Accumulating data indicate that renal uric acid (UA) handling is altered in diabetes and by hypoglycemic agents. In addition, hyperinsulinemia is associated with hyperuricemia and hypouricosuria. However, the underlying mechanisms remain unclear. In this study, we aimed to investigate how diabetes and hypoglycemic agents alter the levels of renal urate transporters. In insulin-depleted diabetic rats with streptozotocin treatment, both UA excretion and fractional excretion of UA were increased, suggesting that tubular handling of UA is altered in this model. In the membrane fraction of the kidney, the expression of urate transporter 1 (URAT1) was significantly decreased, whereas that of ATP-binding cassette subfamily G member 2 (ABCG2) was increased, consistent with the increased renal UA clearance. Administration of insulin to the diabetic rats decreased UA excretion and alleviated UA transporter-level changes, while sodium glucose cotransporter 2 inhibitor (SGLT2i) ipragliflozin did not change renal UA handling in this model. To confirm the contribution of insulin in the regulation of urate transporters, normal rats received insulin and separately, ipragliflozin. Insulin significantly increased URAT1 and decreased ABCG2 levels, resulting in increased UA reabsorption. In contrast, the SGLT2i did not alter URAT1 or ABCG2 levels, although blood glucose levels were similarly reduced. Furthermore, we found that insulin significantly increased endogenous URAT1 levels in the membrane fraction of NRK-52E cells, the kidney epithelial cell line, demonstrating the direct effects of insulin on renal UA transport mechanisms. These results suggest a previously unrecognized mechanism for the anti-uricosuric effects of insulin and provide novel insights into the renal UA handling in the diabetic state.

Significance: Aldosterone produces distinct adaptive responses in volume depletion and hyperkalemia. Mutations in with-no-lysine (WNK) kinases or ubiquitin ligases containing Cullin 3 (CUL3) and Kelch-like 3 (KLHL3) cause a Mendelian disease featuring hypertension and hyperkalemia due to constitutive renal salt reabsorption and inhibited K⁺secretion. WNKs modulate activities of aldosterone-regulated electrolyte flux pathways, and WNK levels are regulated by CUL3/KLHL3; disease-causing mutations prevent WNK degradation. This manuscript shows that angiotensin II (AII), a hormone produced only in volume depletion, induces PKC-mediated phosphorylation of KLHL3, preventing WNK degradation and phenocopying KLHL3 mutations. These findings provide a mechanism by which AII signaling alters WNK4, promoting increased renal salt reabsorption and reduced K⁺secretion.

A key role of aldosterone and mineralocorticoid receptor is to regulate fluid volume and K⁺homeostasis in the body by acting on the renal distal nephron. Global responses of the kidney to elevated aldosterone levels are determined by the coordinate action of different constituent tubule cells, including principal cells, intercalated cells and distal convoluted tubule cells. Recent studies on genetic mutations causing aldosterone overproduction have identified the molecules involved in aldosterone biosynthesis in the adrenal gland, and there is also increasing evidence for mechanisms and signaling pathways regulating the balance between renal NaCl reabsorption and K⁺secretion, the two major effects of aldosterone. In particular, recent studies have demonstrated that mineralocorticoid receptor in intercalated cells is selectively regulated by phosphorylation, which prevents ligand binding and activation. Moreover, the ubiquitin ligase complex composed of Kelch-like 3 and Cullin 3 acts downstream of angiotensin II and plasma K⁺alterations, regulating Na–Cl cotransporter independently of aldosterone in distal convoluted tubule cells. These and other effects are integrated to produce appropriate kidney responses in a high-aldosterone state, and are implicated in fluid and electrolyte disorders in humans. This review summarizes the current knowledge on mechanisms modulating mineralocorticoid receptor and its downstream effectors in the distal nephron.

Significance: Calcineurin inhibitors (CNIs) are potent immunosuppressants; hypertension and hyperkalemia are common adverse effects. Activation of the renal Na-Cl cotransporter (NCC) is implicated in this toxicity; however, the mechanism is unknown. CNIs’ renal effects mimic the hypertension and hyperkalemia resulting from mutations in WNK kinases or in KLHL3-CUL3 ubiquitin ligase. WNKs activate NCC and are degraded by ubiquitylation upon their binding to KLHL3. The binding of WNKs to KLHL3 is prevented by KLHL3 mutations or by PKC-mediated KLHL3 phosphorylation at serine 433. This work shows that calcineurin dephosphorylates KLHL3 ^S433 , promoting WNK4 degradation. Conversely, CNIs inhibit KLHL3 ^S433 dephosphorylation, preventing WNK degradation. These findings implicate calcineurin in the normal regulation of KLHL3’s binding of WNK4 and identify a direct target causing CNI-induced pathology.

Significance: The kinase WNK4 (with-no-lysine kinase 4) is an important regulator of the Na-Cl cotransporter (NCC) in the renal distal convoluted tubule (DCT). Volume depletion induces angiotensin II, activating PKC, which prevents WNK4 degradation by phosphorylating the KLHL3/CUL3 ubiquitin ligase. We now show that PKC also directly phosphorylates WNK4 at multiple sites in cell culture. Phosphorylation of two of these sites, S64 and S1196, promotes increased WNK4 kinase activity by increasing autophosphorylation of the WNK4 T-loop at S332. Volume depletion also induces phosphorylation of WNK4-S64 in the DCT in vivo, promoting NCC activity. These findings provide insights into the mechanisms regulating activity of NCC and the promotion of renal Na-Cl reabsorption without concomitant K⁺secretion in volume depletion.