Background— We previously identified renalase, a secreted novel amine oxidase that specifically degrades circulating catecholamines. Parenteral administration of either native or recombinant renalase lowers blood pressure, heart rate, and cardiac contractility by metabolizing circulating catecholamines. Renalase plasma levels are markedly reduced in patients with chronic kidney disease. It is not known whether endogenous renalase contributes to the regulation of catecholamines. Methods and Results— We show here that circulating renalase lacks significant amine oxidase activity under basal conditions (prorenalase) but that a brief surge of epinephrine lasting <2 minutes causes renalase activity to increase from 48±18 to 2246±98 arbitrary units (n=3; P 5 mm Hg. The catecholamine surge also leads to a 2.8-fold increase in plasma renalase concentration. Cultured cells exposed to dopamine upregulate steady-state renalase gene expression by >10-fold. The time course of prorenalase activation is abnormal in rats with chronic kidney disease. Conclusions— These data identify a novel mechanism for the regulation of circulating catecholamines. In the renalase pathway, excess catecholamine facilitates the conversion of prorenalase, an inactive plasma amine oxidase, to renalase, which can degrade catecholamines. Excess catecholamines not only regulate the activation of prorenalase but also promote its secretion and synthesis. Because chronic kidney disease is associated with a number of systemic abnormalities, including activation of the sympathetic nervous system, increased catecholamines levels, cardiac hypertrophy, and hypertension, renalase replacement is an attractive therapeutic modality owing to its role in catecholamine metabolism.

Kv1.3 is a voltage-gated potassium (K) channel expressed in a number of tissues, including fat and skeletal muscle. Channel inhibition improves experimental autoimmune encephalitis, in part by reducing IL-2 and tumor necrosis factor production by peripheral T lymphocytes. Gene inactivation causes mice (Kv1.3-/-) exposed to a high-fat diet to gain less weight and be less obese than littermate control. Interestingly, although Kv1.3-/- mice on the high-calorie diet gain weight, they remain euglycemic, with low blood insulin levels. This observation prompted us to examine the effect of Kv1.3 gene inactivation and inhibition on peripheral glucose homeostasis and insulin sensitivity. Here we show that Kv1.3 gene deletion and channel inhibition increase peripheral insulin sensitivityin vivo. Baseline and insulin-stimulated glucose uptake are increased in adipose tissue and skeletal muscle of Kv1.3-/- mice. Inhibition of Kv1.3 activity facilitates the translocation of the glucose transporter, GLUT4, to the plasma membrane. It also suppresses c-JUN terminal kinase activity in fat and skeletal muscle and decreases IL-6 and tumor necrosis factor secretion by adipose tissue. We conclude that Kv1.3 inhibition improves insulin sensitivity by increasing the amount of GLUT4 at the plasma membrane. These results pinpoint a pathway through which K channels regulate peripheral glucose homeostasis, and identify Kv1.3 as a pharmacologic target for the treatment of diabetes.

Background: Blood pressure is acutely regulated by the sympathetic nervous system through the action of vasoactive hormones such as epinephrine, norepinephrine, and dopamine. Renalase, a recently described, secreted flavoprotein, acutely decreases systemic pressure when administered in vivo. Single‐nucleotide polymorphisms present in the gene are associated with hypertension, cardiac disease, and diabetes. Although renalase's crystal structure was recently solved, its natural substrate(s) remains undefined. Methods and Results: Using in vitro enzymatic assays and in vivo administration of recombinant renalase, we show that the protein functions as a flavin adenine dinucleotide– and nicotinamide adenine dinucleotide–dependent oxidase that lowers blood pressure by degrading plasma epinephrine. The enzyme also metabolizes the dopamine precursor L‐3,4‐dihydroxyphenylalanine but has low activity against dopamine and does not metabolize norepinephrine. To test if epinephrine and L‐3,4‐dihydroxyphenylalanine were renalase's only substrates, 17 246 unique small molecules were screened. Although the search revealed no additional, naturally occurring compounds, it identified dobutamine, isoproterenol, and α‐methyldopa as substrates of renalase. Mutational analysis was used to test if renalase's hypotensive effect correlated with its enzymatic activity. Single–amino acid mutations that decrease its enzymatic activity to varying degrees comparably reduce its hypotensive effect. Conclusions: Renalase metabolizes circulating epinephrine and L‐3,4‐dihydroxyphenylalanine, and its capacity to decrease blood pressure is directly correlated to its enzymatic activity. These findings highlight a previously unrecognized mechanism for epinephrine metabolism and blood pressure regulation, expand our understanding of the sympathetic nervous system, and could lead to the development of novel therapeutic modalities for the treatment of hypertension. ( J Am Heart Assoc . 2012;1:e002634 doi: 10.1161/ JAHA .112.002634.)

AKI is characterized by increased catecholamine levels and hypertension. Renalase, a secretory flavoprotein that oxidizes catecholamines, attenuates ischemic injury and the associated increase in catecholamine levels in mice. However, whether the amine oxidase activity of renalase is involved in preventing ischemic injury is debated. In this study, recombinant renalase protected human proximal tubular (HK-2) cells against cisplatin- and hydrogen peroxide–induced necrosis. Similarly, genetic depletion of renalase in mice (renalase knockout) exacerbated kidney injury in animals subjected to cisplatin-induced AKI. Interestingly, compared with the intact renalase protein, a 20–amino acid peptide (RP-220), which is conserved in all known renalase isoforms, but lacks detectable oxidase activity, was equally effective at protecting HK-2 cells against toxic injury and preventing ischemic injury in wild-type mice. Furthermore, in vitro treatment with RP-220 or recombinant renalase rapidly activated Akt, extracellular signal-regulated kinase, and p38 mitogen-activated protein kinases and downregulated c-Jun N-terminal kinase. In summary, renalase promotes cell survival and protects against renal injury in mice through the activation of intracellular signaling cascades, independent of its ability to metabolize catecholamines, and we have identified the region of renalase required for these effects. Renalase and related peptides show potential as therapeutic agents for the prevention and treatment of AKI.

Renalase, a recently discovered flavoprotein, which is strongly expressed in the kidney and heart, effectively metabolizes catecholamines. It was discovered during the search to identify proteins secreted by the kidney that could help explain the high incidence of cardiovascular disease in patients with chronic kidney disease. Recent advances have led to more detailed knowledge of its biology, structure, enzymatic activity, mechanisms of action, associations with human disease states and potential therapeutic value. In this study, we review these advances with a focus on hypertension and kidney disease.