Andrej Gorbatenko:
Mechanisms regulating acid-base transporter expression in breast- and pancreatic cancer: contributions to the cancer phenotype

Date: 12-06-2015    Supervisor: Stine Falsig Pedersen

Breast cancer is the most commonly diagnosed cancer in women, and the second most frequent cause of death from cancer in women. Pancreatic ductal adenocarcinoma (PDAC) is one of the deadliest forms of cancer with only a 5% 5-year survival rate. Both types of cancer form solid tumors, characteristics of which are a shift towards glycolytic metabolism and increased acid production. HER2 receptor overexpression in breast cancer leads to further increased glycolysis, invasion and metastasis, drug resistance and poor prognosis. Increased tumor glycolysis requires acquisition of mechanisms for dealing with excess acid production. In this light, evidence accumulates on the importance of pH regulatory proteins to cancer cell survival and motility.

Our group previously demonstrated upregulation of the Na+/HCO3 - co-transporter NBCn1 (SLC4A7) by a constitutively active form of HER2 receptor (p95HER2). In Paper 1 and Paper 3 we demonstrate mechanisms of p95HER2-dependent NBCn1 upregulation. Transcriptionally, NBCn1 expression is controlled by ERK1 and Akt1 kinases modulating Krüppel-like factor 4 (KLF4) transcriptional activity. Additionally, we show that other HER2 family members, when stimulated, are able to upregulate NBCn1 expression as well. Paper 3 demonstrates that p95HER2 further stabilizes NBCn1 mRNA, by preventing its downregulation through a 3’UTR region-dependent mechanism, probably involving miRNAs. In Paper 5 we investigate miRNA expression profiles induced by full-HER2 receptor and p95HER2 and demonstrate that despite major similarities, a group of miRNAs are differentially regulated by full-length HER2 and p95HER2. Additionally, we demonstrate that p95HER2 downregulates a member of the c-myb family of transcription factors, MYBL1, through an oncogenic miR-221/222 pathway.

Paper 2 reviews current findings on two families of bicarbonate transporters – SLC4 and SLC26, in cancer. Moreover, using analysis of publicly available high-throughput data we hypothesize that expression and, perhaps, function, of SLC4 and SLC26 family members, as well as carbonic anhydrase (CA) II, IX and XII is cancer subtype specific. In Paper 4 we show that a3, subunit of another pH-regulatory protein complex - V-ATPase, is upregulated at the mRNA and protein level in a panel of PDAC cancer cell lines. We demonstrate the importance of the a3 subunit for proliferation and survival pathways, motility and invasion in PDAC cells. As increased expression of a3 was detected in every PDAC cell line tested, we propose a3 as a potential target for diagnostics and targeted therapy.

Collectively, our findings demonstrate several novel mechanisms through which the expression of a pH-regulatory protein, NBCn1, can be controlled by an oncogene, p95HER2. Further, they reveal differences in small non-coding RNA (miRNA) expression profiles between full-length HER2 and its truncated p95HER2 form. And finally, we report a potential target for PDAC diagnostics and treatment.