Our data suggest that cardiac SGLT1 expression and/or function are regulated by insulin and leptin, and are perturbed in disease. This is the first study to examine the regulation of cardiac SGLT1 expression by insulin and leptin and to determine changes in SGLT1 expression in cardiac disease.

These data demonstrate that HF increases ObR signalling in cardiomyocytes and that activation of ObR signalling improves functional outcomes in chronic ischaemic injury leading to HF.

these data indicate that leptin signalling mitigates cardiac injury in the post-MI failing heart by acting directly on cardiomyocytes to increase STAT3 and AMPK activation, to decrease cardiac hypertrophy, apoptosis, and inflammation, and to limit deleterious changes in cardiac structure, function, and glycolytic metabolism.

Human mutations in the gene PRKAG2 encoding the γ2 subunit of AMP-activated protein kinase (AMPK) cause a glycogen storage cardiomyopathy. Transgenic mice (TG T400N ) with the human T400N mutation exhibit inappropriate activation of AMPK and consequent glycogen storage in the heart. Although increased glucose uptake and activation of glycogen synthesis have been documented in PRKAG2 cardiomyopathy, the mechanism of increased glucose uptake has been uncertain. Wildtype (WT), TG T400N , and TG α2DN (carrying a dominant negative, kinase dead α2 catalytic subunit of AMPK) mice were studied at ages 2-8 weeks. Cardiac mRNA expression of sodiumdependent glucose transporter 1 (SGLT1), but not facilitated-diffusion glucose transporter 1 (GLUT1) or GLUT4, was increased ~5-7 fold in TG T400N mice relative to WT. SGLT1 protein was similarly increased at the cardiac myocyte sarcolemma in TG T400N mice. Phlorizin, a specific SGLT1 inhibitor, attenuated cardiac glucose uptake in TG T400N mice by ~40%, but not in WT mice. Chronic phlorizin treatment reduced cardiac glycogen content by ~25% in TG T400N mice. AICAR, an AMPK activator, increased cardiac SGLT1 mRNA expression ~3 fold in WT mice. Relative to TG T400N mice, double transgenic (TG T400N /TG α2DN ) mice had decreased (~50%) cardiac glucose uptake and decreased (~70%) cardiac SGLT1 expression. TG T400N hearts had increased binding activity of the transcription factors HNF-1 and Sp1 to the promoter of the gene encoding SGLT1. Our data suggest that upregulation of cardiac SGLT1 is responsible for increased cardiac glucose uptake in the TG T400N mouse. Increased AMPK activity leads to upregulation of SGLT1, which in turn mediates increased cardiac glucose uptake.

These results demonstrate that intact leptin signalling post-MI acts through STAT-3 to increase anti-apoptotic bcl-2 and survivin gene expression and reduces caspase-3 activity, consistent with a cardioprotective role of leptin in the setting of chronic ischaemic injury.

Intermittent hypoxia (IH) commonly occurs in patients with obstructive sleep apnea and can cause a wide range of pathology, including reduced left ventricular (LV) ejection fraction in rats as determined by echocardiography, in rodent models. We utilized echocardiography and pressure-volume (PV) loop analyses to determine whether LV contractility was decreased in inbred C57BL/6J mice exposed to IH and whether blockade of beta-adrenergic receptors modified the response to hypoxia. Adult male 9- to 10-wk-old mice were exposed to 4 wk of IH (nadir inspired O(2) 5-6% at 60 cycles/h for 12 h during the light period) or intermittent air (IA) as control. A second group of animals were exposed to the same regimen of IH or IA, but in the presence of nonspecific beta-blockade with propranolol. Cardiac function was assessed by echocardiography and PV loop analyses, and mRNA and protein expression in ventricular homogenates was determined. Contrary to our expectations, we found with PV loop analyses that LV ejection fraction (63.4 +/- 3.5 vs. 50.5 +/- 2.6%, P = 0.015) and other measures of LV contractility were increased in IH-exposed animals compared with IA controls. There were no changes in contractile proteins, atrial natriuretic peptide levels, LV posterior wall thickness, or heart weight with IH exposure. However, cAMP levels were elevated after IH, and propranolol administration attenuated the increase in LV contractility induced by IH exposure. We conclude that, contrary to our hypothesis, 4 wk of IH exposure in C57BL/6J mice causes an increase in LV contractility that occurs independent of ventricular hypertrophy and is, in part, mediated by activation of cardiac beta-adrenergic pathways.

