Activation of retinoid receptor-mediated signaling ameliorates diabetes-induced cardiac dysfunction in Zucker diabetic rats.

Diabetic cardiomyopathy (DCM) is a significant contributor to the morbidity and mortality associated with diabetes and metabolic syndrome. Retinoids, through activation of retinoic acid receptor (RAR) and retinoid x receptor (RXR), have been linked to control glucose and lipid homeostasis, with effects on obesity and diabetes. However, the functional role of RAR and RXR in the development of DCM remains unclear. Zucker diabetic fatty (ZDF) and lean rats were treated with Am580 (RARα agonist) or LGD1069 (RXR agonist) for 16 weeks, and cardiac function and metabolic alterations were determined. Hyperglycemia, hyperlipidemia and insulin resistance were observed in ZDF rats. Diabetic cardiomyopathy was characterized in ZDF rats by increased oxidative stress, apoptosis, fibrosis, inflammation, activation of MAP kinases and NF-κB signaling and diminished Akt phosphorylation, along with decreased glucose transport and increased cardiac lipid accumulation, and ultimately diastolic dysfunction. Am580 and LGD1069 attenuated diabetes-induced cardiac dysfunction and the pathological alterations, by improving glucose tolerance and insulin resistance; facilitating Akt activation and glucose utilization, and attenuating oxidative stress and interrelated MAP kinase and NF-κB signaling pathways. Am580 inhibited body weight gain, attenuated the increased cardiac fatty acid uptake, ß-oxidation and lipid accumulation in the hearts of ZDF rats. However, LGD1069 promoted body weight gain, hyperlipidemia and cardiac lipid accumulation. In conclusion, our data suggest that activation of RAR and RXR may have therapeutic potential in the treatment of diabetic cardiomyopathy. However, further studies are necessary to clarify the role of RAR and RXR in the regulation of lipid metabolism and homeostasis.

Retinoic acid protects cardiomyocytes from high glucose-induced apoptosis through inhibition of NF-κB signaling pathway.

We have previously shown that retinoic acid (RA) has protective effects on high glucose (HG)-induced cardiomyocyte apoptosis. To further elucidate the molecular mechanisms of RA effects, we determined the interaction between nuclear factor (NF)-κB and RA signaling. HG induced a sustained phosphorylation of IKK/IκBα and transcriptional activation of NF-κB in cardiomyocytes. Activated NF-κB signaling has an important role in HG-induced cardiomyocyte apoptosis and gene expression of interleukin-6 (IL-6), tumor necrosis factor (TNF)-α, and monocyte chemoattractant protein-1 (MCP-1). All-trans RA (ATRA) and LGD1069, through activation of RAR/RXR-mediated signaling, inhibited the HG-mediated effects in cardiomyocytes. The inhibitory effect of RA on NF-κB activation was mediated through inhibition of IKK/IκBα phosphorylation. ATRA and LGD1069 treatment promoted protein phosphatase 2A (PP2A) activity, which was significantly suppressed by HG stimulation. The RA effects on IKK and IκBα were blocked by okadaic acid or silencing the expression of PP2Ac-subunit, indicating that the inhibitory effect of RA on NF-κB is regulated through activation of PP2A and subsequent dephosphorylation of IKK/IκBα. Moreover, ATRA and LGD1069 reversed the decreased PP2A activity and inhibited the activation of IKK/IκBα and gene expression of MCP-1, IL-6, and TNF-α in the hearts of Zucker diabetic fatty rats. In summary, our findings suggest that the suppressed activation of PP2A contributed to sustained activation of NF-κB in HG-stimulated cardiomyocytes; and that the protective effect of RA on hyperglycemia-induced cardiomyocyte apoptosis and inflammatory responses is partially regulated through activation of PP2A and suppression of NF-κB-mediated signaling and downstream targets.

High glucose-induced repression of RAR/RXR in cardiomyocytes is mediated through oxidative stress/JNK signaling.

The biological actions of retinoids are mediated by nuclear retinoic acid receptors (RARs) and retinoid X receptors (RXRs). We have recently reported that decreased expression of RARα and RXRα has an important role in high glucose (HG)-induced cardiomyocyte apoptosis. However, the regulatory mechanisms of HG effects on RARα and RXRα remain unclear. Using neonatal cardiomyocytes, we found that ligand-induced promoter activity of RAR and RXR was significantly suppressed by HG. HG promoted protein destabilization and serine-phosphorylation of RARα and RXRα. Proteasome inhibitor MG132 blocked the inhibitory effect of HG on RARα and RXRα. Inhibition of intracellular reactive oxidative species (ROS) abolished the HG effect. In contrast, H(2)O(2) stimulation suppressed the expression and ligand-induced promoter activity of RARα and RXRα. HG promoted phosphorylation of ERK1/2, JNK and p38 MAP kinases, which was abrogated by an ROS inhibitor. Inhibition of JNK, but not ERK and p38 activity, reversed HG effects on RARα and RXRα. Activation of JNK by over expressing MKK7 and MEKK1, resulted in significant downregulation of RARα and RXRα. Ligand-induced promoter activity of RARα and RXRα was also suppressed by overexpression of MEKK1. HG-induced cardiomyocyte apoptosis was potentiated by activation of JNK, and prevented by all-trans retinoic acid and inhibition of JNK. Silencing the expression of RARα and RXRα activated the JNK pathway. In conclusion, HG-induced oxidative stress and activation of the JNK pathway negatively regulated expression/activation of RAR and RXR. The impaired RAR/RXR signaling and oxidative stress/JNK pathway forms a vicious circle, which significantly contributes to hyperglycemia induced cardiomyocyte apoptosis.