Inhibition of Retinoic Acid Signaling in Proximal Tubular Epithelial cells Protects against Acute Kidney Injury by Enhancing Kim-1-dependent Efferocytosis.

Retinoic acid receptor (RAR) signaling is essential for mammalian kidney development, but in the adult kidney is restricted to occasional collecting duct epithelial cells. We now show there is widespread reactivation of RAR signaling in proximal tubular epithelial cells (PTECs) in human sepsis-associated acute kidney injury (AKI), and in mouse models of AKI. Genetic inhibition of RAR signaling in PTECs protects against experimental AKI but is associated with increased expression of the PTEC injury marker, Kim-1. However, Kim-1 is also expressed by de-differentiated, proliferating PTECs, and protects against injury by increasing apoptotic cell clearance, or efferocytosis. We show that the protective effect of inhibiting PTEC RAR signaling is mediated by increased Kim-1 dependent efferocytosis, and that this is associated with de-differentiation, proliferation, and metabolic reprogramming of PTECs. These data demonstrate a novel functional role that reactivation of RAR signaling plays in regulating PTEC differentiation and function in human and experimental AKI.

Sorting nexin 6, a novel SNX, interacts with the transforming growth factor-beta family of receptor serine-threonine kinases.

Sorting nexins (SNX) comprise a family of proteins with homology to several yeast proteins, including Vps5p and Mvp1p, that are required for the sorting of proteins to the yeast vacuole. Human SNX1, -2, and -4 have been proposed to play a role in receptor trafficking and have been shown to bind to several receptor tyrosine kinases, including receptors for epidermal growth factor, platelet-derived growth factor, and insulin as well as the long form of the leptin receptor, a glycoprotein 130-associated receptor. We now describe a novel member of this family, SNX6, which interacts with members of the transforming growth factor-beta family of receptor serine-threonine kinases. These receptors belong to two classes: type II receptors that bind ligand, and type I receptors that are subsequently recruited to transduce the signal. Of the type II receptors, SNX6 was found to interact strongly with ActRIIB and more moderately with wild type and kinase-defective mutants of TbetaRII. Of the type I receptors, SNX6 was found to interact only with inactivated TbetaRI. SNXs 1-4 also interacted with the transforming growth factor-beta receptor family, showing different receptor preferences. Conversely, SNX6 behaved similarly to the other SNX proteins in its interactions with receptor tyrosine kinases. Strong heteromeric interactions were also seen among SNX1, -2, -4, and -6, suggesting the formation in vivo of oligomeric complexes. These findings are the first evidence for the association of the SNX family of molecules with receptor serine-threonine kinases.

Transcriptional cross-talk between Smad, ERK1/2, and p38 mitogen-activated protein kinase pathways regulates transforming growth factor-beta-induced aggrecan gene expression in chondrogenic ATDC5 cells.

In chondrogenesis, members of the transforming growth factor-beta (TGF-beta) superfamily play critical roles by inducing gene expression of cartilage-specific molecules. By using a chondrogenic cell line, ATDC5, we investigated the TGF-beta-mediated signaling pathways involved in expression of the aggrecan gene (Agc). At confluency, TGF-beta induced Agc expression within 3 h, and cycloheximide blocked this induction, indicating that de novo protein synthesis is essential for this response. At this stage, TGF-beta induced rapid, transient phosphorylation of Smad2, extracellular signal-activated kinase 1/2 (ERK1/2), and p38 mitogen-activated protein kinase (MAPK). Inhibition of the Smad pathways by transfection with a dominant negative Smad4 construct significantly reduced TGF-beta-induced Agc expression, indicating that Smad signaling is essential for this response. Furthermore, an inhibitor of the ERK1/2 pathway, U0126, or inhibitors of the p38 MAPK pathway, SB203580 and SKF86002, repressed TGF-beta-induced Agc expression in a dose-dependent manner, indicating that ERK1/2 or p38 MAPK activation is also required for TGF-beta-induced Agc expression in confluent ATDC5 cells. In differentiated ATDC5 cells, persistently high basal levels of ERK1/2 and p38 MAPK phosphorylation correlated with elevated basal Agc expression, which was inhibited by incubation with inhibitors of these pathways. Whereas Smad2 was rapidly phosphorylated by TGF-beta and involved in the initial activation of Agc expression in confluent cells, Smad2 activation was not required for maintaining the high level of Agc expression. Taken together, these results suggest an important role for transcriptional cross-talk between Smad and MAPK pathways in expression of early chondrocytic phenotypes and identify important changes in the regulation of Agc expression following chondrocyte differentiation.