Background-Increased circulating leptin is present in human heart failure, and leptin deficiency is linked to worse outcomes in chronic ischemic injury. In the present observational study, we tested the hypothesis that cardiac leptin production and signaling are increased in the failing human heart, and that mechanical unloading with a ventricular assist device (VAD) reverses these changes. Methods and Results-All studies were performed using human cardiac tissue obtained from (1) hearts not matched for transplantation (nonfailing), (2) at the time of cardiac transplant (failing), or (3) paired samples at the time of VAD implant (pre-VAD) and removal (post-VAD). The expression of brain naturetic peptide, leptin, leptin receptor, and tumor necrosis factor ␣ mRNA was measured, and the protein expression of leptin and its receptor was examined by Western blot and immunofluorescent staining of cardiac sections. The assessment of leptin signaling was performed by measuring the phosphorylation state of the leptin receptor. The phosphorylation state of signal transducer and activator of transcription-3 and AMP-activated kinase proteins were also measured. All data are expressed as meanϮSEM with a statistical significance in failing relative to nonfailing groups determined by Student independent t test, and the significance between pre-and post-VAD groups determined by paired t test. In failing human hearts, the mRNA expressions of leptin and its receptor were increased 5.4Ϯ0.3-fold (PϽ0.05) and 4.5Ϯ0.3-fold (PϽ0.05), respectively, with similar changes in protein. The phosphorylation state of both the leptin receptor and signal transducer and activator of transcription-3 proteins were increased 1.4Ϯ0.1-fold (PϽ0.05), and the level of phosphorylated AMP-activated kinase protein was increased 1.9Ϯ0.2-fold (PϽ0.05). Mechanical unloading of the failing human heart with a VAD resulted in no change in tumor necrosis factor ␣ expression but a marked decrease in leptin production to 1.7Ϯ0.1% (PϽ0.05) and leptin receptor expression to 3.0Ϯ0.2% (PϽ0.05) of pre-VAD levels. Phosphorylation of the leptin receptor, signal transducer and activator of transcription-3, and AMP-activated kinase were also decreased to 45Ϯ7%, 75Ϯ8%, and 58Ϯ8% of pre-VAD values, respectively (PϽ0.05 for all values). Conclusions-These results indicate that the failing human heart increases expression of leptin and its receptor and that mechanical unloading downregulates this increase. Further, a cardioprotective role for leptin in the failing human heart is suggested through the activation of signal transducer and activator of transcription-3 and AMP-activated kinase signaling. (Circ Heart Fail. 2009;2:676-683.)

Hypoxia manifests in many forms including the short repetitive intermittent hypoxia (IH) of sleep apnoea and the continuous hypoxia (CH) of altitude, both of which may impact metabolic function. Based on our own previous studies and the available literature, we hypothesized that whereas acute exposure to IH and CH would lead to comparable metabolic dysfunction, with longer-term exposure, metabolism would normalize to a greater extent with CH than IH. Studies were conducted in lean C57BL/6J mice exposed to either IH or CH for 1 day or 4 weeks and compared to either intermittent air (IA) or unhandled (UN) controls, respectively. We utilized the frequently sampled intravenous glucose tolerance test and minimal model analyses to determine insulin-dependent (insulin sensitivity; SI) and insulin-independent (glucose effectiveness; Sg) glucose disposal, as well as the insulin response to glucose (acute insulin response to glucose; AIRg). Our data show that 1-day exposure impaired the glucose tolerance and caused reductions in Sg and AIRg in both the IH and CH groups, but only IH caused a significant decrease in SI (7.5 ± 2.7 vs. 17.0 ± 5.3 μU ml−1 min−1; p < 0.05). After 4-week exposure, there was evidence of metabolic adaptation in both hypoxic groups, however, in the CH group, there was a supranormal increase in SI relative to both UN and IH groups. We conclude that in lean mice, the marked metabolic dysfunction that occurs with acute exposure to hypoxia is reversed to a greater extent with chronic CH exposure than chronic IH exposure.