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 2030% 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 mu M 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid in human primary breast carcinomas. At intracellular pH (pHi) levels >6.6, CO2/HCO3--dependent mechanisms accounted for >90% of total net acid extrusion. Na+/H+-exchange activity was prominent only at lower pHi-values. Furthermore, steady-state pHi 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 pHi 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.