Changes in the phenotypic expression of osteoblasts after X irradiation were investigated. Osteoblast-like MC3T3-E1 cells at the actively proliferating, confluent and postproliferation stages were subjected to 10 Gy X irradiation. Irradiation at the confluent stage enhanced accumulation of type I collagen normalized to the DNA content. Irradiation at all stages down-regulated the expression of osteocalcin, but the levels of osteopontin and osteonectin mRNAs were unchanged from the control level. After irradiation at the later stages, the time-dependent increase in alkaline phosphatase activity per cell exceeded that in the control cells. The localization of alkaline phosphatase-positive cells was concordant with that of calcification. In addition, the quality of the calcium deposits was found to be similar to that in control cells as determined by energy dispersive spectrometry and the ratio of calcium to phosphorus, even if the cells were not exactly the same morphologically. The changes in phenotypic expression observed here are closely related to the enhancement of calcification observed in a previous study.

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