The hepatitis B virus encoded oncoprotein pX amplifies TGF-beta family signaling through direct interaction with Smad4: potential mechanism of hepatitis B virus-induced liver fibrosis.

Hepatitis B, one of the most common infectious diseases in the world, is closely associated with acute and chronic hepatitis, cirrhosis, and hepatocellular carcinoma. Many clinical investigations have revealed that hepatic fibrosis is an important component of these liver diseases caused by chronic hepatitis B. TGF-beta signaling plays an important role in the pathogenesis of fibrosis in chronic hepatitis and cirrhosis. As these diseases are associated with hepatitis B virus (HBV) infection, we examined the possibility that the HBV-encoded pX oncoprotein regulates TGF-beta signaling. We show that pX enhances transcriptional activity in response to TGF-beta, BMP-2, and activin by stabilizing the complex of Smad4 with components of the basic transcriptional machinery. Additionally, confocal microscopic studies suggest that pX facilitates and potentiates the nuclear translocation of Smads, further enhancing TGF-beta signaling. Our studies suggest a new paradigm for amplification of Smad-mediated signaling by an oncoprotein and suggest that enhanced Smad-mediated signaling may contribute to HBV-associated liver fibrosis.

The L3 loop and C-terminal phosphorylation jointly define Smad protein trimerization.

Smad proteins mediate the transforming growth factor beta responses. C-terminal phosphorylation of R-Smads leads to the recruitment of Smad4 and the formation of active signaling complexes. We investigated the mechanism of phosphorylation-induced Smad complex formation with an activating pseudo-phosphorylated Smad3. Pseudo-phosphorylated Smad3 has a greater propensity to homotrimerize, and recruits Smad4 to form a heterotrimer containing two Smad3 and one Smad4. The trimeric interaction is mediated through conserved interfaces to which tumorigenic mutations map. Furthermore, a conserved Arg residue within the L3 loop, located near the C-terminal phosphorylation sites of the neighboring subunit, is essential for trimerization. We propose that the phosphorylated C-terminal residues interact with the L3 loop of the neighboring subunit to stabilize the trimer interaction.

Structural basis of Smad1 activation by receptor kinase phosphorylation.

Phosphorylation of Smad1 at the conserved carboxyl terminal SVS sequence activates BMP signaling. Here we report the crystal structure of the Smad1 MH2 domain in a conformation that reveals the structural effects of phosphorylation and a molecular mechanism for activation. Within a trimeric subunit assembly, the SVS sequence docks near two putative phosphoserine binding pockets of the neighboring molecule, in a position ready to interact upon phosphorylation. The MH2 domain undergoes concerted conformational changes upon activation, which signal Smad1 dissociation from the receptor kinase for subsequent heteromeric assembly with Smad4. Biochemical and modeling studies reveal unique favorable interactions within the Smad1/Smad4 heteromeric interface, providing a structural basis for their association in signaling.

Zebrafish nma is involved in TGFbeta family signaling.

Bone morphogenetic proteins (BMP) are members of the TGFbeta superfamily of secreted factors with important regulatory functions during embryogenesis. We have isolated the zebrafish gene, nma, that encodes a protein with high sequence similarity to human NMA and Xenopus Bambi. It is also similar to TGFbeta type I serine/theronine kinase receptors in the extracellular ligand-binding domain but lacks a cytoplasmic kinase domain. During development, nma expression is similar to that of bmp2b and bmp4, and analysis in the dorsalized and ventralized zebrafish mutants swirl and chordino indicates that nma is regulated by BMP signaling. Overexpression of nma during zebrafish and Xenopus development resulted in phenotypes that appear to be based on inhibition of BMP signaling. Biochemically, NMA can associate with TGFbeta type II receptors and bind to TGFbeta ligand. We propose that nma is a BMP-regulated gene whose function is to attenuate BMP signaling during development through interactions with type II receptors and ligands.

