Cardiac myocytes depend on a delicate balance of glucose and free fatty acids as energy sources, a balance that is disrupted in pathological states such as diabetes and myocardial ischaemia.21
Under-stressed conditions such as ischaemia, glucose utilization is activated, resulting in an increase in the amount of ATP generated per mole of oxygen consumed. Therefore, elucidation of the role of SGLT1 in cardiac glucose uptake in health and in disease may inform the development of novel treatments. Classically, it has long been thought that only the GLUT isoforms, GLUT1 and GLUT4, are responsible for glucose uptake in cardiac myocytes.3
However, we have determined that SGLT1 is highly expressed in murine and human cardiac myocytes, with preferential localization in the sarcolemma; SGLT1 expression is altered in diabetic and ischaemic cardiomyopathy; SGLT1 expression may be regulated in part by leptin; and SGLT1 mediates at least part of the increased cardiac glucose uptake in response to insulin and leptin.
One previous study determined unexpectedly high expression of SGLT1 mRNA in the human heart, approximately 10-fold greater than in kidney tissue.6
However, expression of the SGLT1 protein and its cellular localization in heart were not examined. The current study is the first to report the presence of the sodium/glucose cotransporter system in the cardiac myocyte sarcolemma, by both membrane protein fractionation studies and immunofluorescence microscopy. In protein fractions, SGLT1 colocalizes with GLUT1, which is normally localized to the sarcolemma, and with Na+
ATPase, a marker for the sarcolemma; and to a lesser extent with GLUT4, which can be translocated from intracellular stores to the sarcolemma when required. Interestingly, at least in mice, cardiac SGLT1 expression appears to increase with age. The basis for this age dependence is uncertain.
suggest that functional SGLT1 is an oligomer, resulting from homodimerization, or from heterodimerization with RS1, a regulatory protein with a molecular weight very close to that of SGLT1. Our immunoblots of human cardiac protein exhibited two (70 and 140 kDa) SGLT1 bands in all hearts, which could reflect dimerization, but also an intermediate band in at least one heart. Although the identity of the intermediate band is unknown, we speculate that it may represent post-translational modifications in some hearts. For example, it is known that SGLT1 can be phosphorylated. Two SGLT1 bands were also observed previously in rat coronary endothelial cells.25
However, our immunoblots of murine cardiac protein exhibited only monomeric SGLT1. It is unclear at present whether there are species or tissue-specific differences in dimerization and/or post-translational modification.
We next considered whether functional changes in SGLT1 may contribute to the pathophysiology of cardiac diseases, particularly those characterized by increased consumption of glucose. Protein expression of GLUT1 and GLUT4 is decreased in diabetic hearts without a corresponding decrease in cardiac glucose uptake,26
suggesting the presence of other functional cardiac glucose transporters. We observed increased cardiac SGLT1 expression both in human subjects with end-stage cardiomyopathy secondary to type 2 diabetes and in ob/ob
obese mice, a model of type 2 diabetes which exhibits altered myocardial glucose metabolism. Conversely, decreased cardiac expression of SGLT1 was observed in STZ-treated mice, a model of type 1 diabetes. Although the mechanism of divergent SGLT1 expression in type 1 and type 2 diabetes is uncertain, we speculate that increased SGLT1 may be related to chronic hyperinsulinaemia in type 2 diabetes. However, it should be noted that in our study, acute insulin exposure led only to a mild, statistically insignificant increase in SGLT1 expression in WT murine hearts. It is possible that increased SGLT1 in the diabetic heart is an adaptive change in response to a reduction in cardiac GLUT1 and GLUT4 expression.
Ischaemia increases cardiac glucose utilization, requiring a greater capacity for glucose transport across the sarcolemma. In canine hearts subjected to low-flow ischaemia, there is a significant translocation of both GLUT1 and GLUT4 from the intracellular pool to the sarcolemma.27
Several pharmacological agents with demonstrated anti-ischaemic effects have also recently been shown to act by stimulating glucose metabolism in heart.28
In our study, we observed a several-fold increase in SGLT1 expression in both human ischaemic cardiomyopathy and murine hearts subjected to CAL. We observed increased SGLT1 expression associated with the functional recovery in failing human hearts after LVAD insertion, suggesting that upregulation of SGLT1 may be an adaptive response to injury.
Although SGLT1 expression was upregulated by acute administration of leptin, no change was observed with insulin. The role of SGLT1 in the hormonal modulation of cardiac glucose uptake was further addressed by administration of insulin and leptin in WT mice in the presence and absence of phlorizin, a specific inhibitor of SGLT1. Phlorizin inhibited insulin- and leptin-induced increases in cardiac glucose uptake partially to completely. Therefore, SGLT1 appears to be responsible for at least part of, and possibly all of, insulin-stimulated and leptin-stimulated cardiac glucose uptake. Similar to our study, phlorizin was shown to inhibit insulin-stimulated glucose uptake in rat skeletal muscle.25
Although studies to identify the mechanism of increased SGLT1 activity after insulin administration have not yet been performed, insulin may promote trafficking of SGLT1 to the sarcolemma, or may directly stimulate SGLT1 activity. Of note, insulin activates protein kinase C (PKC), and phosphorylation of SGLT1 by PKC29
leads to recruitment of SGLT1 to the plasma membrane.4,30
The relative importance of SGLT1 relative to GLUT1 and GLUT4 in normal and diseased hearts remains uncertain. Mice lacking cardiac GLUT4 exhibited an increase in basal cardiac glucose transport, with a corresponding increase in GLUT1 expression.31
Similar upregulation of SGLT1 as a compensatory mechanism is plausible. Further studies examining the expression and function of SGLT1 in GLUT1 and GLUT4 knockout mice at baseline and in the presence of stressors are warranted.
In conclusion, our data show that SGLT1 is expressed in cardiac myocytes, with preferential localization in the sarcolemma. SGLT1 expression is increased in type 2 diabetes and ischaemia, but decreased in type 1 diabetes. SGLT1 appears to be at least in part responsible for increased cardiac glucose uptake following exposure to insulin and leptin, and leptin appears to act by directly increasing SGLT1 expression. To our knowledge, this is the first study to examine the regulation of SGLT1 by insulin and leptin in the normal heart, and demonstrate changes in SGLT1 expression in disease states. Further studies will be required to determine the therapeutic value of modulation of SGLT1 expression, cellular localization, and activity.