Regulation of the Na+,HCO3‐ cotransporter NBCn1 (SLC4A7) by a constitutively active ErbB2 receptor in MCF‐7 breast cancer cells

Abstract

Net acid production is greatly increased in tumors due to a shift towards glycolytic metabolism. We previously showed that the Na+,HCO3− cotransporter NBCn1 (SLC4A7) is strongly upregulated in MCF‐7 breast cancer cells upon expression of a truncated, constitutively active ErbB2 receptor, ΔNErbB2. Here, we explore the mechanisms involved in this upregulation. NBCn1 mRNA and protein levels were reduced by inhibition of ERK or Src in ΔNErbB2‐, but not vector‐expressing MCF‐7 cells. PI3K‐Akt inhibition reduced NBCn1 expression in both cell types. NBCn1 expression was increased by stimulation of full‐length ErbB receptors. In luciferase promoter assays, the 1.2 kb genomic region upstream of SLC4A7 increased luciferase activity after ΔNErbB2 expression ~2.5 fold. Promoter deletions identified the ΔNErbB2‐ responsive region and putative transcription factor binding sites. NBCn1 expression was increased by expression of dominant‐negative (DN)‐Sp1 or by Sp1 knockdown, and decreased by knockdown of Krüppel‐like factor 4 (KLF4), while DN‐Sp3 or Sp3 knockdown had no effect. Chromatin immunoprecipitation assay (ChIP) demonstrated Sp1, Sp3 and KLF4 binding to the NBCn1 promoter. In conclusion, we identified an NBCn1 promoter and demonstrated that NBCn1 transcriptional regulation involves Sp1 and KLF4, and that ErbB2‐dependent upregulation of NBCn1 is controlled at least in part by ERK‐ and Src signaling.Funding: Danish Cancer Society, Danish Research Council