Role of transforming growth factor-beta signaling in cancer.

Signaling from transforming growth factor-beta (TGF-beta) through its unique transmembrane receptor serine-threonine kinases plays a complex role in carcinogenesis, having both tumor suppressor and oncogenic activities. Tumor cells often escape from the antiproliferative effects of TGF-beta by mutational inactivation or dysregulated expression of components in its signaling pathway. Decreased receptor function and altered ratios of the TGF-beta type I and type II receptors found in many tumor cells compromise the tumor suppressor activities of TGF-beta and enable its oncogenic functions. Recent identification of a family of intracellular mediators, the Smads, has provided new paradigms for understanding mechanisms of subversion of TGF-beta signaling by tumor cells. In addition, several proteins recently have been identified that can modulate the Smad-signaling pathway and may also be targets for mutation in cancer. Other pathways such as various mitogen-activated protein kinase cascades also contribute substantially to TGF-beta signaling. Understanding the interplay between these signaling cascades as well as the complex patterns of cross-talk with other signaling pathways is an important area of investigation that will ultimately contribute to understanding of the bifunctional tumor suppressor/oncogene role of TGF-beta in carcinogenesis.

A novel smad nuclear interacting protein, SNIP1, suppresses p300-dependent TGF-beta signal transduction.

Members of the transforming growth factor-beta superfamily play critical roles in controlling cell growth and differentiation. Effects of TGF-beta family ligands are mediated by Smad proteins. To understand the mechanism of Smad function, we sought to identify novel interactors of Smads by use of a yeast two-hybrid system. A 396-amino acid nuclear protein termed SNIP1 was cloned and shown to harbor a nuclear localization signal (NLS) and a Forkhead-associated (FHA) domain. The carboxyl terminus of SNIP1 interacts with Smad1 and Smad2 in yeast two-hybrid as well as in mammalian overexpression systems. However, the amino terminus of SNIP1 harbors binding sites for both Smad4 and the coactivator CBP/p300. Interaction between endogenous levels of SNIP1 and Smad4 or CBP/p300 is detected in NMuMg cells as well as in vitro. Overexpression of full-length SNIP1 or its amino terminus is sufficient to inhibit multiple gene responses to TGF-beta and CBP/p300, as well as the formation of a Smad4/p300 complex. Studies in Xenopus laevis further suggest that SNIP1 plays a role in regulating dorsomedial mesoderm formation by the TGF-beta family member nodal. Thus, SNIP1 is a nuclear inhibitor of CBP/p300 and its level of expression in specific cell types has important physiological consequences by setting a threshold for TGF-beta-induced transcriptional activation involving CBP/p300.

The MSG1 non-DNA-binding transactivator binds to the p300/CBP coactivators, enhancing their functional link to the Smad transcription factors.

The MSG1 nuclear protein has a strong transcriptional activating activity but does not bind directly to DNA. When cotransfected, MSG1 enhances transcription mediated by the Smad transcription factors in mammalian cells in a manner dependent on ligand-induced Smad hetero-oligomerization. However, the mechanism of this MSG1 effect has been unknown. We now show that MSG1 directly binds to the p300/cAMP-response element-binding protein-binding protein (CBP) transcriptional coactivators, which in turn bind to the Smads, and enhances Smad-mediated transcription in a manner dependent on p300/CBP. The C-terminal transactivating domain of MSG1 is required for binding to p300/CBP and enhancement of Smad-mediated transcription; the viral VP16 transactivating domain could not substitute for it. In the N-terminal region of MSG1, we identified a domain that is necessary and sufficient to direct the specific interaction of MSG1 with Smads. We also found that the Hsc70 heat-shock cognate protein also forms complex with MSG1 in vivo, suppressing both binding of MSG1 to p300/CBP and enhancement of Smad-mediated transcription by MSG1. These results indicate that MSG1 interacts with both the DNA-binding Smad proteins and the p300/CBP coactivators through its N- and C-terminal regions, respectively, and enhances the functional link between Smads and p300/CBP.

