Activation of Protein Tyrosine Kinases byCoxiella burnetii: Role in Actin Cytoskeleton Reorganization and Bacterial Phagocytosis

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Abstract
Coxiella burnetii, the agent of Q fever, is an obligate intracellular microorganism that grows in monocytes/macrophages. The internalization of virulent organisms by monocytes is lower than that of avirulent variants and is associated with actin cytoskeleton reorganization. We studied the activation of protein tyrosine kinases (PTKs) byC. burnetiiin THP-1 monocytes. Virulent organisms induced early PTK activation and the tyrosine phosphorylation of several endogenous substrates, including Hck and Lyn, two Src-related kinases. PTK activation reflectsC. burnetiivirulence since avirulent variants were unable to stimulate PTK. We also investigated the role of PTK activation inC. burnetii-stimulated F-actin reorganization. Tyrosine-phosphorylated proteins were colocalized with F-actin inside cell protrusions induced byC. burnetii, and PTK activity was increased in Triton X-100-insoluble fractions. In addition, lavendustin A, a PTK inhibitor, and PP1, a Src kinase inhibitor, preventedC. burnetii-induced cell protrusions and F-actin reorganization. We finally assessed the role of PTK activation in bacterial phagocytosis. Pretreatment of THP-1 cells with lavendustin A and PP1 upregulated the uptake of virulentC. burnetiibut had no effect on the phagocytosis of avirulent organisms. Thus, it is likely that PTK activation byC. burnetiinegatively regulates bacterial uptake by interfering with cytoskeleton organization.