Differentiation impairs low pH-induced Ca2+ signaling and ERK phosphorylation in granule precursor tumour cells

Extracellular acidification is a hallmark of a number of debilitating pathologies including cancer, ischemia and inflammation. We have recently shown that in human granule precursor tumour cells a fall in extracellular pH triggers increases in intracellular Ca2+ concentration through activation of G-protein coupled proton-sensing receptors coupling to phospholipase C. This pH-dependent rise in cytosolic Ca2+ led to activation of the extracellular signal-regulated kinase ERK, providing a mechanistic explanation of how extracellular acidification can promote tumour growth. We now find that differentiation of granule precursor tumour cells profoundly affects their ability to respond to extracellular acidification with gene transcription. Differentiating cells have a lower Ca2+ release probability from intracellular Ca2+ stores upon acidification and cells that respond have a significantly smaller and slower Ca2+ signal than proliferating cells. Importantly, Ca2+ release in differentiating cells fails to evoke ERK phosphorylation. This altered responsiveness of differentiating cells is not due to reduced proton-sensing receptor expression or diminished Ca2+ store content. Rather, our results suggest that in differentiating cells, the proton-sensing receptor couples less effectively to phospholipase C activation and IP3 formation. Hence, the ability of human granule cells to respond to extracellular acidification by generating Ca2+ signals and ERK activation is state-dependent, being lost upon differentiation.