Hypoxia has been proposed as an important microenvironmental factor in the development of tissue fibrosis; however, the underlying mechanisms are not well defined. To examine the role of hypoxia-inducible factor-1 (HIF-1), a key mediator of cellular adaptation to hypoxia, in the development of fibrosis in mice, we inactivated Hif-1alpha in primary renal epithelial cells and in proximal tubules of kidneys subjected to unilateral ureteral obstruction (UUO) using Cre-loxP-mediated gene targeting. We found that Hif-1alpha enhanced epithelial-to-mesenchymal transition (EMT) in vitro and induced epithelial cell migration through upregulation of lysyl oxidase genes. Genetic ablation of epithelial Hif-1alpha inhibited the development of tubulointerstitial fibrosis in UUO kidneys, which was associated with decreased interstitial collagen deposition, decreased inflammatory cell infiltration, and a reduction in the number of fibroblast-specific protein-1-expressing (FSP-1-expressing) interstitial cells. Furthermore, we demonstrate that increased renal HIF-1alpha expression is associated with tubulointerstitial injury in patients with chronic kidney disease. Thus, we provide clinical and genetic evidence that activation of HIF-1 signaling in renal epithelial cells is associated with the development of chronic renal disease and may promote fibrogenesis by increasing expression of extracellular matrix-modifying factors and lysyl oxidase genes and by facilitating EMT.

CTGF plays a significant role in the development of renal fibrosis by mediating the fibrotic effects of transforming growth factor (TGF)-β1and has been shown to be hypoxia inducible in human breast cancer cells. It has been suggested that hypoxia is an important underlying cause for the development of renal fibrosis through the modulation of profibrotic genes. One of the key mediators of the cell's response to lowered oxygen environments is hypoxia-inducible-factor-1 (HIF-1), a basic helix-loop-helix transcription factor, which enables cells to adapt to hypoxia by regulating the expression of genes involved in increasing oxygen availability ( VEGF, erythropoietin) and enhancing glucose uptake and metabolism ( Glut-1, PGK). In this paper, we have used primary tubular epithelial cell cultures from a tetracycline-inducible- Hif- 1α knockout murine model to further elucidate the role of Hif-1 in the hypoxic-induction of Ctgf expression. We show that hypoxia response elements present upstream of Ctgf enable direct interaction of Hif-1 transcription factor with the Ctgf promoter, resulting in increased transcription of Ctgf mRNA. Cells deficient in Hif- 1α were incapable of inducing Ctgf mRNA in response to hypoxia, suggesting an absolute requirement of Hif-1. Furthermore, the observed Hif-1-mediated hypoxic stimulation of Ctgf expression was found to occur independently of TGF-β1signaling. Our findings have important implications for a number of fibrotic disorders in which hypoxia, CTGF, and TGF-β1are involved, including renal, dermal, hepatic, and pulmonary fibrosis.

Chronic hypoxia accelerates renal fibrosis. The chief mediator of the hypoxic response is hypoxia-inducible factor 1 (HIF-1) and its oxygen-sensitive component HIF-1alpha. HIF-1 regulates a wide variety of genes, some of which are closely associated with tissue fibrosis. To determine the specific role of HIF-1 in renal fibrosis, we generated a knockout mouse in which tubular epithelial expression of von Hippel-Lindau tumor suppressor (VHL), which acts as a ubiquitin ligase to promote proteolysis of HIF-1alpha, was targeted. We investigated the effect of VHL deletion (i.e., stable expression of HIF-1alpha) histologically and used the anti-HIF-1alpha agent [3-(5'-hydroxymethyl-2'-furyl)-1-benzyl indazole] (YC-1) to test whether inhibition of HIF-1alpha could represent a novel approach to treating renal fibrosis. The area of renal fibrosis was significantly increased in a 5/6 renal ablation model of VHL-/- mice and in all VHL-/- mice at least 60 wk of age. Injection of YC-1 inhibited the progression of renal fibrosis in unilateral ureteral obstruction model mice. In conclusion, HIF-1alpha appears to be a critical contributor to the progression of renal fibrosis and could be a useful target for its treatment.

