Role of clathrin-mediated endocytosis of surfactant protein A by alveolar macrophages in intracellular signaling

Moulakakis C, Stamme C. Role of clathrin-mediated endocytosis of surfactant protein A by alveolar macrophages in intracellular signaling. Am J Physiol Lung Cell Mol Physiol 296: L430-L441, 2009. First published January 9, 2009; doi: 10.1152/ajplung.90458.2008.-We recently provided evidence that anti-inflammatory macrophage activation, i.e., the inhibition of constitutive and signal-induced NF-kappa B activity by the pulmonary collectin surfactant protein (SP)-A, critically involves a promoted stabilization of I kappa B-alpha,the predominant inhibitor of NF-kappa B, via posttranscriptional mechanisms comprising the activation of atypical (a) PKC zeta. SP-A uptake and degradation by alveolar macrophages (AM phi) occur in a receptor-mediated, clathrin-dependent manner. However, a mutual link between endocytosis of and signaling by SP-A remains elusive. The aim of this study was to investigate whether clathrin-mediated endocytosis (CME) of SP-A by AM phi is a prerequisite for its modulation of the I kappa B-alpha/NF-kappa B pathway. The inhibition of clathrin-coated pit (CCP) formation and clathrin-coated vesicle (CCV) formation/budding abrogates SP-A-mediated I kappa B-alpha stabilization and SP-A-mediated inhibition of LPS-induced NF-kappa B activation in freshly isolated rat AM phi, as determined by Western analysis, fluorescence-activated cell sorting, confocal microscopy, and EMSA. Actin depolymerization and inhibition of CCP formation further abolished SP-A-mediated inhibition of LPS-induced TNF-alpha release, as determined by ELISA. In addition, SP-A-induced atypical PKC zeta activation was abolished by pretreatment of AM phi with CCV inhibitors as determined by in vitro immunocomplex kinase assay. Although CME is classically considered as a means to terminate signaling, our results demonstrate that SP-A uptake via CME by AM phi has to precede the initiation of SP-A signaling.