Effect of acidosis on the properties of the glutaminase mRNA pH-response element binding protein

The pH-responsive stabilization of the rat renal glutaminase (GA) mRNA during metabolic acidosis is mediated by a pH-response element (pH-RE). The primary pH-RE within the GA mRNA consists of a direct repeat of an 8-base adenosine and uridine-rich sequence, which binds a specific cytosolic protein, the pH-response element binding protein (REBP). The functional analysis of this system was performed in LLC-PK(1)-F(+) cells, a pH-responsive line of porcine proximal tubule-like cells. Cytosolic extracts of LLC-PK(1)-F(+) cells also contain a protein that binds with high affinity to the rat GA mRNA pH-RE. The apparent binding of this protein is increased threefold in cytosolic extracts prepared from LLC-PK(1)-F(+) cells that were grown in acidic medium (pH = 6.9, HCO(3)- = 10 mM). Extracts prepared from the renal cortex of rats that were made acutely acidotic also exhibit a similar increase in binding to the RNA probe that contains the direct repeat of the pH-RE. The temporal increase in binding correlates with the temporal increase in GA mRNA. Scatchard analysis indicates that the increased binding is due to an increase in both the affinity and the maximal binding of the pH-REBP. Thus, increased binding of the pH-REBP to the GA mRNA may initiate its stabilization and increased expression during acidosis.