FAs, derived from the plasma-free FA pool, from uptake of circulating lipoproteins (chylomicron remnants), and from de novo hepatic lipogenesis, are esterified via fatty acyl CoA to diacylglyceride (DAG) derivatives and subsequently into TG droplets by diacylglycerol acyl transferase 1 (DGAT1). Lipogenic enzyme expression is regulated by transcription factors including sterol regulatory element binding protein-1c (SREBP-1c) and carbohydrate response element–binding protein (ChREBP), which can be induced in the hyperinsulinemic environment. Mttp gene expression is regulated by FoxO1 as described in the study in this issue of the JCI by Kamagate et al. (7). Insulin acts by binding to its receptor, and activation of the receptor tyrosine kinase, which leads to multiple tyrosine phosphorylation of IRS and subsequent activation of PI3K to produce PIP3. PIP3, in turn, activates phosphoinositide-dependent kinases (PDKs), which phosphorylate and activate Akt, leading to phosphorylation and nuclear exclusion of FoxO1. ER membrane–localized apoB and Mttp mRNAs generate corresponding proteins through translation on ribosomes into the rough ER lumen. Active MTP, in complex with protein disulfide isomerase (PDI), transfers lipids (yellow), including phospholipids, to stabilize nascent apoB during translation, thereby leading to the formation of an emulsion of apoB and lipid in pre-VLDL particles. Cytosolic TG droplets are lipolyzed by TG hydrolases (TGHs), resulting in the release of diacylglyceride and FAs, which are then esterified by DGAT2 to form TG in close association with ER membranes. Pre-VLDL particles fuse with luminal TG droplets, also formed by MTP activity, to form large VLDL particles. Insulin is believed to block VLDL assembly in a post-ER compartment at the fusion stage, possibly through a mechanism involving PI3K-generated PIP3. VLDL particles move through the secretory pathway, where additional remodeling may occur, and are eventually secreted into plasma. Failure of pre-VLDL apoB to attain a proper conformation and to successfully fuse with TG results in the shunting of defective particles (dotted arrow) to autophagolysosomes, the presumptive degradation compartment.