Autophosphorylation docking site Tyr-867 in Mer receptor tyrosine kinase allows for dissociation of multiple signaling pathways for phagocytosis of apoptotic cells and down-modulation of lipopolysaccharide-inducible NF-kappaB transcriptional activation.
Efficient clearance of apoptotic cells is essential for tissue homeostasis, allowing for cellular turnover without inflammatory consequences. The Mer (Nyk and c-Eyk) receptor tyrosine kinase (Mertk) is involved in two aspects of apoptotic cell clearance by acting as a receptor for Gas6, a gamma-carboxylated phosphatidylserine-binding protein that bridges apoptotic and viable cells. First, Mertk acts in a bona fide engulfment pathway in concert with alphavbeta5 integrin by regulating cytoskeletal assemblages, and second, it acts as a negative regulator for inflammation by down-modulating pro-inflammatory signals mediated from bacterial lipopolysaccharide-Toll-like receptor 4 (TLR4) signaling, and hence recapitulating anti-inflammatory immune modulation by apoptotic cells. Here we describe Mertk post-receptor events that govern phagocytosis and cytoskeletal signaling are principally mediated by autophosphorylation site Tyr-867. Using the Mertk Y867F mutant and pharmacological inhibitors, we show that Tyr-867 is required for phosphatidylinositol 3-kinase and phospholipase Cgamma2 activation; their activation in turn elicits protein kinase C-dependent signals that act on the actin cytoskeleton. Although Mertk(Y867F) blocked the tyrosine phosphorylation of FAK on Tyr-861 and p130(cas) and also abrogated the phagocytosis of apoptotic cells, this mutant did not suppress lipopolysaccharide-inducible NF-kappaB transcription, nor was NF-kappaB activation dependent on the protein kinase C inhibitor, calphostin C. Finally, unlike the cytoskeletal events associated with Tyr-867 autophosphorylation, the trans-inhibition of NF-kappaB occurred in a postnuclear-dependent fashion independent of cytosolic IkappaB phosphorylation and p65/RelA sequestration. Taken together, these data suggest that Mertk has distinct and separable effects for phagocytosis and for resolving inflammation, providing a molecular rationale for how immune licensing and inflammation can be dissociated from phagocytosis in a single phagocytic receptor.
Tibrewal
Nitu
N
Department of Biochemistry & Molecular Biology, University of Medicine and Dentistry of New Jersey (UMDNJ)-New Jersey Medical School, Newark New Jersey 07103.
Wu
Yi
Y
Department of Biochemistry & Molecular Biology, University of Medicine and Dentistry of New Jersey (UMDNJ)-New Jersey Medical School, Newark New Jersey 07103.
D'mello
Veera
V
Department of Biochemistry & Molecular Biology, University of Medicine and Dentistry of New Jersey (UMDNJ)-New Jersey Medical School, Newark New Jersey 07103.
Akakura
Reiko
R
Department of Biochemistry & Molecular Biology, University of Medicine and Dentistry of New Jersey (UMDNJ)-New Jersey Medical School, Newark New Jersey 07103.
George
Thaddeus C
TC
Amnis Corporation, Seattle, Washington 98121.
Varnum
Brian
B
Inflammation Department, Amgen Pharmaceuticals, Amgen, Inc., Thousand Oaks, California 91320.
Birge
Raymond B
RB
Department of Biochemistry & Molecular Biology, University of Medicine and Dentistry of New Jersey (UMDNJ)-New Jersey Medical School, Newark New Jersey 07103. Electronic address: birgera@umdnj.edu.
D017209:Apoptosis; D003599:Cytoskeleton / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005786:Gene Expression Regulation; D006801:Humans; D008070:Lipopolysaccharides / Q000378:metabolism*; D008954:Models, Biological; D016328:NF-kappa B / Q000378:metabolism*; D009281:Naphthalenes / Q000494:pharmacology; D010587:Phagocytosis; D010766:Phosphorylation; D011518:Proto-Oncogene Proteins / Q000737:chemistry* / Q000378:metabolism*; D020794:Receptor Protein-Tyrosine Kinases / Q000737:chemistry* / Q000378:metabolism*; D015398:Signal Transduction; D015533:Transcriptional Activation; D014443:Tyrosine / Q000737:chemistry*; D000076202:c-Mer Tyrosine Kinase
D004791:Enzyme Inhibitors; D008070:Lipopolysaccharides; D016328:NF-kappa B; D009281:Naphthalenes; D011518:Proto-Oncogene Proteins; D014443:Tyrosine; C097072:MERTK protein, human; D020794:Receptor Protein-Tyrosine Kinases; D000076202:c-Mer Tyrosine Kinase; C058819:calphostin C