A difference in the regulation of mRNA expression between the phenotypic and the embryonic alkaline phosphatase genes in human cancer cells.

The steady-state levels of mRNAs encoding alkaline phosphatase isoenzymes were examined in two human breast carcinoma cell lines. MDA-MB-157 cells expressed the phenotypic breast alkaline phosphatase and BT20 cells expressed the nonphenotypic placental alkaline phosphatase isoenzyme, frequently reexpressed in neoplasms. Dexamethasone (DEX), which elicits a general effect on phosphatase expression, and 1,25-dihydroxy vitamin D3 (1,25(OH)2D3), a promoter of cell differentiation that correspondingly effects embryonic phosphatase expression, were chosen as perturbing agents for these experiments. RNA blot analysis showed a single RNA species of approximately 2.6 kb under all treatment conditions in BT20 cells and a single RNA species of 2.6 kb under each condition in MDA-MB-157 cells. The results showed that the expression of both the AP isoenzyme mRNA phenotypic of breast produced by MDA-MB-157 cells and the embryonic alkaline phosphatase isoenzyme (PLAP) mRNA produced by BT20 cells was increased by treatment with DEX. By comparison 1,25(OH)2D3 caused an increase in the tissue-unspecific AP mRNA in the MDA-MB-157 cells, but caused a decrease in PLAP mRNA levels in BT20 cells. The level of each isoenzyme mRNA species is altered by either hormone in a dose- and time-dependent manner in both cell lines. In BT20 cells, treatment with cycloheximide showed that ongoing protein synthesis is not required to potentiate the PLAP mRNA response to DEX, but is required for the action of 1,25(OH)2D3. However, protein synthesis is required for the action of both hormones in the MDA-MB-157 cells which make the breast phenotypic AP. These data demonstrate that the DEX- and 1,25(OH)2D3-regulated expression of both of these alkaline phosphatase isoenzymes occurs via a complex mechanism involving control of mRNA abundance, not translational control of constant message levels.