Cardiomyocyte glycophagy is regulated by insulin and exposure to high extracellular glucose

Abstract

Disturbed systemic glycemic and insulinemic status elicits cardiomyocyte metabolic stress and altered glucose handling. In diabetes, pathological myocardial glycogen accumulation occurs. Recently, evidence of a specific myocardial autophagic degradation pathway for glycogen ("glycophagy") has been reported, differentiated from the more well-characterized protein "macrophagy" pathway. The goal of this study was to identify potential mechanisms involved in cardiac glycogen accumulation, glycophagy, and macrophagy regulation using cultured neonatal rat ventricular myocytes (NRVMs). In NRVMs, insulin-induced Akt phosphorylation was evident with 5 mM-glucose conditions (∼2.3-fold increased). Under high-glucose (30 mM) conditions, insulin-augmented phosphorylation was not observed. Accumulation of glycogen was observed in response to insulin only in high-glucose conditions (∼2-fold increase). Increased expression of the glycophagy marker starch-binding domain-containing protein-1 (STBD1, 25% increase) was observed under high-glucose and insulin conditions. Expression levels of the macrophagy markers p62 and light chain protein 3BII:I were not increased by insulin at either glucose level. Preliminary results from hearts of streptozotocin-treated diabetic rats are supportive of the findings obtained in NRVMs, suggesting diabetes induced elevated expression of STBD1 and of an additional glycophagy marker GABA(A) receptor-associated protein-like 1. Confocal microscopy demonstrated that light chain protein 3B and STBD1 immunomarkers were not colocalized in NRVMs. These findings provide the first evidence that cardiomyocyte glycophagy induction occurs under the influence of insulin and is responsive to extracellular high glucose. This study suggests that the regulation of glycogen content and glycophagy induction in the cardiomyocyte may be linked, and it is speculated that glycogen pathology in diabetic cardiomyopathy has glycophagic involvement.

Cardiac Insulin Signaling Regulates Glycolysis Through Phosphofructokinase 2 Content and Activity

“…However, prolonged rerouting of glucose to alternative pathways, especially when circulating glucose levels are elevated, may be detrimental for cardiac health. For example, glycogen content is counterintuitively elevated during fasting40; during the long‐term, hyperglycemia leads to overt glycogen accumulation, a process associated with cardiomyocyte stress and cardiac pathological features 41, 42. Future experiments will test the hypothesis that sustaining PFK‐2 content and activity may be a means of mitigating long‐term rerouting of EGIs to alternative pathways.…”

Myocardial autophagic energy stress responses—macroautophagy, mitophagy, and glycophagy

“…In cultured neonatal rat cardiomyocytes, protein expression of the glycophagy marker STBD1 was increased by extracellular insulin concentration linked with activation of the insulin-regulated PI3K/Akt signaling pathway (61). Similarly, fasting-induced upregulation of STBD1 and GABARAPL1 protein content was associated with activation of Akt in vivo (76).…”

“…Since our first report of increased cardiac autophagy in the type 2 diabetic fructose-fed mouse (59), the experimental literature in this field has expanded considerably, but it is not yet possible to synthesize a comprehensive understanding. Relying on a variable selection of molecular tools, states of increased (4,12,49,50,59,61,78,93,102), unchanged (47,48,57,62), and decreased (6,23,28,29,73,82,101,103,104,110) basal cardiac autophagy activity have all been reported in diabetic/ insulin resistant contexts (see Table 1). These discrepancies are not necessarily attributable to the different models of diabetes, since contrasting findings have been observed within the same diabetic model [e.g., STZ-mouse: increased LC3BII (12) vs. decreased LC3BII (103)].…”

“…Recently we have demonstrated that the autophagic/lysosomal glycogen degradation pathway (glycogen-specific autophagy, 'glycophagy') is operational in the adult heart and is upregulated by diabetes in vivo (61,76). Glycophagy is well characterized in the newborn liver and heart where a transient upregulation of lysosomal acid ␣-glucosidase mediates an increase in autophagic glycogen degradation (43).…”

A systems genetics approach identifiesTrp53inp2as a link between cardiomyocyte glucose utilization and hypertrophic response

“…An assay for quantification of cardiomyocyte glycogen content was adapted from a previous study (18,40). Briefly, cardiomyocytes were scraped in PBS and pelleted cells lysed (50 mM Tris·HCl, 2% SDS, and 5% glycerol).…”