Elucidation of Smad requirement in transforming growth factor-beta type I receptor-induced responses

Integration of TGF-beta/Smad and Jagged1/Notch signalling in epithelial-to-mesenchymal transition

Epithelial-to-mesenchymal transitions (EMTs) underlie cell plasticity required in embryonic development and frequently observed in advanced carcinogenesis. Transforming growth factor-beta (TGF-beta) induces EMT phenotypes in epithelial cells in vitro and has been associated with EMT in vivo. Here we report that expression of the hairy/enhancer-of-split-related transcriptional repressor Hey1, and the Notch-ligand Jagged1 (Jag1), was induced by TGF-beta at the onset of EMT in epithelial cells from mammary gland, kidney tubules, and epidermis. The HEY1 expression profile was biphasic, consisting of immediate-early Smad3-dependent, Jagged1/Notch-independent activation, followed by delayed, indirect Jagged1/Notch-dependent activation. TGF-beta-induced EMT was blocked by RNA silencing of HEY1 or JAG1, and by chemical inactivation of Notch. The EMT phenotype, biphasic activation of Hey1, and delayed expression of Jag1 were induced by TGF-beta in wild-type, but not in Smad3-deficient, primary mouse kidney tubular epithelial cells. Our findings identify a new mechanism for functional integration of Jagged1/Notch signalling and coordinated activation of the Hey1 transcriptional repressor controlled by TGF-beta/Smad3, and demonstrate functional roles for Smad3, Hey1, and Jagged1/Notch in mediating TGF-beta-induced EMT.

TGFbeta3 signaling activates transcription of the LEF1 gene to induce epithelial mesenchymal transformation during mouse palate development

Epithelial mesenchymal transformation (EMT) of the medial edge epithelial (MEE) seam creates palatal confluence. This work aims to elucidate the molecular mechanisms by which TGFβ₃ brings about palatal seam EMT. We collected mRNA for PCR analysis from individual transforming MEE cells by laser microdissection techniques and demonstrated that TGFβ₃ stimulates lymphoid-enhancing factor 1 (LEF1) mRNA synthesis in MEE cells. We show with antisense β-catenin oligonucleotides that up-regulated LEF1 is not activated by β-catenin in palate EMT. We ruled out other TGFβ₃ targets, such as RhoA and MEK1/2 pathways, and we present evidence using dominant-negative Smad4 and dominant-negative LEF1 showing that TGFβ₃ uses Smads both to up-regulate synthesis of LEF1 and to activate LEF1 transcription during induction of palatal EMT. When phospho-Smad2 and Smad4 are present in the nucleus, LEF1 is activated without β-catenin. Our paper is the first to show that the Smad2,4/LEF1 complex replaces β-catenin/LEF1 during activation of EMT in vivo by TGFβ₃.

The role of epithelial-to-mesenchymal transition in renal fibrosis

Epithelial-to-mesenchymal transition (EMT) involving injured epithelial cells plays an important role in the progression of fibrosis in the kidney. Tubular epithelial cells can acquire a mesenchymal phenotype, and enhanced migratory capacity enabling them to transit from the renal tubular microenvironment into the interstitial space and escape potential apoptotic cell death. EMT is a major contributor to the pathogenesis of renal fibrosis, as it leads to a substantial increase in the number of myofibroblasts, leading to tubular atrophy. However, recent findings suggest that EMT involving tubular epithelial cell is a reversible process, potentially determined by the surviving cells to facilitate the repopulation of injured tubules with new functional epithelia. Major regulators of renal epithelial cell plasticity in the kidney are two multifunctional growth factors, bone morphogenic protein-7 (BMP-7) and transforming growth factor beta1 (TGF-beta1). While TGF-beta1 is a well-established inducer of EMT involving renal tubular epithelial cells, BMP-7 reverses EMT by directly counteracting TGF-beta-induced Smad-dependent cell signaling in renal tubular epithelial cells. Such antagonism results in the repair of injured kidneys, suggesting that modulation of epithelial cell plasticity has therapeutic advantages.

