The hypoxic and acidic
tumor environment necessitates intracellular pH (pHi) regulation for
tumor progression.
Carbonic anhydrase IX (CA IX;
hypoxia-induced) is known to facilitate CO2 export and generate HCO3(-) in the extracellular
tumor space. It has been proposed that HCO3(-) is re-captured by the cell to maintain an alkaline pHi . A diverse range of HCO3(-) transporters, coupled with a lack of a clear over-expression in
cancers have limited molecular identification of this cellular process. Here, we report that
hypoxia induces the Na(+)/HCO3(-)
co-transporter (NBCe1) SLC4A4
mRNA expression exclusively in the LS174
colon adenocarcinoma cell line in a HIF1α dependent manner. HCO3(-) dependent pHi recovery observations revealed the predominant use of an NBC mechanism suggesting that reversal of a
Cl(-)/HCO3(-) exchanger is not utilized for
tumor cell pHi regulation. Knockdown of SLC4A4 via
shRNA reduced cell proliferation and increased mortality during external
acidosis and spheroid growth. pHi recovery from
acidosis was partially reduced with knockdown of SLC4A4. In MDA-MB-231
breast cancer cells expressing high levels of SLC4A4 compared to LS174 cells, SLC4A4 knockdown had a strong impact on cell proliferation, migration, and invasion. SLC4A4 knockdown also altered expression of other
proteins including CA IX. Furthermore the Na(+)/HCO3(-) dependent pHi recovery from
acidosis was reduced with SLC4A4 knockdown in MDA-MB-231 cells. Combined our results indicate that SLC4A4 contributes to the HCO3(-) transport and
tumor cell phenotype. This study complements the on-going molecular characterization of the HCO3(-) re-uptake mechanism in other
tumor cells for future strategies targeting these potentially important
drug targets.