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FIH1 (factor inhibiting HIF-1) in the kidney: more than an oxygen sensor?
co m m e nt a r y
see original article on page 857
FIH1 (factor inhibiting HIF-1) in the kidney: more than an oxygen sensor? Yasemin Sirin1 and Hermann Pavenstädt1 Factor inhibiting HIF-1 (FIH1) inhibits the activity of hypoxia-inducible factor 1 (HIF-1) by preventing HIF-1 from binding to p300/CBP. Schödel et al. demonstrate that, in the kidney, FIH1 is expressed selectively in the distal tubules and podocytes. Although FIH1 functions as a transcriptional repressor of HIF target genes in tubular cells, it does not do so in podocytes. This study suggests that FIH1 may have diverse implications for distal tubular function and podocyte biology. Kidney International (2010) 78, 836–837. doi:10.1038/ki.2010.282
The maintenance of oxygen homeostasis is an important biological necessity that requires coordinated regulation of a range of different genes. The hypoxiainducible transcription factors (HIFs) control the expression of most genes involved in cellular processes during adaptation to hypoxic conditions, including angiogenesis, metabolism, vasodilation, and cell migration.1 Within the kidney, hypoxia plays a critical role in the pathogenesis of tubular and glomerular injury. It acts as an important factor contributing to tubular atrophy, interstitial fibrosis, and induction of HIF or inhibition of prolyl-4-hydroxylases (PHDs), thus protecting the kidney against injury.2 Under aerobic conditions, HIF-1 is bound by the von Hippel–Lindau tumor suppressor protein (Vhlh), which targets the protein for ubiquitination and degradation. However, under hypoxic conditions, Vhlh cannot bind to HIF-1α, which leads to its stabilization. Thus, HIF-1α can translocate to the nucleus, where it 1Medizinische Klinik und Poliklinik D, University of Münster, Münster, Germany Correspondence: Hermann Pavenstädt, Medizinische Klinik und Poliklinik D, University of Münster, Albert-Schweitzer Strasse 33, 48149 Münster, Germany. E-mail: [email protected]
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dimerizes with HIF-1 and activates the transcription of downstream target genes. Vhlh loss of function results in consecutive stabilization of HIF-1 and upregulat i o n o f H I F- 1 t a r g e t g e n e s . 1 Podocyte-specific deletion of Vhlh has been shown to initiate a necrotizing crescentic glomerulonephritis accompanied by the expression of CXCR4, one of the HIF target genes. The presence of chemokine receptors such as CXCR4 in podocytes has been demonstrated before.3,4 Treatment with antibodies to CXCR4 has resulted in the delayed onset and decreased severity of renal disease in podocyte-specific Vhlh knockout mice.3 In concordance with these findings, it could be shown that the expression of hypoxia-inducible factors (HIFs) and CXCR4 is significantly altered in human nephrosclerosis. Moreover, a plasticity of podocytes during hypoxic glomerular damage has been suggested.5 The activity or stability of HIF-1 is maintained by prolyl- and asparagyl-4hydroxylases (PHDs/FIH1), which belong to the superfamily of iron (II) and 2-oxoglutarate–dependent dioxygenases. Hydroxylation of the HIF-1 subunit by PHDs and FIH1 blocks the association of HIFs with the transcriptional coactivators, thus inhibiting transcriptional activation. Both prolyl hydroxylases, which
determine HIF-1 abundance, and the asparagyl hydroxylase, which controls transcriptional activity, execute their functions in an oxygen-dependent manner and thus serve as cellular oxygen sensors.1 FIH1 knockout mice did not show critical alterations in HIF functions, such as angiogenesis, erythropoiesis, or development. However, mice lacking FIH1 exhibited reduced body weight, an elevated metabolic rate, hyperventilation, and improved glucose and lipid homeostasis. They were resistant to high-fat-diet– induced weight gain and hepatic steatosis. Neuron-specific loss of FIH1 mimicked some of the major metabolic phenotypes of the FIH1 knockout mice, indicating that FIH1 regulates metabolism through effects on neurons.6 In terms of the kidney, our understanding of the expression pattern of FIH1 and its functional relevance is incomplete. Schödel and colleagues for the first time show that, in the kidney, tubular FIH1 expression is limited to the thick ascending limb, distal convoluted tubules, and collecting ducts. In the collecting ducts, both principal and intercalated cells expressed FIH1 (Schödel et al.7). In the distal tubules, HIF-1, PHD2, and FIH1 were coexpressed, suggesting a synergistic role in the control of HIF accumulation. Whereas HIF-1 was activated in the proximal tubules under hypoxic conditions, FIH1 was not expressed in this segment of the kidney, suggesting that HIF-1 might be regulated in a different manner in the proximal tubules. Within the glomerula, FIH1 was expressed selectively in podocytes.7 FIH1 knockdown had no significant effect on HIF-1 expression under hypoxic conditions. To better understand the functional role of FIH1 in HIF-controlled gene regulation in tubular cells and podocytes, the authors silenced FIH1 in both cell types in vitro. Thereafter, they investigated transcript levels of the HIF-1 target genes GLUT1, PHD3, VEGF, CXCR4, and TGF1, which are known to play a role in HIF-1–dependent functions.7 Under atmospheric oxygen levels (i.e., in the absence of HIF-1), FIH1 knockdown did not affect HIF-1 target gene expression in tubular cells, whereas it induced the expression of several target genes, such as Kidney International (2010) 78
co m m e nt a r y
Inflammation CXCR4 OH FIH1
Signaling proteins, NICD, etc.
