Metformin, an AMPK activator, stimulates the phosphorylation of aquaporin 2 and urea transporter A1 in inner medullary collecting ducts

Abstract

Nephrogenic diabetes insipidus (NDI) is characterized by production of very large quantities of dilute urine due to an inability of the kidney to respond to vasopressin. Congenital NDI results from mutations in the type 2 vasopressin receptor (V2R) in ∼90% of families. These patients do not have mutations in aquaporin-2 (AQP2) or urea transporter UT-A1 (UT-A1). We tested adenosine monophosphate kinase (AMPK) since it is known to phosphorylate another vasopressin-sensitive transporter, NKCC2 (Na-K-2Cl cotransporter). We found AMPK expressed in rat inner medulla (IM). AMPK directly phosphorylated AQP2 and UT-A1 in vitro. Metformin, an AMPK activator, increased phosphorylation of both AQP2 and UT-A1 in rat inner medullary collecting ducts (IMCDs). Metformin increased the apical plasma membrane accumulation of AQP2, but not UT-A1, in rat IM. Metformin increased both osmotic water permeability and urea permeability in perfused rat terminal IMCDs. These findings suggest that metformin increases osmotic water permeability by increasing AQP2 accumulation in the apical plasma membrane but increases urea permeability by activating UT-A1 already present in the membrane. Lastly, metformin increased urine osmolality in mice lacking a V2R, a mouse model of congenital NDI. We conclude that AMPK activation by metformin mimics many of the mechanisms by which vasopressin increases urine-concentrating ability. These findings suggest that metformin may be a novel therapeutic option for congenital NDI due to V2R mutations.

Metformin improves urine concentration in rodents with nephrogenic diabetes insipidus

“…The beneficial effect of metformin was sustained for up to 10 days. Metformin, given by gavage, also enhanced urine osmolality in V2R KO mice, starting from the third hour and peaking at the seventh hour, consistent with our previous study (12). These results indicate that metformin, a well-known and safe oral antidiabetic drug, might be a novel treatment option for congenital NDI and potentially for other types of NDI as well.…”

“…We also showed that AMPK directly phosphorylates AQP2 and UT-A1 in vitro (12). In inner medullary suspensions from control animals, metformin increased AQP2 membrane association but did not increase UT-A1 in the membrane (12). These experiments suggested regulation of transporter function by acute AMPK stimulation.…”

“…In previous studies, we showed that metformin treatment of isolated perfused inner medullary collecting ducts increased urea and water permeability (12). We also showed that AMPK directly phosphorylates AQP2 and UT-A1 in vitro (12).…”

“…Metformin is an oral antidiabetic drug that stimulates both AMPK catalytic subunits (13). Our previous data have shown that metformin can stimulate AQP2 membrane accumulation, and water and urea permeability, in rat inner medullary collecting duct cells (12). In addition, AMPK was found to increase the phosphorylation of NKCC2 at Ser126 in vivo and in vitro (14) (Figure 1).…”

“…Although present throughout the body, it has only recently been shown to be present in the kidney medulla (12). Metformin is an oral antidiabetic drug that stimulates both AMPK catalytic subunits (13).…”