Abstract
Recent studies indicate that the amount of alkaline phosphatase (ALP) activity in human osteoblast-line cells is proportional to the concentration of phosphate in the culture medium. The current studies were intended to extend those observations and to determine whether the effects of phosphate (and phosphate esters and analogs) to alter the cellular level of ALP activity, in human osteosarcoma SaOS-2 cells, reflected regulation at the level of transcription. Consistent with previous findings, we found direct, time- and dose-dependent correlations between the concentration of phosphate and the amount of ALP activity/mg cell protein (P<0.05). Surprisingly, we also found a negative correlation between the phosphate concentration in the medium and the level of skeletal ALP mRNA (e.g., r=-0.98, P<0.01 at 24 hours). As the highest cellular levels of skeletal ALP activity were associated with the lowest levels of ALP mRNA, these data indicated that the phosphate-dependent increase in ALP activity was not mediated by an increase in transcription and, conversely, that the effect of phosphate withdrawal to decrease ALP activity was not mediated by a decrease in transcription. Two additional studies have further suggested that these paradoxical effects of phosphate—increasing ALP activity while decreasing ALP mRNA—may be unique to inorganic phosphate: (1) β-glycerol-phosphate (at 10 mmol/liter) mimicked the action of phosphate to increase the cellular level of ALP activity (P<0.001), but did not mimick the action of phosphate to decrease the level of ALP mRNA; and (2) neither phenylphosphonate or molybdate (both of which, like phosphate, increased ALP activity, P<0.05) or borate (which decreased ALP activity, P<0.05) altered the level of ALP mRNA. In summary, these studies show that the effect of phosphate to regulate the level of ALP activity in human osteoblast-line cells is not determined by effects on ALP mRNA synthesis, suggesting that the regulation may depend on phosphate-dependent changes in the translation of ALP mRNA and/or a modification of ALP activity.
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Kyeyune-Nyombi, E., Nicolas, V., Strong, D.D. et al. Paradoxical effects of phosphate to directly regulate the level of skeletal alkaline phosphatase activity in human osteosarcoma (SaOS-2) cells and inversely regulate the level of skeletal alkaline phosphatase mRNA. Calcif Tissue Int 56, 154–159 (1995). https://doi.org/10.1007/BF00296348
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DOI: https://doi.org/10.1007/BF00296348