Pasteurella multocida toxin is a mitogen for bone cells in primary culture

The effect of recombinant Pasteurella multocida toxin (PMT) on primary cultures of embryonic chick bone-derived osteoblastic cells was investigated. It was found that PMT was a potent mitogen for primary derived chicken osteoblasts. The toxin stimulated DNA synthesis and cell proliferation in quiescent osteoblasts at the first passage and accelerated cell growth in subconfluent cultures. Cell viability was not affected by PMT, even at relatively high concentrations. Osteoblast numbers increased in a dose-dependent manner in response to PMT. Intracellular inositol phosphates were elevated in response to PMT, but no elevation in cyclic AMP (cAMP) levels was evident. Indeed, PMT inhibited cAMP elevation in osteoblasts in response to cholera toxin at a stage before other PMT-mediated events take place. In addition to increased cell turnover, PMT down-regulated the expression of several markers of osteoblast differentiation. Both alkaline phosphatase and type I collagen were reduced, but osteonectin was not affected. The in vitro deposition of mineral in cultures of primary osteoblasts and osteoblast-like osteosarcoma cells was also inhibited by the presence of PMT. This suggests that PMT interferes with differentiation at a preosteoblastic stage.