Mechanisms of the assembly of hepatic very low density lipoprotein in rat hepatoma McA-RH7777 cells.

Abstract

The role of apolipoprotein B (apoB) and microsomal triglyceride transfer protein (MTP) in hepatic triglyceride (TG)-rich very low density lipoprotein (VLDL) assembly has been investigated. Rat hepatoma McA-RH7777 cells were used to characterize structural determinants within apoB that are required for VLDL assembly/secretion, and to define mechanisms through which MTP facilitates VLDL assembly/secretion. Working with McA-RH7777 cells stably expressing recombinant human apoB100 (representing the full-length apoB) and various carboxyl-terminally truncated apoB proteins, we found that the apoB forms equal to or shorter than apoB29 (representing the amino-terminal 29% of the full-length apoB) were unable to assemble VLDL, whereas those equal to or longer than apoB34 could. Detailed biochemical analysis using apoB100- and apoB48-transfected cells revealed that assembly of VLDL, irrespective of the apoB length, was achieved through the same general pathway. Three notable features associated with the VLDL assembly process in McA-RH7777 cells were: (a) VLDL assembly was absolutely dependent upon exogenous oleate, (b) VLDL assembly was achieved post-translationally, and (c) the hallmark of VLDL assembly was the incorporation of bulk TG. Using chemical inhibitors of MTP, we found that MTP activity was required in the assembly/secretion of TG-rich VLDL, and that the requirement of MTP activity was determined by the amount of TG utilized for lipoprotein assembly. Moreover, the accumulation and attainment of TG within microsomal lumen was a function of MTP activity. Thus, MTP plays a role in facilitating the accumulation of TG within microsomes, an event that can be separated from TG incorporation into mature VLDL but represents an indispensable requisite for the post-translational assembly of TG-enriched VLDL.