HIF-1α
Future studies have to show the functional significance of these observations. Conditional knockout mice for FIH1 are now available.6 Using these, it will be fascinating to investigate the functional consequences of a specific FIH1 gene deletion in the distal tubules and podocytes, respectively. DISCLOSURE The authors declared no competing interests. REFERENCES 1. 2.
Figure 1 | Hypothetical model of the function of FIH1 in podocytes under atmospheric oxygen levels. Under atmospheric oxygen levels, HIF-1 is suppressed in podocytes. FIH1 may then hydroxylate signaling proteins, such as the Notch intracellular domain (NICD), which in turn induces a downregulation of the CXCR4 receptor, leading to the inhibition of podocyte inflammation.
CXCR4, in podocytes.7 This unexpected observation might be explained by an activation of HIF to levels below the detection limit. Alternatively and more probably, FIH1 might regulate podocyte function via HIF-1-independent mechanisms. FIH1 regulates several other signaling proteins; for example, it has been shown that FIH1 hydroxylates two residues of the Notch intracellular domain (NICD), leading to an attenuation of Notch function.8 Since upregulation of NICD in podocytes has been shown to play an important role in the pathogenesis of diabetic nephropathy and focal segmental glomerulosclerosis,9 it could be interesting to study a putative interaction between FIH1 and Notch in podocytes.
Kidney International (2010) 78
Under hypoxic conditions (i.e., in the presence of HIF-1), FIH1 knockdown in tubular cells enhanced HIF target gene expression, revealing the transcriptional inhibition of HIF by FIH1. However, an increase of HIF-1α target gene expression could not be observed in podocytes. In addition to the in vitro data, the authors showed a decrease of glomerular FIH1 expression in a model of acute proteinuric glomerulonephritis. This was accompanied by increased CXCR4 expression, suggesting that FIH1 might inhibit CXCR4 expression in vivo and thus might have implications in the development and / or propagation of glomerular inflammation (Figure 1).
3.
4.
5.
6.
7.
8.
9.
Nakayama K. Cellular signal transduction of the hypoxia response. J Biochem 2009; 146: 757–765. Tanaka T, Nangaku M. The role of hypoxia, increased oxygen consumption, and hypoxiainducible factor-1 alpha in progression of chronic kidney disease. Curr Opin Nephrol Hypertens 2010; 19: 43–50. Ding M, Cui S, Li C et al. Loss of the tumor suppressor Vhlh leads to upregulation of Cxcr4 and rapidly progressive glomerulonephritis in mice. Nat Med 2006; 12: 1081–1087. Huber TB, Reinhardt HC, Exner M et al. Expression of functional CCR and CXCR chemokine receptors in podocytes. J Immunol 2002; 168: 6244–6252. Neusser MA, Lindenmeyer MT, Moll AG et al. Human nephrosclerosis triggers a hypoxia-related glomerulopathy. Am J Pathol 2010; 176: 594–607. Zhang N, Fu Z, Linke S et al. The asparaginyl hydroxylase factor inhibiting HIF-1alpha is an essential regulator of metabolism. Cell Metab 2010; 11: 364–378. Schödel J, Bohr D, Klanke B et al. Factor inhibiting HIF limits the expression of hypoxia inducible genes in podocytes and distal tubular cells. Kidney Int 2010; 78: 857–867. Zheng X, Linke S, Dias JM et al. Interaction with factor inhibiting HIF-1 defines an additional mode of cross-coupling between the Notch and hypoxia signaling pathways. Proc Natl Acad Sci USA 2008; 105: 3368–3373. Niranjan T, Bielesz B, Gruenwald A et al. The Notch pathway in podocytes plays a role in the development of glomerular disease. Nat Med 2008; 14: 290–298.