The Smad4 activation domain (SAD) is a proline-rich, p300-dependent transcriptional activation domain.

Transforming growth factor-beta (TGF-beta) family members signal through a unique set of intracellular proteins called Smads. Smad4, previously identified as the tumor suppressor DPC4, is functionally distinct among the Smad family, and is required for the assembly and transcriptional activation of diverse, Smad-DNA complexes. We previously identified a 48-amino acid proline-rich regulatory element within the middle linker domain of this molecule, the Smad4 activation domain (SAD), which is essential for mediating these signaling activities. We now characterize the functional activity of the SAD. Mutants lacking the SAD are still able to form complexes with other Smad family members and associated transcription factors, but cannot activate transcription in these complexes. Furthermore, the SAD itself is able to activate transcription in heterologous reporter assays, identifying it as a proline-rich transcriptional activation domain, and indicating that the SAD is both necessary and sufficient to activate Smad-dependent transcriptional responses. We show that transcriptional activation by the SAD is p300-dependent, and demonstrate that this activity is associated with a physical interaction of the SAD with the amino terminus of p300. These data identify a novel function of the middle linker region of Smad4, and define the role of the SAD as an important locus determining the transcriptional activation of the Smad complex.

A novel mitochondrial septin-like protein, ARTS, mediates apoptosis dependent on its P-loop motif.

Here we describe a protein product of the human septin H5/PNUTL2/CDCrel2b gene, which we call ARTS (for apoptosis-related protein in the TGF-beta signalling pathway). ARTS is expressed in many cells and acts to enhance cell death induced by TGF-beta or, to a lesser extent, by other apoptotic agents. Unlike related septin gene products, ARTS is localized to mitochondria and translocates to the nucleus when apoptosis occurs. Mutation of the P-loop of ARTS abrogates its competence to activate caspase 3 and to induce apoptosis. Taken together, these observations expand the functional attributes of septins previously described as having roles in cytokinesis and cellular morphogenesis.

The transforming growth factor-beta/SMAD signaling pathway is present and functional in human mesangial cells.

BACKGROUND: Transforming growth factor-beta (TGF-beta) signals through a unique set of intracellular proteins, called SMADs, that have been characterized mainly in transient overexpression systems. Because several models of glomerulosclerosis suggest a role for TGF-beta in the extracellular matrix accumulation, we sought to characterize the role of SMAD proteins in mediating TGF-beta1 responses in a more physiological system using nontransformed human mesangial cells. METHODS: Endogenous SMAD expression and its modulation by TGF-beta1 were evaluated by Western and Northern blot analyses. Phosphorylation of Smad2 and Smad3 was determined by both phospholabeling and immunoblot. SMAD function and its role in type I collagen transcription were investigated in cotransfection experiments using promoter-luciferase reporter gene constructs. RESULTS: Cultured human mesangial cells express Smad2, Smad3, and Smad4 proteins. TGF-beta1 down-regulated Smad3 mRNA and protein expression, respectively, after 4 and 24 hours of treatment, whereas Smad2 and Smad4 were less affected. Both Smad2 and Smad3 were phosphorylated in response to TGF-beta1 beginning at 5 minutes, with maximal phosphorylation at 15 minutes, and decreasing phosphorylation by 2 hours. Smad2/3 and Smad4 coimmunoprecipitate only after TGF-beta1 treatment. The activity of a transiently transfected, TGF-beta-responsive construct, p3TP-Lux, was stimulated 3.6-fold by TGF-beta1. Overexpressed wild-type Smad3 increased basal luciferase activity, which was further stimulated by TGF-beta1. A dominant negative mutant form of Smad3 lacking the C-terminal serine phosphoacceptor sites (Smad3A) inhibited TGF-beta1-induced luciferase activity. TGF-beta1 also increased the activation of an alpha2(I) collagen promoter-luciferase reporter construct transfected into mesangial cells. This activation was inhibited by cotransfection with the Smad3A mutant. CONCLUSIONS: Smad2, Smad3, and Smad4 are present and activated by TGF-beta1 in human mesangial cells. The SMAD pathway is functional in these cells and appears to be involved in TGF-beta1-induced type I collagen gene transcription. These findings raise the possibility that SMAD signaling plays a role in glomerular matrix accumulation.

