Small fat droplets interact with structural filaments in macrophages with cytoskeletal dysfunction contributing to perturbed phagocytosis and migration.

Abstract

Fat droplets (FDs) are cellular organelles composed of triglycerides/sterols encompassed within a phospholipid monolayer. The presence of large FDs in macrophages (i.e. foam cells) is well-established across a variety of pathological conditions including atherosclerosis, tuberculosis, and obesity. However, the biophysical nature of interactions between small, high curvature fat droplets and various structural filaments and the subsequent consequences are less understood than that of large diameter lipid bodies. Here, micron-sized oleic acid-derived FDs are shown to be rigid and indent stiff organelles like the nucleus and as well as actin filaments in macrophages, similar to results seen in U2OS osteosarcoma cells where FDs can cause nuclear rupture. Notably, FD-loaded macrophages demonstrate impaired phagocytosis, as exhibited by decreased uptake of IgG-opsonized microbeads, nanobeads, and lentivirus. There was no observed difference in the quantity of adhered but non-internalized objects, highlighting that FDs perturb the cytoskeletal-driven uptake process. Inhibition of either myosin-II or actin in macrophages only shows decreased uptake in the absence of FDs, further indicating that FDs already predispose macrophages to heightened internal actomyosin dysfunction. Surprisingly, phagocytosis was less suppressed in macrophages where fat droplets comprised a higher area fraction within the cell, possibly reflecting beneficial use of energy reserves which overcome cytoskeletal disruption in macrophages. Macrophage migration, another cytoskeletal-driven process, is also impacted by FD presence. FD-containing macrophages display reduced infiltration into a cancer cell spheroid model, instead tending to remain at the spheroid periphery. Combined, this data may provide additional insights into why tumor associated macrophages (TAMs), a macrophage population associated with a population of small FDs, show reduced efficacy in phagocytosing and clearing tumors.