Germ line mutations in the VHL tumor suppressor gene result in von Hippel-Lindau (VHL) disease, a familial tumor syndrome that predisposes affected patients to the development of highly vascularized neoplasms. These include hemangioblastomas of the retina and central nervous system (CNS), renal-cell carcinomas (RCC) of the clear-cell type, and endocrine and exocrine pancreatic tumors, as well as pheochromocytomas (34). In addition, VHL has also been found to be inactivated in the majority of sporadic RCC (8, 13).VHL deficiency leads to constitutive activation of hypoxiainducible factor (HIF) and increased expression of its target genes irrespective of the oxygen concentration (38). The VHL gene product (pVHL), together with elongins B and C (6, 25), Cullin-2 (43), and Rbx1 (23), forms an E3 ubiquitin ligase (19), which targets the hydroxylated, oxygen-sensitive ␣ subunits of HIF-1, -2, and -3 for ubiquitination and subsequent degradation by the 26S proteasome (18,20,39). As a normal physiological response to hypoxia, HIF-1 and HIF-2 facilitate both oxygen delivery and adaptation to oxygen deprivation by regulating genes that are involved in glucose uptake and metabolism, angiogenesis, erythropoiesis, cell proliferation, and apoptosis (50, 62). HIFs belong to the PAS (Per-Arnt-Sim) family of basic helix-loop-helix (bHLH) transcription factors that bind to DNA as heterodimers composed of an oxygensensitive ␣ subunit and a constitutively expressed ␤ subunit, also known as the arylhydrocarbon receptor nuclear translocator (ARNT). ARNT is the general binding partner for the bHLH/PAS domain-containing proteins. In addition to forming heterocomplexes with HIF, ARNT also heterodimerizes with single minded (SIM), which is involved in neural development, and with the arylhydrocarbon receptor (AhR), which is involved in the xenobiotic response to environmental toxins (for a review, see reference 24).The expression patterns of the HIF subunits differ within embryonic and adult tissues. In the adult, Hif-1␣ mRNA is ubiquitously expressed, and Hif-1␣ protein can be detected at baseline levels within multiple cell types in various tissues under normoxia and is significantly enhanced under conditions of hypoxia (55). In contrast, while Hif-2␣ mRNA expression has been detected within many tissues, Hif-2␣ protein has been found to be restricted to specific cell types within various tissues. In addition to being expressed in endothelial cells, Hif-2␣ is also expressed in glial cells of the brain, type II pneumocytes of the lung, cardiomyocytes, fibroblasts of the kidney, intersitial cells of the pancreas and duodenum, and hepatocytes (26,63). Arnt expression is ubiquitous, and it seems to be the only Hif-␤ subunit present in the liver (21).pVHL appears to have multiple functions besides regulating HIF, and its contributions to the development of VHL-associated tumors are presently subject to intense investigations. As a result of these efforts, it has been shown that pVHL plays an important role in fibronectin extracellular-matrix...

Capillary rarefaction is a hallmark of fibrotic diseases and results in reduced blood perfusion and oxygen delivery. In the kidney, tubulointerstitial fibrosis, which leads to the destruction of renal tissue and the irreversible loss of kidney function, is associated with hypoxia and the activation of Hypoxia-Inducible-Factor (HIF) signaling. HIF-1 and HIF-2 are basic-helix-loop-helix transcription factors that allow cells to survive in a low oxygen environment by regulating energy metabolism, vascular remodeling, erythropoiesis, cellular proliferation and apoptosis. Recent studies suggest that HIF activation promotes epithelial to mesenchymal transition (EMT) and renal fibrogenesis. These findings raise the possibility that the spectrum of HIF activated biological responses to hypoxic stress may differ under conditions of acute and chronic hypoxia. Here we discuss the role of HIF signaling in the pathogenesis and progression of chronic kidney disease.

