Contribution of Na+,HCO3−‐cotransport to cellular pH control in human breast cancer: A role for the breast cancer susceptibility locus NBCn1 (SLC4A7)

Genome‐wide association studies recently linked the locus for Na⁺,HCO₃^?‐cotransporter NBCn1 (SLC4A7) to breast cancer susceptibility, yet functional insights have been lacking. To determine whether NBCn1, by transporting HCO₃^? into cells, may dispose of acid produced during high metabolic activity, we studied the expression of NBCn1 and the functional impact of Na⁺,HCO₃^?‐cotransport in human breast cancer. We found that the plasmalemmal density of NBCn1 was 20–30% higher in primary breast carcinomas and metastases compared to matched normal breast tissue. The increase in NBCn1 density was similar in magnitude to that observed for Na⁺/H⁺‐exchanger NHE1 (SLC9A1), a transporter previously implicated in cell migration, proliferation and malignancy. In primary breast carcinomas, the apparent molecular weight for NBCn1 was increased compared to normal tissue. Using pH‐sensitive fluorophores, we showed that Na⁺,HCO₃^?‐cotransport is the predominant mechanism of acid extrusion and is inhibited 34 ± 9% by 200 μM 4,4′‐diisothiocyanatostilbene‐2,2′‐disulfonic acid in human primary breast carcinomas. At intracellular pH (pH_i) levels >6.6, CO₂/HCO₃^?‐dependent mechanisms accounted for >90% of total net acid extrusion. Na⁺/H⁺‐exchange activity was prominent only at lower pH_i‐values. Furthermore, steady‐state pH_i was 0.35 ± 0.06 units lower in the absence than in the presence of CO₂/HCO₃^?. In conclusion, expression of NBCn1 is upregulated in human primary breast carcinomas and metastases compared to normal breast tissue. Na⁺,HCO₃^?‐cotransport is a major determinant of pH_i in breast cancer and the modest DIDS‐sensitivity is consistent with NBCn1 being predominantly responsible. Hence, our results suggest a major pathophysiological role for NBCn1 that may be clinically relevant.