Hsp40s play distinct roles during the initial stages of apolipoprotein B biogenesis

Apolipoprotein B (ApoB) is the primary component of atherogenic lipoproteins, which transport serum fats and cholesterol. Therefore, elevated levels of circulating ApoB are a primary risk factor for cardiovascular disease. During ApoB biosynthesis in the liver and small intestine under nutrient-rich conditions, ApoB cotranslationally translocates into the endoplasmic reticulum (ER) and is lipidated and ultimately secreted. Under lipid-poor conditions, ApoB is targeted for ER Associated Degradation (ERAD). Although prior work identified select chaperones that regulate ApoB biogenesis, the contributions of cytoplasmic Hsp40s are undefined. To this end, we screened ApoB-expressing yeast and determined that a class A ER-associated Hsp40, Ydj1, associates with and facilitates the ERAD of ApoB. Consistent with these results, a homologous Hsp40, DNAJA1, functioned similarly in rat hepatoma cells. DNAJA1 deficient cells also secreted hyperlipidated lipoproteins, in accordance with attenuated ERAD. In contrast to the role of DNAJA1 during ERAD, DNAJB1—a class B Hsp40—helped stabilize ApoB. Depletion of DNAJA1 and DNAJB1 also led to opposing effects on ApoB ubiquitination. These data represent the first example in which different Hsp40s exhibit disparate effects during regulated protein biogenesis in the ER, and highlight distinct roles that chaperones can play on a single ERAD substrate.

Clofibrate Attenuates ROS Production by Lipid Overload in Cultured Rat Hepatoma Cells

PURPOSE: To investigate the effect of clofibrate on inducing liver fatty acid binding protein (FABP1) following a high-fat load in a hepatocyte cell culture model. METHODS: Rat hepatoma cells (CRL-1548) were treated with a fatty acid (FA) mixture consisting of oleate:palmitate (2:1) in the presence of 3% albumin. Cells were treated with 0, 0.5, 1, 2, or 3 mM FA for 24 and 48 hr, or further treated with 500 µM clofibrate (CLO) to induce FABP1 levels. Cytotoxicity was determined using the WST-1 assay. Intracellular lipid droplets were quantitated following staining with Nile Red. Dichlorofluorescein (DCF) was used to assess the extent of intracellular reactive oxygen species (ROS). RESULTS: Cell viability decreased (p < 0.01) with an increase in lipid concentration. Intracellular lipid droplets accumulated significantly (p < 0.001) with an increase in long-chain fatty acid load, which was associated with a statistical increase (p < 0.05) in ROS levels. Early clofibrate treatment showed significant increases in intracellular FABP1 levels with significant decreases in ROS levels (p < 0.05). Silencing FABP1 expression using siRNA revealed that FABP1 was the main contributor for the observed intracellular ROS clearance. CONCLUSIONS: Characteristic cellular damage resulted from released ROS following a high fat load to hepatoma cells. The damage was attenuated through early treatment with clofibrate, which may act as a hepatoprotectant by inducing FABP1 expression and in this manner, suppress intracellular ROS levels. This article is open to POST-PUBLICATION REVIEW. Registered readers (see “For Readers”) may comment by clicking on ABSTRACT on the issue’s contents page.