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see original article on page 857
FIH1 (factor inhibiting HIF-1) in the kidney: more than an oxygen sensor? Yasemin Sirin1 and Hermann Pavenstädt1 Factor inhibiting HIF-1 (FIH1) inhibits the activity of hypoxia-inducible factor 1 (HIF-1) by preventing HIF-1 from binding to p300/CBP. Schödel et al. demonstrate that, in the kidney, FIH1 is expressed selectively in the distal tubules and podocytes. Although FIH1 functions as a transcriptional repressor of HIF target genes in tubular cells, it does not do so in podocytes. This study suggests that FIH1 may have diverse implications for distal tubular function and podocyte biology. Kidney International (2010) 78, 836–837. doi:10.1038/ki.2010.282
The maintenance of oxygen homeostasis is an important biological necessity that requires coordinated regulation of a range of different genes. The hypoxiainducible transcription factors (HIFs) control the expression of most genes involved in cellular processes during adaptation to hypoxic conditions, including angiogenesis, metabolism, vasodilation, and cell migration.1 Within the kidney, hypoxia plays a critical role in the pathogenesis of tubular and glomerular injury. It acts as an important factor contributing to tubular atrophy, interstitial fibrosis, and induction of HIF or inhibition of prolyl-4-hydroxylases (PHDs), thus protecting the kidney against injury.2 Under aerobic conditions, HIF-1 is bound by the von Hippel–Lindau tumor suppressor protein (Vhlh), which targets the protein for ubiquitination and degradation. However, under hypoxic conditions, Vhlh cannot bind to HIF-1α, which leads to its stabilization. Thus, HIF-1α can translocate to the nucleus, where it 1Medizinische Klinik und Poliklinik D, University of Münster, Münster, Germany Correspondence: Hermann Pavenstädt, Medizinische Klinik und Poliklinik D, University of Münster, Albert-Schweitzer Strasse 33, 48149 Münster, Germany. E-mail: [email protected]
836
dimerizes with HIF-1 and activates the transcription of downstream target genes. Vhlh loss of function results in consecutive stabilization of HIF-1 and upregulat i o n o f H I F- 1 t a r g e t g e n e s . 1 Podocyte-specific deletion of Vhlh has been shown to initiate a necrotizing crescentic glomerulonephritis accompanied by the expression of CXCR4, one of the HIF target genes. The presence of chemokine receptors such as CXCR4 in podocytes has been demonstrated before.3,4 Treatment with antibodies to CXCR4 has resulted in the delayed onset and decreased severity of renal disease in podocyte-specific Vhlh knockout mice.3 In concordance with these findings, it could be shown that the expression of hypoxia-inducible factors (HIFs) and CXCR4 is significantly altered in human nephrosclerosis. Moreover, a plasticity of podocytes during hypoxic glomerular damage has been suggested.5 The activity or stability of HIF-1 is maintained by prolyl- and asparagyl-4hydroxylases (PHDs/FIH1), which belong to the superfamily of iron (II) and 2-oxoglutarate–dependent dioxygenases. Hydroxylation of the HIF-1 subunit by PHDs and FIH1 blocks the association of HIFs with the transcriptional coactivators, thus inhibiting transcriptional activation. Both prolyl hydroxylases, which
determine HIF-1 abundance, and the asparagyl hydroxylase, which controls transcriptional activity, execute their functions in an oxygen-dependent manner and thus serve as cellular oxygen sensors.1 FIH1 knockout mice did not show critical alterations in HIF functions, such as angiogenesis, erythropoiesis, or development. However, mice lacking FIH1 exhibited reduced body weight, an elevated metabolic rate, hyperventilation, and improved glucose and lipid homeostasis. They were resistant to high-fat-diet– induced weight gain and hepatic steatosis. Neuron-specific loss of FIH1 mimicked some of the major metabolic phenotypes of the FIH1 knockout mice, indicating that FIH1 regulates metabolism through effects on neurons.6 In terms of the kidney, our understanding of the expression pattern of FIH1 and its functional relevance is incomplete. Schödel and colleagues for the first time show that, in the kidney, tubular FIH1 expression is limited to the thick ascending limb, distal convoluted tubules, and collecting ducts. In the collecting ducts, both principal and intercalated cells expressed FIH1 (Schödel et al.7). In the distal tubules, HIF-1, PHD2, and FIH1 were coexpressed, suggesting a synergistic role in the control of HIF accumulation. Whereas HIF-1 was activated in the proximal tubules under hypoxic conditions, FIH1 was not expressed in this segment of the kidney, suggesting that HIF-1 might be regulated in a different manner in the proximal tubules. Within the glomerula, FIH1 was expressed selectively in podocytes.7 FIH1 knockdown had no significant effect on HIF-1 expression under hypoxic conditions. To better understand the functional role of FIH1 in HIF-controlled gene regulation in tubular cells and podocytes, the authors silenced FIH1 in both cell types in vitro. Thereafter, they investigated transcript levels of the HIF-1 target genes GLUT1, PHD3, VEGF, CXCR4, and TGF1, which are known to play a role in HIF-1–dependent functions.7 Under atmospheric oxygen levels (i.e., in the absence of HIF-1), FIH1 knockdown did not affect HIF-1 target gene expression in tubular cells, whereas it induced the expression of several target genes, such as Kidney International (2010) 78
co m m e nt a r y
Inflammation CXCR4 OH FIH1
Signaling proteins, NICD, etc.
