The likely role of SIRT1 in mediating the effects of SGLT2 inhibitors is noteworthy, since SIRT1 (but not AMPK) can stimulate HIF-2α [
20,
21], the primary transactivator of the gene for erythropoietin synthesis [
142]. Interestingly, SGLT2 inhibitors have been strongly linked to the enhanced production of erythropoietin and to an increase in red blood cell mass in clinical trials [
121,
143‐
145]. More importantly, activation of HIF-2α can be expected to exert its own effects to promote autophagy and mute cellular stress and inflammation [
19,
29‐
31]. In contrast, as a result of AMPK agonism, metformin suppresses the activity of HIF-2α [
89], and thus, the drug decreases the hematocrit [
88]. The potential differences in HIF-2α signaling between SGLT2 inhibitors and metformin may be clinically relevant, since (in statistical mediation analyses) the erythrocytosis produced by SGLT2 inhibitors is the most powerful predictor of the ability of these drugs to reduce the risk of serious heart failure events in large-scale clinical trials [
144,
145].
Interestingly, in the mediation analyses of large-scale cardiovascular outcomes trials, the effect of SGLT2 inhibitors to decrease serum uric acid is also a major independent predictor of the drug-related reduction in serious heart failure events [
144,
145]. Previous work attributed the urate-lowering effects of SGLT2 inhibitors to an effect of these drugs to simultaneously inhibit glucose and uric acid reabsorption in the proximal renal tubule [
146], since glycosuria may directly enhance fractional excretion of uric acid [
147]. However, urate is also a biomarker of oxidative stress in the stressed myocardium [
148‐
150], i.e., the increase in reactive oxygen species in patients with diabetes leads to activation of xanthine oxidase, the enzyme that catalyzes the synthesis of uric acid [
151]. Interestingly, the depletion of nicotinamide adenine dinucleotide (NAD+) in diabetes not only causes upregulation of xanthine oxidase but also downregulation of SIRT1 [
152,
153]. There is an inverse relationship between the activities of SIRT1 and xanthine oxidase. Upregulation of xanthine oxidase suppresses SIRT1 [
154] and inhibition of xanthine oxidase activates SIRT1 [
155]; thus, serum levels of uric acid are inversely related to the activity of SIRT1 in states of energy overabundance [
156]. Therefore, by enhancing SIRT1-mediated suppression of oxidative stress or by a direct consequence of SIRT1 activation [
157‐
159], SGLT2 inhibitors may suppress the activity of xanthine oxidase and reduce serum uric acid [
145,
160]. Thus, activation of SIRT1 may explain the observed statistical link between the urate-lowering and cardioprotective effects of SGLT2 inhibitors. In contrast, metformin (which does not enhance signaling through SIRT1) increases serum uric acid [
161].