Hydrogen sulfide (H₂S), a novel endogenous gaseous bioactive substance, has recently been implicated in the regulation of cardiovascular and neuronal functions. However, its role in the control of renal function is unknown. In the present study, incubation of renal tissue homogenates with l-cysteine (l-Cys) (as a substrate) produced H₂S in a concentration-dependent manner. This H₂S production was completely abolished by inhibition of both cystathionine β-synthetase (CBS) and cystathionine γ-lyase (CGL), two major enzymes for the production of H₂S, using amino-oxyacetic acid (AOAA), an inhibitor of CBS, and propargylglycine (PPG), an inhibitor of CGL. However, inhibition of CBS or CGL alone induced a small decrease in H₂S production. In anesthetized Sprague-Dawley rats, intrarenal arterial infusion of an H₂S donor (NaHS) increased renal blood flow, glomerular filtration rate (GFR), urinary sodium (U_Na·V), and potassium (U_K·V) excretion. Consistently, infusion of both AOAA and PPG to inhibit the endogenous H₂S production decreased GFR, U_Na·V, and U_K·V, and either one of these inhibitors alone had no significant effect on renal functions. Infusion of l-Cys into renal artery to increase the endogenous H₂S production also increased GFR, U_Na·V, and U_K·V, which was blocked by AOAA plus PPG. It was shown that H₂S had both vascular and tubular effects and that the tubular effect of H₂S might be through inhibition of Na⁺/K⁺/2Cl^- cotransporter and Na⁺/K⁺/ATPase activity. These results suggest that H₂S participates in the control of renal function and increases urinary sodium excretion via both vascular and tubular actions in the kidney.

To examine whether hypoxia-inducible factor (HIF)-1α mediates the profibrotic effects of angiotensin II, we treated cultured renal medullary interstitial cells with angiotensin II and found that it increased HIF-1α levels. This was accompanied by a significant upregulation of collagen I/III, the tissue inhibitor of metalloproteinase-1, elevation of the proliferation marker proliferating cell nuclear antigen, and a transdifferentiation marker vimentin. All these effects of angiotensin II were completely blocked by siRNA for HIF-1α but not HIF-2α. Overexpression of a prolyl-hydroxylase domain-containing protein 2 (PHD2) transgene, the predominant renal HIF prolyl-hydroxylase, attenuated the effects of angiotensin II and its gene silencing enhanced the effects of angiotensin II. Removal of hydrogen peroxide eliminated angiotensin II-induced profibrotic effects. A 2-week infusion of rats with angiotensin II increased the expression of HIF-1α and α-smooth muscle actin, another marker of transdifferentiation, in renal medullary interstitial cells in vivo. Thus, our study suggests that HIF-1α mediates angiotensin II-induced profibrotic effects through activation of cell transdifferentiation. We propose that redox regulation of prolyl-PHD2 plays a critical role in angiotensin II-induced activation of HIF-1α in renal cells.

Although it has been shown that upregulation of hypoxia-inducible factor (HIF)-1α is protective in acute ischemic renal injury, long-term overactivation of HIF-1α is implicated to be injurious in chronic kidney diseases. Angiotensin II (Ang II) is a well-known pathogenic factor producing chronic renal injury and has also been shown to increase HIF-1α. However, the contribution of HIF-1α to Ang II–induced renal injury has not been evidenced. The present study tested the hypothesis that HIF-1α mediates Ang II–induced renal injury in Sprague-Dawley rats. Chronic renal injury was induced by Ang II infusion (200 ng/kg per minute) for 2 weeks in uninephrectomized rats. Transfection of vectors expressing HIF-1α small hairpin RNA into the kidneys knocked down HIF-1α gene expression by 70%, blocked Ang II–induced HIF-1α activation, and significantly attenuated Ang II–induced albuminuria, which was accompanied by inhibition of Ang II–induced vascular endothelial growth factor, a known glomerular permeability factor, in glomeruli. HIF-1α small hairpin RNA also significantly improved the glomerular morphological damage induced by Ang II. Furthermore, HIF-1α small hairpin RNA blocked Ang II–induced upregulation of collagen and α-smooth muscle actin in tubulointerstitial region. There was no difference in creatinine clearance and Ang II–induced increase in blood pressure. HIF-1α small hairpin RNA had no effect on Ang II–induced reduction in renal blood flow and hypoxia in the kidneys. These data suggested that overactivation of HIF-1α–mediated gene regulation in the kidney is a pathogenic pathway mediating Ang II–induced chronic renal injuries, and normalization of overactivated HIF-1α may be used as a treatment strategy for chronic kidney damages associated with excessive Ang II.