HIF-1α
Future studies have to show the functional significance of these observations. Conditional knockout mice for FIH1 are now available.6 Using these, it will be fascinating to investigate the functional consequences of a specific FIH1 gene deletion in the distal tubules and podocytes, respectively. DISCLOSURE The authors declared no competing interests. REFERENCES 1. 2.
Figure 1 | Hypothetical model of the function of FIH1 in podocytes under atmospheric oxygen levels. Under atmospheric oxygen levels, HIF-1 is suppressed in podocytes. FIH1 may then hydroxylate signaling proteins, such as the Notch intracellular domain (NICD), which in turn induces a downregulation of the CXCR4 receptor, leading to the inhibition of podocyte inflammation.
CXCR4, in podocytes.7 This unexpected observation might be explained by an activation of HIF to levels below the detection limit. Alternatively and more probably, FIH1 might regulate podocyte function via HIF-1-independent mechanisms. FIH1 regulates several other signaling proteins; for example, it has been shown that FIH1 hydroxylates two residues of the Notch intracellular domain (NICD), leading to an attenuation of Notch function.8 Since upregulation of NICD in podocytes has been shown to play an important role in the pathogenesis of diabetic nephropathy and focal segmental glomerulosclerosis,9 it could be interesting to study a putative interaction between FIH1 and Notch in podocytes.
Kidney International (2010) 78
Under hypoxic conditions (i.e., in the presence of HIF-1), FIH1 knockdown in tubular cells enhanced HIF target gene expression, revealing the transcriptional inhibition of HIF by FIH1. However, an increase of HIF-1α target gene expression could not be observed in podocytes. In addition to the in vitro data, the authors showed a decrease of glomerular FIH1 expression in a model of acute proteinuric glomerulonephritis. This was accompanied by increased CXCR4 expression, suggesting that FIH1 might inhibit CXCR4 expression in vivo and thus might have implications in the development and / or propagation of glomerular inflammation (Figure 1).
3.
4.
5.
6.
7.
8.
9.
Nakayama K. Cellular signal transduction of the hypoxia response. J Biochem 2009; 146: 757–765. Tanaka T, Nangaku M. The role of hypoxia, increased oxygen consumption, and hypoxiainducible factor-1 alpha in progression of chronic kidney disease. Curr Opin Nephrol Hypertens 2010; 19: 43–50. Ding M, Cui S, Li C et al. Loss of the tumor suppressor Vhlh leads to upregulation of Cxcr4 and rapidly progressive glomerulonephritis in mice. Nat Med 2006; 12: 1081–1087. Huber TB, Reinhardt HC, Exner M et al. Expression of functional CCR and CXCR chemokine receptors in podocytes. J Immunol 2002; 168: 6244–6252. Neusser MA, Lindenmeyer MT, Moll AG et al. Human nephrosclerosis triggers a hypoxia-related glomerulopathy. Am J Pathol 2010; 176: 594–607. Zhang N, Fu Z, Linke S et al. The asparaginyl hydroxylase factor inhibiting HIF-1alpha is an essential regulator of metabolism. Cell Metab 2010; 11: 364–378. Schödel J, Bohr D, Klanke B et al. Factor inhibiting HIF limits the expression of hypoxia inducible genes in podocytes and distal tubular cells. Kidney Int 2010; 78: 857–867. Zheng X, Linke S, Dias JM et al. Interaction with factor inhibiting HIF-1 defines an additional mode of cross-coupling between the Notch and hypoxia signaling pathways. Proc Natl Acad Sci USA 2008; 105: 3368–3373. Niranjan T, Bielesz B, Gruenwald A et al. The Notch pathway in podocytes plays a role in the development of glomerular disease. Nat Med 2008; 14: 290–298.
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