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Staphylococcus aureus invades osteoblasts and can persist in the intracellular environment. The present study examined the role of osteoblast mitogen-activated protein kinase (MAPK) pathways in bacterial invasion. S. aureus infection of normal human and mouse osteoblasts resulted in an increase in the phosphorylation of the extracellular signal-regulated protein kinases (ERK 1 and 2). This stimulation of ERK 1 and 2 correlated with the time course of S. aureus invasion, and bacterial adherence induced the MAPK pathway. ERK 1 and 2 phosphorylation was time and dose dependent and required active S. aureus gene expression for maximal induction. The nonpathogenic Staphylococcus carnosus was also able to induce ERK 1 and 2 phosphorylation, albeit at lower levels than S. aureus. Phosphorylation of the stress-activated protein kinases was increased in both infected human and mouse osteoblasts; however, the p38 MAPK pathway was not activated in response to S. aureus. Finally, the transcription factor c-Jun, but not Elk-1 or ATF-2, was phosphorylated in response to S. aureus infection.

Staphylococcus aureus and Salmonella spp. are common causes of bone diseases; however, the immune response during such infections is not well understood. Colony-stimulating factors (CSF) have a profound influence on osteoclastogenesis, as well as the development of immune responses following infection. Therefore, we questioned whether interaction of osteoblasts with two very different bacterial pathogens could affect CSF expression by these cells. Cultured mouse and human osteoblasts were exposed to various numbers of S. aureus or Salmonella dublin bacteria, and a comprehensive analysis of granulocyte-macrophage (GM)-CSF, granulocyte (G)-CSF, macrophage (M)-CSF, and interleukin-3 (IL-3) mRNA expression and cytokine secretion was performed. Expression of M-CSF and IL-3 mRNAs by mouse osteoblasts was constitutive and did not increase significantly following bacterial exposure. In contrast, GM-CSF and G-CSF mRNA expression by mouse osteoblasts was dramatically upregulated following interaction with either viable S. aureus or Salmonella. This increased mRNA expression also translated into high levels of GM-CSF and G-CSF secretion by mouse and human osteoblasts following bacterial exposure. Viable S. aureus and Salmonella induced maximal levels of CSF mRNA expression and cytokine secretion compared to UV-killed bacteria. Furthermore, GM-CSF and G-CSF mRNA expression could be induced in unexposed osteoblasts separated by a permeable Transwell membrane from bacterially exposed osteoblasts. M-CSF secretion was increased in cultures of exposed human osteoblasts but not in exposed mouse osteoblast cultures. Together, these studies are the first to define CSF expression and suggest that, following bacterial exposure, osteoblasts may influence osteoclastogenesis, as well as the development of an immune response, via the production of these cytokines.