Lipoxins, which are endogenously produced lipid mediators, promote the resolution of inflammation, and may inhibit fibrosis, suggesting a possible role in modulating renal disease. Here, lipoxin A4 (LXA 4 ) attenuated TGF-b1-induced expression of fibronectin, N-cadherin, thrombospondin, and the notch ligand jagged-1 in cultured human proximal tubular epithelial (HK-2) cells through a mechanism involving upregulation of the microRNA let-7c. Conversely, TGF-b1 suppressed expression of let-7c. In cells pretreated with LXA 4 , upregulation of let-7c persisted despite subsequent stimulation with TGF-b1. In the unilateral ureteral obstruction model of renal fibrosis, let-7c upregulation was induced by administering an LXA 4 analog. Bioinformatic analysis suggested that targets of let-7c include several members of the TGF-b1 signaling pathway, including the TGF-b receptor type 1. Consistent with this, LXA 4 -induced upregulation of let-7c inhibited both the expression of TGF-b receptor type 1 and the response to TGF-b1. Overexpression of let-7c mimicked the antifibrotic effects of LXA 4 in renal epithelia; conversely, anti-miR directed against let-7c attenuated the effects of LXA 4 . Finally, we observed that several let-7c target genes were upregulated in fibrotic human renal biopsies compared with controls. In conclusion, these results suggest that LXA 4 -mediated upregulation of let-7c suppresses TGF-b1-induced fibrosis and that expression of let7c targets is dysregulated in human renal fibrosis.

Treatment with the lipoxin analog 15-epi-16-(FPhO)-LXA4-Me prior to injury modified the expression of many differentially expressed pathogenic mediators, including cytokines, growth factors, adhesion molecules, and proteases, suggesting a renoprotective action at the core of the pathophysiology of acute renal failure (ARF). Importantly, this lipoxin-modulated transcriptomic response included many genes expressed by renal parenchymal cells and was not merely a reflection of a reduced renal mRNA load resulting from attenuated leukocyte recruitment. The data presented herein suggest a framework for understanding drivers of kidney injury in ischemia/reperfusion and the molecular basis for renoprotection by lipoxins in this setting.

Unresolved inflammation underlies the development of fibrosis and organ failure. Here, we investigate the potential of the proresolving eicosanoid lipoxinA₄ (LXA₄) and its synthetic analog benzo-LXA₄ to prophylactically modulate fibrotic and inflammatory responses in a model of early renal fibrosis, unilateral ureteric obstruction (UUO). Male Wistar rats (Animalia, Chordata, Rattus norvegicus) were injected intravenously with vehicle (0.1% ethanol), LXA₄ (45 μg/250-g rat), or benzo-LXA₄ (15 μg/250-g rat) 15 min prior to surgery and sacrificed 3 d postligation. Renal gene and protein expression, collagen deposition, macrophage infiltration, and apoptosis were analyzed using manipulated kidneys from sham operations as control. Lipoxins (LXs) attenuated collagen deposition and renal apoptosis (P<0.05) and shifted the inflammatory milieu toward resolution, inhibiting TNF-α and IFN-γ expression, while stimulating proresolving IL-10. LXs attenuated UUO-induced activation of MAP kinases, Akt, and Smads (P<0.05) in injured kidneys. We explored whether the underlying mechanism reflected LX-induced modulation of fibroblast activation. Using cultured rat renal NRK-49F fibroblasts, we report that LXA₄ (1 nM) inhibits TGF-β1 (10 ng/ml)-induced activation of Smad2 and MAP-kinases (P<0.05), and furthermore, LXA₄ reduced TGF-β1-stimulated PAI-1 luciferase activation (P<0.05) relative to vehicle-stimulated cells. We propose that LXs may represent a potentially useful and novel therapeutic strategy for consideration in the context of renal fibrosis.