Mechanism of pH regulation of connexin 43 expression in MC3T3-E1 cells

Gap junction (GJ) expression and function allowing cell-cell communication among osteoblasts may be important in new bone formation. An alkaline milieu stimulates mineralization, while extracellular acidification leads to demineralization of bone. It was previously demonstrated that alkaline pH increases, while acid pH decreases GJ intercellular communication by an increase in steady-state GJ connexin 43 (Cx43) mRNA and protein expression. At pH 7.6, transcription of new Cx43 mRNA was significantly higher than that at pH 6.9 but not significantly different at pH 7.2, as assessed by nuclear run-on assay. Transcription of new Cx43 mRNA was higher at pH 7.2 compared to that at pH 6.9. Although Cx43 mRNA half-life tended to be longer at pH 7.6, analysis of variance did not yield a significant difference of the Cx43 mRNA half-life at any of the pHs tested. Likewise, the half-life of Cx43 protein at pHs of 6.9, 7.2, and 7.6 was not significantly different. Plasma membrane and cytosolic Cx43 fractions were proportionately similar at pH 7.2 and 6.9. Thus, the decrease in Cx43 mRNA at low pH compared to high pH is due to a decrease in the transcription rate of Cx43 but not due to an alteration of message stability. The early uncoupling of gap junctions by low pH found previously does not appear to be due to changes in the half-life nor distribution of Cx43 protein between cytosolic and plasma membrane compartments.