Overactivation of hypoxia-inducible factor (HIF)-1α is implicated as a pathogenic factor in chronic kidney diseases (CKD). However, controversy exists regarding the roles of HIF-1α in CKD. Additionally, although hypoxia and HIF-1α activation are observed in various CKD and HIF-1α has been shown to stimulate fibrogenic factors, there is no direct evidence whether HIF-1α is an injurious or protective factor in chronic renal hypoxic injury. The present study determined whether knocking down the HIF-1α gene can attenuate or exaggerate kidney damage using a chronic renal ischemic model. Chronic renal ischemia was induced by unilaterally clamping the left renal artery for 3 wk in Sprague-Dawley rats. HIF-1α short hairpin (sh) RNA or control vectors were transfected into the left kidneys. Experimental groups were sham+control vector, clip+control vector, and clip+HIF-1α shRNA. Enalapril was used to normalize blood pressure 1 wk after clamping the renal artery. HIF-1α protein levels were remarkably increased in clipped kidneys, and this increase was blocked by shRNA. Morphological examination showed that HIF-1α shRNA significantly attenuated injury in clipped kidneys: glomerular injury indices were 0.71 ± 0.04, 2.50 ± 0.12, and 1.34 ± 0.11, and the percentage of globally damaged glomeruli was 0.02, 34.3 ± 5.0, and 6.3 ± 1.6 in sham, clip, and clip+shRNA groups, respectively. The protein levels of collagen and α-smooth muscle actin also dramatically increased in clipped kidneys, but this effect was blocked by HIF-1α shRNA. In conclusion, long-term overactivation of HIF-1α is a pathogenic factor in chronic renal injury associated with ischemia/hypoxia.

Hypoxia inducible factor (HIF)-1α, a transcription factor, is abundantly expressed in the renal medulla and regulates many oxygen-sensitive genes such as nitric oxide synthase, cyclooxygenase-2, and heme oxygenase-1. Given the important roles of these genes in the control of arterial pressure, the present study was to test the hypothesis that HIF-1α–mediated gene activation serves as an antihypertensive pathway by regulating renal medullary function and sodium excretion. HIF-1α decoy oligodeoxynucleotides (ODNs) or scrambled ODNs were transfected into the renal medulla in uninephrectomized Sprague–Dawley rats. Two weeks after ODN transfection, the HIF-1α binding activities were significantly inhibited by 45%, and high salt–induced increases of nitric oxide synthase-2 and heme oxygenase-1 transcriptions were also inhibited by 70% and 61% in the renal medulla from decoy rats. The natriuretic responses and increases of renal medullary blood flow responding to the elevations of renal perfusion pressure were significantly blunted by 50% and 37% in decoy rats. Intravenously acute sodium loading increased medullary blood flow and urinary sodium excretion, which was remarkably attenuated in decoy rats. In decoy rats, high salt intake caused a greater positive sodium balance. Consequently, arterial pressure was remarkably increased (from 118±1.9 to 154±6.3 mm Hg) in decoy rats but not in control rats when the rats were challenged with a high salt diet. There was no blood pressure change in decoy rats that were maintained in normal salt diet. In conclusion, HIF-1α–mediated gene activation importantly participates in the regulation of renal medullary function and long-term arterial blood pressure.