Activation of signaling pathways by putative scavenger receptor class A (SR-A) ligands requires CD14 but not SR-A

Macrophage scavenger class A type I and type II receptors (SR-A) are trimeric, integral membrane glycoproteins that bind an unusually broad array of macromolecular ligands. These ligands include modified proteins and lipoproteins, nucleic acids, and a variety of plant and microbial cell wall constituents, such as fucoidan and lipoteichoic acid. Early studies of SR-A functions indicated that the receptors bound, internalized, and degraded their ligands without provoking any macrophage activating signaling events. More recent studies have provided evidence that several SR-A ligands can activate macrophage gene expression via utilization of a receptor-linked, PI3-kinase pathway. To investigate the role of SR-A in engaging signal transduction events, we employed macrophages taken from mice lacking these receptors. Using either fucoidan or lipoteichoic acid, we confirm that both ligands stimulate tyrosine phosphorylation of PI3-kinase and production of modest levels of the cytokine, TNFalpha. However, macrophages taken from SR-A null mice did not differ from wild type macrophages in these responses, indicating that these signaling events arise independently of SR-A activity. Employing mice lacking CD14, a GPI anchored receptor that binds bacterial lipopolysaccharide and signals via activation of Toll-like receptors, we show that the fucoidan and lipoteichoic acid responses are largely abrogated when CD14 is absent. These data do not provide support for direct SR-A involvement in signal transduction events and suggest that the early characterization of these receptors as initiators of a non-phlogistic, pathogen clearance pathway was correct.