Proteasomal and autophagic pathways converge on lipid droplets

Apolipoprotein B (apoB) is the primary protein of very low-density lipoproteins (VLDL). We found that apoB accumulated on the surface of cytoplasmic lipid droplets (LDs) of hepatocytes when the proteasomal or autophagic processes were suppressed. ApoB associated with LDs was poly-ubiquitinated. and surrounded by autophagic vacuoles. Moreover, proteasomal subunits were concentrated around LDs. Our data suggest that apoB that is destined to be degraded remains adhered to LDs until it is broken down by the proteasomal and autophagic pathways. We speculate that the LD surface serves as a platform to prevent hydrophobic apoB from forming aggregates, and that LDs may play a similar role for other aggregation-prone hydrophobic proteins.