5-Aminoimidazole-4-carboxamide ribonucleoside stabilizes low density lipoprotein receptor mRNA in hepatocytes via ERK-dependent HuR binding to an AU-rich element.

OBJECTIVE: 5-Aminoimidazole-4-carboxamide ribonucleoside (AICAR) has pleiotropic and beneficial effects on metabolic disorders. However, the effects of AICAR on low density lipoprotein (LDL) metabolism are poorly understood. METHODS AND RESULTS: AICAR induces increased LDLR mRNA levels and increased LDLR protein production in hepatocytes. The AICAR-dependent LDLR mRNA increase was partially mediated by mRNA stabilization in an extracellular signal-regulated kinase1/2 (ERK1/2)-dependent manner, but not by the AMP-activated protein kinase (AMPK) activation. Reporter assays using a variety of constructs harboring the 3'-untranslated region (UTR) of human LDLR mRNA revealed that the most upstream AU-rich element (ARE) was critical for these AICAR effects. Using UV cross-linking assays, we found increased binding of three cytoplasmic proteins to this ARE region in response to AICAR and identified a 35-kDa protein as Human antigen R (HuR). Blocking ERK signaling pathway activation resulted in attenuated HuR binding. Silencing HuR expression by RNA interference hindered AICAR-induced LDLR mRNA stability, whereas its overexpression stabilized this mRNA. CONCLUSIONS: AICAR-dependent LDLR mRNA stabilization is mediated, at least in part, by HuR binding to the ARE1 region. Given that AICAR enhanced LDL uptake in hepatocytes, our findings warrant further studies using animal models to develop a novel LDL-cholesterol lowering agent as a possible strategy to treat atherosclerosis-related cardiovascular diseases.

Resveratrol increases the expression and activity of the low density lipoprotein receptor in hepatocytes by the proteolytic activation of the sterol regulatory element-binding proteins.

OBJECTIVE: The hepatocyte low density lipoprotein receptor (LDLR) plays a pivotal role in lipoprotein metabolism by lowering plasma LDL-cholesterol, a risk factor for atherosclerosis. The present study was conducted to investigate the effects of grape polyphenols on LDLR gene expression in human hepatocyte models. METHODS AND RESULTS: Among the 14 phenolic compounds in red wine, we found that a stilbene trans-resveratrol most strongly up-regulated LDLR gene expression in HepG2 cells. Trans-resveratrol increased the LDLR protein and uptake of fluorescent-labeled LDL. Moreover, it enhanced LDLR gene promoter activity through the proteolytic activation of the sterol regulatory element-binding protein-2 (SREBP-2) as well as SREBP-1. However, sterols completely abolished trans-resveratrol-induced SREBP activation and LDLR gene expression. Finally, AMP-activated protein kinase (AMPK) knockdown analyses by siRNA revealed that AMPK activation was unnecessary for the effects of trans-resveratrol. CONCLUSIONS: Trans-resveratrol up-regulated hepatic LDLR expression via proteolytic activation of SREBPs. We concluded that trans-resveratrol exhibits the anti-atherogenic effect, at least in part, by increased hepatic LDLR expression and subsequent LDL uptake.

Stabilization of small heterodimer partner mRNA by grape seed procyanidins extract in cultured hepatocytes.

SCOPE: Consumption of dietary grape seed procyanidins extract (GSPE) has improved the plasma lipid profile in humans and experimental animals. The effect of GSPE on the reduction of the postprandial plasma triglyceride (TG) levels has been attributed to the activation of the small heterodimer partner (SHP). GSPE increases SHP gene expression in rat liver and the TG-lowering effect of GSPE is abolished in SHP-deficient mice. However, the mechanism by which GSPE increases SHP mRNA levels remains unclear. This study addressed the effect of GSPE on SHP mRNA stability. METHODS AND RESULTS: The present study shows for the first time that SHP mRNA is rapidly degraded, as measured by actinomycin D-based mRNA chase experiments, and GSPE transiently stabilizes SHP mRNA in HepG2 cells. This degradative effect was completely abolished with 2 h of prolonged treatment with GSPE. However, treatment of fresh HepG2 cells with a pretreated GSPE-containing medium also stabilized SHP mRNA, indicating that GSPE inactivation is not responsible for the transient effects that GSPE has on SHP mRNA stability. CONCLUSION: SHP expression is intricately controlled by mRNA stabilization, which is transiently increased by GSPE, along with at the transcriptional and posttranslational levels.