Dexamethasone downregulates lysosomal secretion in mouse macrophages: involvement of signaling through protein kinase C.

Spectrofluorimetric methods were used to investigate the effects of dexamethasone (dex) on cytosolic and lysosomal pH and on macrophage secretion of lysosomal contents. Secretion of N-acetyl-beta-D-glucosaminidase (NAG) in response to zymosan particles, lysosomotropic methylamine, or the H(+)-ATPase inhibitor bafilomycin A1 was inhibited by pretreatment with dex. The inhibition was not reversed by mannan and was seen also when secretion of preloaded fluorescein-labelled dextran was monitored, demonstrating that dex did not exert its effect by enhancing the reuptake of lysosomal enzyme. The binding of zymosan particles to macrophages was diminished after dex treatment, as was the zymosan-induced phospholipase C activation. However, the decreased binding of zymosan did not alone account for the inhibition of phospholipase C activation. Also, cytosolic pH was lowered by dex treatment. This might contribute to the inhibition of lysosomal secretion, but restoration of cytosolic pH by an increase in extracellular pH did not restore the secretory response. Lysosomal secretion induced by a combination of protein kinase C (PKC)-activating phorbol ester and methylamine was more resistant to dex than secretion induced by methylamine alone, or other secretagogues. We interpret this, together with previous data, to indicate that dex inhibits macrophage lysosomal secretion by attenuating one or more step(s) in a PKC-mediated signaling pathway necessary for the secretory response.