Silencing of HIF prolyl-hydroxylase 2 gene in the renal medulla attenuates salt-sensitive hypertension in Dahl S rats.

Salt-sensitive hypertension accounts for 50% of hypertension cases1 and exhibits a much higher risk for development of organ damage than salt-resistant hypertension.2,3 The mechanism regulating salt sensitivity of blood pressure is not very clear. Renal medullary function is well known to play a critical role in the regulation of sodium excretion and blood pressure, and it is known that dysfunction in the renal medulla is involved in salt-sensitive hypertension.4,5 We have recently shown that transcription factor hypoxia-inducible factor (HIF) 1α–mediated activation of antihypertensive genes in the renal medulla enhances the production of a variety of protective factors in the renal medulla, which promotes the excretion of extra sodium load and regulates the renal adaptation to high salt intake.6 HIF-1α and many HIF-1α target genes, such as hemeoxygenase 1 (HO-1), cyclooxygenase 2 (COX-2), nitric oxide synthase 2 (NOS-2), and endothelin 1, are highly expressed in the renal medulla and significantly upregulated in response to high salt intake.7–13 The products of these HIF-1α target genes importantly participate in the regulation of blood flow and/or tubular activity in the renal medulla and play critical roles in sodium balance and long-term control of arterial blood pressure as well as salt sensitivity of blood pressure.7,8,11–15 We have demonstrated that high salt diet upregulates HIF-1α levels in the renal medulla6,16 and that blockade of HIF-1α function to inhibit the expression of its target genes in the renal medulla induces sodium retention after high-salt challenge, producing a salt-sensitive hypertension.6 These results suggest that HIF-1α–mediated gene activation in the renal medulla represents an important molecular adaptive mechanism to maintain sodium balance in response to high salt intake. Interestingly, it has been shown that the above protective genes regulated by HIF-1α are defective in Dahl salt-sensitive hypertensive (Dahl S) rats12,13,16–18 and that the deficiencies of these HIF-1α target genes in the renal medulla are considered to be responsible for the development of hypertension in this animal model.12,13,17 We recently showed that upregulation of renal medullary HIF-1α levels in response to high salt intake was blunted in Dahl S rats,16,19 indicating that the abnormal responses of the above protective genes may be due to a defect of HIF-1α in the renal medulla and that impairment in HIF-1α–mediated gene activation in the renal medulla may be responsible for salt-sensitive hypertension in Dahl S rats. Indeed, correction of HIF-1α deficiency in the renal medulla increased the expression of antihypertensive genes in the renal medulla, enhanced the urinary sodium excretion, reduced sodium retention, and consequently, attenuated salt-sensitive hypertension in Dahl S rats.19 Furthermore, it has been demonstrated that HIF prolyl-hydroxylase 2 (PHD2), an enzyme that promotes the degradation of HIF-1α, is the most abundant isoform of PHDs in the kidneys20,21 and is highly expressed in the renal medulla.16,20,21 We have shown that high salt intake suppresses the expression of PHD2 in the renal medulla and that this high salt–induced inhibition of PHD2 is an upstream signal that increases HIF-1α–mediated gene expression in the renal medulla in response to high-salt challenge.16 Notably, the high salt–induced inhibition in PHD2 in the renal medulla is also defective in Dahl S rats.16 This study sought to test the hypothesis that deficiency in PHD2/HIF-1α–mediated molecular adaptation in response to high salt intake in the renal medulla may be the pathogenic mechanism responsible for salt-sensitive hypertension and that silencing the PHD2 gene to increase the levels of HIF-1α and its target genes in the renal medulla enhances the sodium excretion and attenuates salt-sensitive hypertension in Dahl S rats. We first transfected PHD2 short hairpin RNA (shRNA) plasmids into the renal medulla and then detected the pressure natriuresis, the renal sodium excretion after sodium overload, and the arterial blood pressure after high-salt challenge in Dahl S rats. Our data showed that correction of the defect in PHD2 response to high salt intake attenuated salt-sensitive hypertension in Dahl S rats.