Targeted disruption of TGF-beta1/Smad3 signaling protects against renal tubulointerstitial fibrosis induced by unilateral ureteral obstruction

Tubulointerstitial fibrosis is the final common result of a variety of progressive injuries leading to chronic renal failure. Transforming growth factor-beta (TGF-beta) is reportedly upregulated in response to injurious stimuli such as unilateral ureteral obstruction (UUO), causing renal fibrosis associated with epithelial-mesenchymal transition (EMT) of the renal tubules and synthesis of extracellular matrix. We now show that mice lacking Smad3 (Smad3ex8/ex8), a key signaling intermediate downstream of the TGF-beta receptors, are protected against tubulointerstitial fibrosis following UUO as evidenced by blocking of EMT and abrogation of monocyte influx and collagen accumulation. Culture of primary renal tubular epithelial cells from wild-type or Smad3-null mice confirms that the Smad3 pathway is essential for TGF-beta1-induced EMT and autoinduction of TGF-beta1. Moreover, mechanical stretch of the cultured epithelial cells, mimicking renal tubular distention due to accumulation of urine after UUO, induces EMT following Smad3-mediated upregulation of TGF-beta1. Exogenous bone marrow monocytes accelerate EMT of the cultured epithelial cells and renal tubules in the obstructed kidney after UUO dependent on Smad3 signaling. Together the data demonstrate that the Smad3 pathway is central to the pathogenesis of interstitial fibrosis and suggest that inhibitors of this pathway may have clinical application in the treatment of obstructive nephropathy.

Peroxisome proliferator-activated receptor-gamma ligands inhibit TGF-beta 1-induced fibronectin expression in glomerular mesangial cells

The thiazolidinedione (TZD) class of antidiabetic drugs, which are ligands for peroxisome proliferator-activated receptor (PPAR)-gamma, has been shown to possess potent anti-inflammatory and antineoplastic actions. Here, we show in mesangial cells that PPAR-gamma agonists inhibit fibronectin expression by transforming growth factor (TGF)-beta 1. TGF-beta 1 enhanced fibronectin mRNA expression, and this enhancement was abrogated by pretreatment with pioglitazone. Electrophoretic mobility shift assay identified that pioglitazone inhibited TGF-beta 1-induced DNA binding of activator protein-1 (AP-1). Pioglitazone inhibited AP-1 reporter activity but not Smad binding elements reporter activity without affecting TGF-beta 1-induced activation of mitogen-activated protein kinases (MAPKs) or Smad2. PPAR-gamma overexpression inhibited TGF-beta 1-induced fibronectin expression as well as the activation of AP-1. 15-Deoxy-Delta(12,14)-prostaglandin J(2) (15d-PGJ(2)), a natural PPAR-gamma ligand, also inhibited TGF-beta1-induced fibronectin expression by suppressing AP-1 activation by TGF-beta 1. 15d-PGJ(2) inhibited the TGF-beta 1-induced MAPK activation. Dominant-negative PPAR-gamma (Delta PPAR-gamma) completely abrogated the inhibitory effect of pioglitazone and incompletely blocked its effect of 15d-PGJ(2) on TGF-beta 1-induced AP-1 reporter activity. Delta PPAR-gamma overexpression did not affect the inhibitory effect of 15d-PGJ(2) on TGF-beta 1-induced MAPK activation. In conclusion, pioglitazone inhibits TGF-beta 1-induced fibronectin expression by inhibiting AP-1 activation dependent on PPAR-gamma, while 15d-PGJ(2) acts through a dual mechanism independent of and dependent on PPAR-gamma activation in mouse mesangial cells.

