Deficiency of Renal Cortical EGF Increases ENaC Activity and Contributes to Salt-Sensitive Hypertension

Various stimuli, including hormones and growth factors, modulate epithelial sodium channels (ENaCs), which fine-tune Na⁺ absorption in the kidney. Members of the EGF family are important for maintaining transepithelial Na⁺ transport, but whether EGF influences ENaC, perhaps mediating salt-sensitive hypertension, is not well understood. Here, the ENaC inhibitor benzamil attenuated the development of hypertension in Dahl salt-sensitive rats. Feeding these salt-sensitive rats a high-salt diet led to lower levels of EGF in the kidney cortex and enhanced the expression and activity of ENaC compared with feeding a low-salt diet. To directly evaluate the role of EGF in the development of hypertension and its effect on ENaC activity, we infused EGF intravenously while continuously monitoring BP of the salt-sensitive rats. Infusion of EGF decreased ENaC activity, prevented the development of hypertension, and attenuated glomerular and renal tubular damage. Taken together, these findings indicate that cortical EGF levels decrease with a high-salt diet in salt-sensitive rats, promoting ENaC-mediated Na⁺ reabsorption in the collecting duct and the development of hypertension.More than 76 million American adults have high BP¹ and the likelihood of developing hypertension significantly increases with age. Nearly 40% of African Americans aged >20 years exhibit hypertension and nearly 70% of these individuals have a form of hypertension that is highly sensitive to salt intake. A reduced ability to maintain sodium homeostasis and normal levels of arterial pressure is a hallmark of all forms of hypertension.² In the kidney, discretionary Na⁺ reabsorption in response to endocrine input to the aldosterone-sensitive distal nephron (ASDN) is a determinant of the pressure-natriuresis relationship, which is of fundamental importance in the long-term control of arterial pressure.^2,3 Although sodium transport in ASDN accounts for a small proportion of renal sodium transport (<10%), ENaC activity is the rate-limiting step for this discretionary Na⁺ reabsorption.⁴The Dahl salt-sensitive (SS) rat strain used in this study is a genetic animal model of hypertension and kidney disease that reveals disease traits similar to those observed in humans. This inbred strain exhibits a low-renin, sodium-sensitive form of hypertension that is associated with severe and progressive proteinuria, glomerulosclerosis, and renal interstitial fibrosis.^5–7The EGF and related hormones are multipotent agents^8–11 involved in regulation of various renal functions and, particularly, ion channel activity. For instance, EGF stimulation rapidly induces TRPC5¹¹ and TRPM6¹⁰ channel translocation to the plasma membrane. Members of the EGF family play an important role in the expansion of renal cysts¹² and promote glomerular injury and renal failure in rapidly progressive crescentic glometulonephritis.¹³ Moreover, Groenestege et al. described a mutation in the pro-EGF encoding gene, which is responsible for development of isolated autosomal recessive hypomagnesemia, associated with renal Mg²⁺ wasting.¹⁴A role for EGF and its related growth factors in the regulation of ENaC-mediated sodium absorption has been proposed, although contradictory results have been observed with respect to ENaC activity and sodium transport.^15–17 Although EGF was shown to stimulate ENaC-mediated renal salt absorption in some studies, others reported that EGF decreases sodium transport and ENaC activity.¹⁷ Our recent data show that EGF and its related growth factors (TGF-α, HB-EGF, and amphiregulin) have a biphasic effect on sodium absorption as represented by the experiments on cultured murine mpkCCD_c14 principal cells.¹⁵ Basolateral application of the EGF family growth factors to polarized mpkCCD_c14 principal cells grown on permeable supports acutely increases Na⁺ reabsorption, whereas chronic treatment of the monolayers with EGF and its related growth factors leads to significant inhibition of ENaC-mediated transport.¹⁵ Similar observations were made in Xenopus laevis A6 principal cells.¹⁶ There are a number of potential mechanisms mediating downstream signaling. Thus, it was proposed that EGF effects could be mediated by either the extracellular signal–regulated kinase 1/2 and mitogen-activated protein kinase pathway, Akt,^16,18 or reactive oxygen species production.¹⁹ENaC dysfunction leads to disturbances in total body Na⁺ homeostasis associated with abnormal regulation of BP as observed in patients with Liddle’s syndrome and pseudohypoaldosteronism type 1.²⁰ However, the exact role of ENaC in mechanisms mediating salt-sensitive hypertension remains unclear. It was proposed that the expression of ENaC is abnormally regulated by dietary sodium in SS rats, and this abnormal expression is one of the factors causing salt-sensitive hypertension.^21–24 Here we confirm that ENaC expression is upregulated on a high-salt (HS) diet and provide direct evidence that ENaC activity is abnormally upregulated by dietary sodium in hypertensive SS rats, and this enhanced activity is one of the major factors causing salt-sensitive hypertension. In addition, our studies identify EGF as a key molecular substrate for a mechanism that diminishes development of salt-sensitive hypertension.