Expression and function of Na⁺/Ca²⁺ exchangers 1 and 3 in human macrophages and monocytes

Abstract

The Na(+)/Ca(2+) exchanger (NCX) is a membrane transporter that can switch Na(+) and Ca(2+) in either direction to maintain the homeostasis of intracellular Ca(2+). Three isoforms (NCX1, NCX2, and NCX3) have been characterized in excitable cells, e.g. neurons and muscle cells. We examined the expression of these NCX isoforms in primary human lung macrophages (HLM) and blood monocytes. NCX1 and NCX3, but not NCX2, are expressed in HLM and monocytes at both mRNA and protein levels. Na(+)-free medium induced a significant increase in intracellular calcium concentration ([Ca(2+)](i)) in both cell types. This response was completely abolished by the NCX inhibitor 5-(N-4-chlorobenzyl)-20,40-dimethylbenzamil (CB-DMB). Moreover, inhibition of NCX activity during Ca(2+)-signaling induced by histamine caused a delay in restoring baseline [Ca(2+)](i). Na(+)-free medium induced TNF-alpha expression and release in HLM comparable to that caused by LPS. TNF-alpha release induced by Na(+)-free medium was blocked by CB-DMB and greatly reduced by RNAi-mediated knockdown of NCX1. These results indicate that human macrophages and monocytes express NCX1 and NCX3 that operate in a bidirectional manner to restore [Ca(2+)](i), to generate Ca(2+)-signals, and to induce TNF-alpha production. Therefore, NCX may contribute to regulate Ca(2+) homeostasis and proinflammatory functions in human macrophages and monocytes.