Hyperglycemia in Mice with Early Diabetes Predisposes for Cardiac Dysfunction

Abstract

Introduction Cardiac glucose uptake and oxidation are reduced in diabetes despite hyperglycemia, and aberrant fatty acid (FA) utilization by hearts accounts, at least partially, for diabetic cardiomyopathy (DbCM). Recently, we found that FOXO1, which is activated during insulin resistance, increases KLF5 expression that causes DbCM through lipotoxicity and oxidative stress. Herein, we investigated the metabolic profile of cardiomyocytes (CMs) in early and late type-1 diabetes (T1D) and the potential involvement of FOXO1 activation in cardiac complications. Hypothesis In the early stage of diabetes, high glucose accounts for the onset of DbCM. Methods and Results We induced T1D in mice via 5 daily intraperitoneal injections of streptozotocin (STZ). Cardiac protein analysis 4 weeks post-STZ injection (early T1D), showed lower pser256 FOXO1 and increased FOXO1 protein levels in nuclear extracts compared to non-T1D mice, indicating increased nuclear FOXO1 translocation. Despite increased nuclear presence, FOXO1 was transcriptionally inactive as shown by normal FOXO1 acetylation levels and expression of FOXO1 target genes. On the other hand, the same analyses 12 weeks post-STZ injection (late T1D) showed that nuclear FOXO1 translocation coincided with higher transcriptional activity. CoIP analysis suggests that SIRT1 deacetylates FOXO1 in late T1D since high glucose upregulated SIRT1 in a human CM cell line (AC16). In contrast to FOXO1 transcriptional status, echocardiography revealed that CM-specific FOXO1 deleted mice were protected from mild cardiac dysfunction that occurs in early T1D, indicating FOXO1 as a minor contributing factor in early-T1D cardiac complications. Seahorse analysis in adult CMs isolated from early T1D-mice showed higher glucose and lower FA dependence compared to non-T1D mice. In accordance with previous studies, the metabolic profile of adult CMs from mice with late T1D shifted to higher FA and lower glucose dependence compared to non-T1D mice. Then, we explored whether increased glucose dependence in early T1D may account for the initiation of DbCM. To this end, we treated T1D mice with an inhibitor of the sodium/glucose cotransporter (SGLT)2 (dapagliflozin). Dapagliflozin normalized plasma glucose levels, improved insulin sensitivity (GTT and ITT), reduced lipolysis in white adipose tissue and prevented DbCM without affecting hepatic VLDL-triglyceride secretion. Adult CMs of mice treated with STZ and dapagliflozin for 4 weeks showed higher FA and lower glucose dependence compared to mice with early T1D. Accordingly, cardiac lipidome analysis showed that dapagliflozin restored acylcarnitines, which is an essential component of FA oxidation, to normal levels as opposed to T1D-mice that had lower acylcarnitines compared to non-T1D mice. Conclusion Hyperglycemia in early T1D facilitates higher glucose dependence of CMs, which leads to DbCM independent of FOXO1 activation.