SMAD3/4-dependent transcriptional activation of the human type VII collagen gene (COL7A1) promoter by transforming growth factor beta.

The human type VII collagen gene (COL7A1) recently has been identified as an immediate-early response gene for transforming growth factor beta (TGF-beta)/SMAD signaling pathway. In this study, by using MDA-MB-468 SMAD4-/- breast carcinoma cells, we demonstrate that expression of SMAD4 is an absolute requirement for SMAD-mediated promoter activity. We also demonstrate that the SMAD binding sequence (SBS) representing the TGF-beta response element in the region -496/-444 of the COL7A1 promoter functions as an enhancer in the context of a heterologous promoter. Electrophoretic mobility-shift assays with nuclear extracts from COS-1 cells transfected with expression vectors for SMADs 1-5 indicate that SMAD3 forms a complex with a migration similar to that of the endogenous TGF-beta-specific complex observed in fibroblast extracts. Electrophoretic mobility-shift assays using recombinant glutathione S-transferase-SMAD fusion proteins indicate that both SMAD4 and C-terminally truncated SMAD3, but not SMAD2, can bind the COL7A1 SBS. Coexpression of SMAD3 and SMAD4 in COS-1 cells leads to the formation of two complexes: a DNA/protein complex containing SMAD3 alone and another slower-migrating complex containing both SMAD3 and SMAD4, the latter complex not being detected in fibroblasts. Maximal transactivation of COL7A1 SBS-driven promoters in either MDA-MB-468 carcinoma cells or fibroblasts requires concomitant overexpression of SMAD3 and SMAD4. These data may represent the first identification of a functional homomeric SMAD3 complex regulating a human gene.

Transcriptional activating activity of Smad4: roles of SMAD hetero-oligomerization and enhancement by an associating transactivator.

Smad4 plays a pivotal role in signal transduction of the transforming growth factor beta superfamily cytokines by mediating transcriptional activation of target genes. Hetero-oligomerization of Smad4 with the pathway-restricted SMAD proteins is essential for Smad4-mediated transcription. We provide evidence that SMAD hetero-oligomerization is directly required for the Smad4 C-terminal domain [Smad4(C)] to show its transcriptional transactivating activity; this requirement obtains even when Smad4(C) is recruited to promoters by heterologous DNA-binding domains and in the absence of the inhibitory Smad4 N-terminal domain. Defined mutations of GAL4 DNA-binding domain fusion of Smad4(C) that disrupt SMAD hetero-oligomerization suppressed transcriptional activation. Importantly, we found that an orphan transcriptional activator MSG1, a nuclear protein that has strong transactivating activity but apparently lacks DNA-binding activity, functionally interacted with Smad4 and enhanced transcription mediated by GAL4 DNA-binding domain-Smad4(C) and full-length Smad4. Transcriptional enhancement by MSG1 depended on transforming growth factor beta signaling and was suppressed by Smad4(C) mutations disrupting SMAD hetero-oligomerization or by the presence of Smad4 N-terminal domain. Furthermore, Smad4(C) did not show any detectable transactivating activity in yeast when fused to heterologous DNA-binding domains. These results demonstrate additional roles of SMAD hetero-oligomerization in Smad4-mediated transcriptional activation. They also suggest that the transcriptional-activating activity observed in the presence of Smad4 in mammalian cells may be derived, at least in part, from endogenously expressed separate transcriptional activators, such as MSG1.

Smad2 transduces common signals from receptor serine-threonine and tyrosine kinases.

