Role of endogenously derived leukotrienes in the regulation of lysosomal enzyme expression in macrophages exposed to beta 1,3-glucan.

The expression of the lysosomal enzyme hexosaminidase has been shown to be stimulated by the exposure of mouse macrophages to beta 1,3-glucan, a particulate component of yeast cell walls and zymosan particles. Exposure of mouse peritoneal macrophages to particulate beta 1,3-glucan (100 micrograms/ml) was also found to stimulate the production of eicosanoids from both the cyclooxygenase (prostaglandin E2) and 5'-lipoxygenase (leukotriene C4) pathways. The objective of this study was to determine the relationship, if any, between the production of arachidonic acid metabolites and the increased expression of lysosomal enzymes. To determine if products of the cyclooxygenase or 5'-lipoxygenase pathway were involved in the regulation of hexosaminidase expression, macrophages were exposed to beta 1,3-glucan in the presence of indomethacin, an inhibitor of cyclooxygenase; 4,7,10,13 ETYA, an inhibitor of both cyclooxygenase and 5'-lipoxygenase; or AA861, a selective inhibitor of 5'-lipoxygenase. While the increased expression of hexosaminidase was not affected in macrophages stimulated with beta 1,3-glucan in the presence of indomethacin, both 4,7,10,13 ETYA and AA861 completely blocked the response, suggesting a role for products of the 5'-lipoxygenase pathway in the regulation of hexosaminidase expression. To further explore the relationship between arachidonate release and the increase expression of hexosaminidase, macrophages were exposed to phospholipase A2 in an attempt to circumvent the interaction between beta 1,3-glucan and the macrophage membrane. Incubation with phospholipase A2 was found both to induce the accumulation of LTC4 in the culture supernatant and to stimulate the increased expression of hexosaminidase. The mechanism of regulation of hexosaminidase expression by products of the 5'-lipoxygenase pathway was investigated by incubating macrophages with purified luekotrienes in either the presence or absence of beta 1,3-glucan. Incubation of macrophages with purified LTC4 or LTB4 in the absence of beta 1,3-glucan failed to stimulate the expression of hexosaminidase. However, challenging macrophage monolayers with LTC4 or LTB4 in the presence of a suboptimal concentration of beta 1,3-glucan (1 microgram/ml) led to a synergistic increase in the expression of hexosaminidase. Collectively these data suggest that the leukotriene products of the 5'-lipoxygenase pathway, LTC4 and LTB4, regulate the expression of lysosomal enzymes by apparently priming macrophages, thereby increasing their sensitivity to triggering agents such as beta 1,3-glucan. Since macrophages produce LTC4, and the increased expression of hexosaminidase is prevented by inhibitors of the 5'-lipoxygenase pathway, the data further suggest that LTC4 may prime macrophages in an autocrine or paracrine fashion.