CD5L (CD5 molecule-like) is a secreted glycoprotein that participates in host response to bacterial infection. CD5L influences the monocyte inflammatory response to the bacterial surface molecules lip Show more
CD5L (CD5 molecule-like) is a secreted glycoprotein that participates in host response to bacterial infection. CD5L influences the monocyte inflammatory response to the bacterial surface molecules lipopolysaccharide (LPS) and lipoteichoic acid (LTA) by inhibiting TNF secretion. Here we studied the intracellular events that lead to macrophage TNF inhibition by human CD5L. To accomplish this goal, we performed functional analyses with human monocytic THP1 macrophages, as well as with peripheral blood monocytes. Inhibition of phosphatidylinositol 3-kinase (PtdIns3K) reversed the inhibitory effect of CD5L on TNF secretion. Among the various PtdIns3K isoforms, our results indicated that CD5L activates PtdIns3K (whose catalytic subunit is termed PIK3C3), a key modulator involved in autophagy. Further analysis revealed a concomitant enhancement of autophagy markers such as cellular LC3-II content, increased LC3 puncta, as well as LC3-LysoTracker Red colocalization. Moreover, electron microscopy showed an increased presence of cytosolic autophagosomes in THP1 macrophages overexpressing CD5L. Besides preventing TNF secretion, CD5L also inhibited IL1B and enhanced IL10 secretion. This macrophage anti-inflammatory pattern of CD5L was reverted upon silencing of autophagy protein ATG7 by siRNA transfection. Additional siRNA experiments in THP1 macrophages indicated that the induction of autophagy mechanisms by CD5L was achieved through cell-surface scavenger receptor CD36, a multiligand receptor expressed in a wide variety of cell types. Our data represent the first evidence that CD36 is involved in autophagy and point to a significant contribution of the CD5L-CD36 axis to the induction of macrophage autophagy. Show less
Macrophages serve essential functions as regulators of immunity and homeostasis, and their proliferation contributes to pathogenesis of certain disorders. In this report, we show that induction of mac Show more
Macrophages serve essential functions as regulators of immunity and homeostasis, and their proliferation contributes to pathogenesis of certain disorders. In this report, we show that induction of macrophage proliferation by the growth factor M-CSF is negatively modulated by agonists that activate the nuclear receptor liver X receptor (LXR), both in vitro and in vivo. Both isoforms LXR α and β are involved in the antiproliferative actions of LXR ligands in macrophages. In contrast, M-CSF does not exert negative effects on LXR-mediated gene expression. Treatment with LXR agonists results in the accumulation of macrophages in the G(0)/G(1) phase of the cell cycle without affecting ERK-1/2 phosphorylation. The use of small interfering RNA or genetically modified mice revealed that, in contrast to other cellular models, functional expression of either the cyclin-dependent kinase inhibitor p27KIP1 or the cholesterol transporters ATP-binding cassette A1 or ATP-binding cassette G1 was not required for the antiproliferative effects of LXR agonists in macrophages. Western blot analysis revealed that protein expression of key molecules that regulate progression through the cell cycle, such as cyclins D1 and B1 and cyclin-dependent kinases 2 and 4, was downregulated upon LXR activation. These observations suggest a role for LXR agonists in limiting macrophage proliferative responses associated to pathogenic disorders. Show less