SMAD proteins mediate signals from receptor serine-threonine kinases (RSKs) of the TGF-beta superfamily. We demonstrate here that HGF and EGF, which signal through RTKs, can also mediate SMAD-dependent reporter gene activation and induce rapid phosphorylation of endogenous SMAD proteins by kinase(s) downstream of MEK1. HGF induces phosphorylation and nuclear translocation of epitope-tagged Smad2 and a mutation that blocks TGF-beta signaling also blocks HGF signal transduction. Smad2 may thus act as a common positive effector of TGF-beta- and HGF-induced signals and serve to modulate cross talk between RTK and RSK signaling pathways.

Characterization of functional domains within Smad4/DPC4.

Smad proteins are a family of highly conserved, intracellular proteins that signal cellular responses downstream of transforming growth factor-beta (TGF-beta) family serine/threonine kinase receptors. One of these molecules, Smad4, originally identified as the candidate tumor suppressor gene dpc-4, reconstitutes TGF-beta- and activin-dependent transcriptional responses in Smad4 null cell lines and interacts in a ligand-dependent manner with other Smad family members in both TGF-beta, activin, and bone morphogenetic protein-2/-4 pathways. Here, we used an assay based on the restoration of ligand-dependent transcriptional responses in a Smad4 null cell line to characterize functional domain structures within Smad4. We showed that restoration of TGF-beta-induced transcriptional responses by Smad4 was inhibited by co-transfection with a kinase dead TGF-beta type II receptor and that constitutive activation was blocked with TGF-beta neutralizing antibodies, confirming the essential role of Smad4 in TGF-beta signaling. Using a series of Smad4 mutation, deletion, and Smad1/Smad4 chimera constructs we identified a 47-amino acid deletion within the middle-linker region of Smad4 that is essential for the mediation of signaling responses. In addition, we showed that the NH2-terminal domain of Smad4 augments ligand-dependent activation associated with the middle-linker region, indicating that there is a distinct ligand-response domain within the N terminus of this molecule.

Serine phosphorylation, chromosomal localization, and transforming growth factor-beta signal transduction by human bsp-1.

The transforming growth factor-beta (TGF-beta) superfamily regulates a multitude of cellular and developmental events. TGF-beta family ligands signal through transmembrane serine/threonine kinase receptors whose downstream effectors are largely unknown. Using genetic data from the fruit fly, we have identified a downstream effector of TGF-beta-induced signaling. TGF-beta signaling protein-1 (BSP-1) is rapidly phosphorylated in response to TGF-beta. Localization of bsp-1 to chromosome 4q28 suggests a role in carcinogenesis. These data suggest that BSP-1 is the prototype of a new class of signaling molecules.

Detection of abnormal peripheral blood mononuclear cell cytokine networks in human IgA nephropathy.

Dysregulated cytokine expression has been implicated in the pathogenesis of IgA nephropathy, but the mechanisms and selectivity of this response are poorly understood. In this study we have examined the expression of a range of immunoregulatory cytokine mRNAs by peripheral blood mononuclear cells (PBMNCs) from 45 patients with IgA nephropathy stratified empirically according to urinary red cell excretion: 10 in remission, and 35 with active disease (21 mild, 14 moderate), and 17 normal, and 15 disease, controls. We used a semi-quantitative polymerase chain reaction (PCR) technique. None of the patients had experienced recent episodes of macroscopic hematuria. Simultaneous analysis of monocyte class II antigen (DR) expression was also performed by two-color immunoflow cytometry. TGF-beta 1 mRNA was detected in 68% (24 of 35) of patients with active, and 70% (7 of 10) inactive IgA nephropathy, but in only 18% (3 of 17) normal (P < 0.005), and 27% (4 of 15) disease controls. IL-6 transcripts were identified in 37% (13 of 35) of patients with active IgA nephropathy, compared with 6% (1 of 17) normal controls (P = 0.015), with no significant increase in IgA remission, or disease control groups. TNF-alpha mRNA was detected in 29% (5 of 17) of normal and 53% (8 of 15) disease controls, but in only 7% (3 of 35) of patients with IgA nephropathy (P = 0.015). There was no significant change in TGF-beta 2, gamma-IFN, IL-2, IL-4, IL-1 alpha or IL-1 beta detection between groups.(ABSTRACT TRUNCATED AT 250 WORDS)