Exogenous glucocorticoids increase macrophage secretion of apo E by cholesterol-independent pathways

Abstract

Macrophage apo E synthesis and secretion has been previously demonstrated to be regulated by intracellular free cholesterol levels and is decreased by cytokines and other inflammatory stimuli associated with macrophage activation. In a recent study, the opposing effects of TGF beta and GM-CSF were reported with the former increasing and the latter decreasing apo E secretion and apo E mRNA levels. In an attempt to further understand the mechanisms by which TGF beta increased apo E expression in mouse peritoneal macrophages, the present study was performed to determine whether pharmacological agents could up-regulate apo E secretion by a mechanism independent of intracellular free cholesterol levels. Agents which resulted in increased apo E secretion were subdivided based on their effects on cAMP elevation. In addition to TGF beta, dexamethasone resulted in significant increases in apo E secretion. The 2-4-fold enhancement in apo E secretion by both TGF beta and dexamethasone occurred without concomitant changes in intracellular cAMP or free cholesterol. Other agents which increased apo E secretion included cholera toxin and 8-bromo-cAMP. While these agents did not affect intracellular cholesterol levels, cholera toxin did increase macrophage cAMP. The changes in apo E secretion by dexamethasone and 8-bromo-cAMP were associated with elevations in apo E mRNA. Dexamethasone-treated macrophages had 6-fold increases in apo E mRNA by 48 h when compared with control macrophages. Macrophages stimulated with 8-bromo-cAMP for 48 h demonstrated a more modest but statistically significant (P < 0.001) 2.2-fold increase. Similar effects of dexamethasone, cholera toxin, TGF beta, and 8-bromo-cAMP on apo E secretion were also apparent in macrophage-derived foam cells. In addition to increasing apo E secretion in macrophages and foam cells, dexamethasone and 8-bromo-cAMP inhibited the down-regulation of apo E secretion mediated by LPS and GM-CSF. Finally, the increased apo E secretion by exogenous glucocorticoids or TGF beta was not species specific as similar effects were observed in rabbit peritoneal macrophages. Therefore, while macrophage activation results in decreased apo E synthesis, macrophages exposed to anti-inflammatory agents including dexamethasone, TGF beta, or following cAMP elevation demonstrate increased apo E secretion by a cholesterol-independent mechanism.