Abstract 432: MEK/ERK Inhibition Corrects the Defect in VLDL Assembly and Secretion in HepG2 Cells via Activation of Cell Death--Inducing DFFA-Like Effector B (Cide B), ApoCIII and Lipin-1

Abstract

Suppression of MEK/ERK signalling in HepG2 cells promotes the assembly and secretion of VLDL-sized apoB100-containing lipoproteins via an unknown mechanism (Tsai et al, Arterioscler Thromb Vasc Biol 2007; 27:211-218). In this report, we investigated the mechanisms underlying the profound stimulatory effects of MEK/ERK inhibition on VLDL assembly and secretion in HepG2 cells. Following MEK/ERK inhibition with the inhibitor U0126, the mRNA levels of lipin-1α, -1β, apoCIII and CideB were significantly increased by 1.22, 1.25, 3.6, and 1.9 fold, respectively, all key factors involved in intracellular lipid metabolism. Gain-of-function studies were then used to mimic the effects seen with MEK/ERK inhibition. Upon transient overexpression of lipin-1α, and-1β, secretion of VLDL-apoB was increased by 2.07 and 2.23 fold, respectively. Similarly, overexperession of wild type apoCIII (C3WT) significantly increased the secretion of VLDL-apoB. By contrast, loss-of-function approaches using RNA interference showed that knockdown of apoCIII or CideB significantly decreased secretion of VLDL-apoB by 22% or 69%, respectively. Overexpression of MTP could not block the inhibitory effects of CideB knockdown on VLDL secretion in MEK/ERK-inhibited HepG2 cells. Taken together, our data suggest that increased expression of a number of key protein factors, particularly CideB, may mediate the profound stimulatory effect of MEK/ERK inhibition on VLDL assembly and secretion. Enhanced expression of CideB, apo CIII, and lipin-1 may alter and promote availability of TG pools available for VLDL assembly in a process independent of MTP.