TLR-dependent Bim phosphorylation in macrophages is mediated by ERK and is connected to proteasomal degradation of the protein.
The pro-apoptotic Bcl-2 homology domain 3-only protein Bim has been shown to play an important role in immune cell homeostasis and various forms of apoptosis in the immune system. Bim is expressed in most immune cells, and regulation of Bim activity can occur on both transcriptional and post-translational levels. Toll-like receptor (TLR) stimulation has previously been shown to increase Bim expression and to cause Bim phosphorylation in the absence of apoptosis in mouse macrophages. Here we identify extracellular signal-regulated kinase as the major kinase responsible for TLR-dependent Bim phosphorylation. Three TLR-dependent serine phosphorylation sites, S55, S65 and S100, on mouse Bim were identified, two of them unique to the splice form Bim(EL) and one also present on Bim(L). A Bim mutant defective in these three phosphorylation sites showed slightly enhanced pro-apoptotic activity, which might indicate a protective effect of Bim phosphorylation in this system. Phosphorylation did not alter the association of Bim protein with the microtubule cytoskeleton. However, TLR-mediated phosphorylation led to accelerated degradation of Bim via the proteasome. Thus, TLR stimulation of macrophages can regulate Bim levels in opposing ways, namely by transcriptional induction and by phosphorylation-dependent degradation of the protein.
Häcker
Georg
G
Institute for Medical Microbiology, Immunology and Hygiene, Technical University Munich, Trogerstrasse 30, D-81675 Munich, Germany.
Suttner
Kathrin
K
Harada
Hisashi
H
Kirschnek
Susanne
S
D000818:Animals; D051017:Apoptosis Regulatory Proteins / Q000378:metabolism*; D000072224:Bcl-2-Like Protein 11; D002460:Cell Line; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism*; D006367:HeLa Cells; D006801:Humans; D008264:Macrophages / Q000378:metabolism*; D008565:Membrane Proteins / Q000378:metabolism*; D051379:Mice; D010766:Phosphorylation; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D011518:Proto-Oncogene Proteins / Q000378:metabolism*; D051193:Toll-Like Receptors / Q000378:metabolism*
D051017:Apoptosis Regulatory Proteins; C000606775:BCL2L11 protein, human; D000072224:Bcl-2-Like Protein 11; C000606776:Bcl2l11 protein, mouse; D008565:Membrane Proteins; D011518:Proto-Oncogene Proteins; D051193:Toll-Like Receptors; D048049:Extracellular Signal-Regulated MAP Kinases; D046988:Proteasome Endopeptidase Complex