Autophagy differentially regulates macrophage lipid handling depending on the lipid substrate (oleic acid versus acetylated-LDL) and inflammatory activation state

The regulation of macrophage lipid droplet dynamics by autophagy is complex:

Inhibiting autophagosome initiation steps attenuates oleic acid (OA) induced lipid

droplet (LD) (OA-LD) biogenesis, whereas interfering with later-autophagosome

maturation/lysosomal steps accelerates OA-LD biogenesis rate, but not OA-LD

degradation. Here we hypothesized that regulation of macrophage lipid handling by

autophagy may be lipid-substrate and activation-state-specific. Using automated

quantitative live-cell imaging, initial LD biogenesis rate was ~30% slower when the lipid

source was acetylated low density lipoprotein (acLDL) compared to OA. Yet, both were

similarly affected by triacsin-C, an inhibitor of acyl-CoA synthase, which inhibited, and

etomoxir, an inhibitor of acylcarnitine palmitoyl transferase (fatty acid oxidation), which

augmented, LD biogenesis rates. An autophagy inducing peptide, Tat-Beclin1,

enhanced the degradation, and inhibited (by 37%) the biogenesis of acLDL induced LD

(acLDL-LD). Yet, Tat-Beclin1 increased OA-LD biogenesis rate by 70%. When

macrophages were pre-activated with LPS+INFG they exhibited increased

autophagosome number and area, and reduced BECN1 and ATG14 protein levels.

Yet, autophagosome growth-rate and autophagic flux were markedly attenuated.

Concomitantly, OA-LD and acLDL-LD biogenesis rates increased 3 and 7.4-fold,

respectively, but could not be further modulated by Tat-Beclin1, as observed in non-

activated/naïve macrophages. We propose that macrophage autophagy, and/or

components of its machinery, regulate LD biogenesis, degradation and foam cell

formation in a highly lipid-source and activation-state dependent manner.