activation. Expression of one of variants bestowed cancer cells with a growth advantage in the presence of TGFβ. These results highlight the potential roles of some naturally occurring TβRI variants on the promotion of tumor malignancy.

Identification of CD109 as part of the TGF-beta receptor system in human keratinocytes

We have previously reported that keratinocytes defective in glycosylphosphatidylinositol (GPI)-anchor biosynthesis display enhanced TGF-beta responses. These studies implicated the involvement of a 150 kDa GPI-anchored TGF-beta1 binding protein, r150, in modulating TGF-beta signaling. Here, we sought to determine the molecular identity of r150 by affinity purification and microsequencing. Our results identify r150 as CD109, a novel member of the alpha2-macroglobulin (alpha2M)/complement superfamily, whose function has remained obscure. In addition, we have identified a novel CD109 isoform that occurs in the human placenta but not keratinocytes. Biochemical studies show that r150 contains an internal thioester bond, a defining feature of the alpha2M/complement family. Loss and gain of function studies demonstrate that CD109 is a component of the TGF-beta receptor system, and a negative modulator of TGF-beta responses in keratinocytes, as implicated for r150. Our data suggest that CD109 can inhibit TGF-beta signaling independently of ligand sequestration and may exert its effect on TGF-beta signaling by direct modulation of receptor activity. Together, our results linking CD109 function to regulation of TGF-beta signaling suggest that CD109 plays a unique role in the regulation of isoform-specific TGF-beta signaling in keratinocytes.

Stimulation of pro-alpha(1)(I) collagen by TGF-beta(1) in mesangial cells: role of the p38 MAPK pathway

Transforming growth factor-beta(1) (TGF-beta(1)) is a potent inducer of extracellular matrix protein synthesis and a key mediator of renal fibrosis. However, the intracellular signaling mechanisms by which TGF-beta(1) stimulates this process remain incompletely understood. In this report, we examined the role of a major stress-activated intracellular signaling cascade, belonging to the mitogen-activated protein kinase (MAPK) superfamily, in mediating TGF-beta(1) responses in rat glomerular mesangial cells, using dominant-negative inhibition of TGF-beta(1) signaling receptors. We first stably transfected rat glomerular mesangial cells with a kinase-deleted mutant TGF-beta type II receptor (TbetaR-II(M)) designed to inhibit TGF-beta(1) signaling in a dominant-negative fashion. Next, expression of TbetaR-II(M) mRNA was confirmed by Northern analysis. Cell surface expression and ligand binding of TbetaR-II(M) protein were demonstrated by affinity cross-linking with (125)I-labeled-TGF-beta(1). TGF-beta(1) rapidly induced p38 MAPK phosphorylation in wild-type and empty vector (pcDNA3)-transfected control mesangial cells. Interestingly, transfection with dominant-negative TbetaR-II(M) failed to block TGF-beta(1)-induced p38 MAPK phosphorylation. Moreover, dominant-negative TbetaR-II(M) failed to block TGF-beta(1)-stimulated pro-alpha(1)(I) collagen mRNA expression and cellular protein synthesis, whereas TGF-beta(1)-induced extracellular signal-regulated kinase (ERK) 1/ERK2 activation and antiproliferative responses were blocked by TbetaR-II(M). In the presence of a specific inhibitor of p38 MAPK, SB-203580, TGF-beta(1) was unable to stimulate pro-alpha(1)(I) collagen mRNA expression in the control and TbetaR-II(M)-transfected mesangial cells. Finally, we confirmed that both p38 MAPK activation and pro-alpha(1)(I) collagen stimulation were TGF-beta(1) effects that were abrogated by dominant-negative inhibition of TGF-beta type I receptor. Thus we show first demonstration of p38 MAPK activation by TGF-beta(1) in mesangial cells, and, given the rapid kinetics, this TGF-beta(1) effect is likely a direct one. Furthermore, our findings suggest that the p38 MAPK pathway functions as a component in the signaling of pro-alpha(1)(I) collagen induction by TGF-beta(1) in mesangial cells.