Activin receptor-like kinase (ALK)1 is an antagonistic mediator of lateral TGFbeta/ALK5 signaling

Transforming growth factor-beta (TGFbeta) regulates the activation state of the endothelium via two opposing type I receptor/Smad pathways. Activin receptor-like kinase-1 (ALK1) induces Smad1/5 phosphorylation, leading to an increase in endothelial cell proliferation and migration, while ALK5 promotes Smad2/3 activation and inhibits both processes. Here, we report that ALK5 is important for TGFbeta/ALK1 signaling; endothelial cells lacking ALK5 are deficient in TGFbeta/ALK1-induced responses. More specifically, we show that ALK5 mediates a TGFbeta-dependent recruitment of ALK1 into a TGFbeta receptor complex and that the ALK5 kinase activity is required for optimal ALK1 activation. TGFbeta type II receptor is also required for ALK1 activation by TGFbeta. Interestingly, ALK1 not only induces a biological response opposite to that of ALK5 but also directly antagonizes ALK5/Smad signaling.

Mechanisms of TGF-beta signaling from cell membrane to the nucleus

TGF-beta signaling controls a plethora of cellular responses and figures prominently in animal development. Recent cellular, biochemical, and structural studies have revealed significant insight into the mechanisms of the activation of TGF-beta receptors through ligand binding, the activation of Smad proteins through phosphorylation, the transcriptional regulation of target gene expression, and the control of Smad protein activity and degradation. This article reviews these latest advances and presents our current understanding on the mechanisms of TGF-beta signaling from cell membrane to the nucleus.

Smad3: a key player in pathogenetic mechanisms dependent on TGF-beta

Transforming growth factor-beta (TGF-beta), a key player in a large variety of physiological and disease processes, signals through transmembrane receptor serine/threonine kinases to activate novel signaling intermediates called Smad proteins, which then modulate transcription of target genes. We have utilized mice with a targeted deletion of Smad3, one of two homologous proteins involved in signaling from TGF-beta/activin, to investigate the function of this particular pathway in transducing such effects of TGF-beta. The dramatic results of the absence of Smad3 on parameters of healing of cutaneous wounds, such as reepithelialization and influx of inflammatory cells, as well as on fibrosis as modeled by radiation fibrosis of skin in mice, suggest that signaling flux through Smad3 is critical for chemotactic activity of TGF-beta, inhibitory effects of TGF-beta on keratinocyte proliferation and migration, and chemoattraction and elaboration of extracellular matrix by fibroblasts in fibrotic diseases. We recently identified a novel molecule, TLP for TRAP-1-like protein, which selectively interferes with Smad3 signaling, and are currently investigating whether levels of this protein might be altered in disease to change the relative flow of information from Smad2 and Smad3.

Interfering with TGFbeta-induced Smad3 nuclear accumulation differentially affects TGFbeta-dependent gene expression

BACKGROUND

Transforming growth factor-beta (TGFbeta) plays an important role in late-stage carcinogenesis by stimulating invasive behavior of cancer cells, promoting neo-angiogenesis and by helping cancer cells to escape surveillance by the immune system. It also supports colonization of the bone by metastatic breast cancer cells by increasing expression of osteolytic parathyroid hormone-related protein (PTHrP). Interfering with TGFbeta signalling may thus weaken the malignant properties of cancer cells. We investigated to what extent two inhibitors, SB-202190 and SB-203580, interfere with TGFbeta-signalling in invasive MDA-MB-231 breast cancer cells. These compounds, formerly used as p38-MAPK-specific inhibitors, were recently also demonstrated to inhibit TGFbeta type I receptor kinase.

RESULTS

Our results show that these inhibitors delay the onset of TGFbeta-induced nuclear accumulation of Smad3 and reduces its amplitude. This effect was accompanied by a strong reduction in TGFbeta-responsivess of the slow-responder genes pthrp, pai-1 and upa, while the reactivity of the fast-responder gene smad7 to TGFbeta remained almost unchanged. Neither was the TGFbeta response of the fast-responder ese-1/esx gene, whose expression we found to be strongly downregulated by TGFbeta, affected by the inhibitors.

CONCLUSION

The data show that SB-202190 and SB-203580 suppress TGFbeta-dependent activation of genes that are important for the acquisition of invasive behavior, while having no effect on the expression of the natural TGFbeta inhibitor Smad7. This suggests that these compounds are potent inhibitors of malignant behavior of cancer cells.