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Cellular and Molecular Biology

Loss of RPTPγ primes breast tissue for acid extrusion, promotes malignant transformation and results in early tumour recurrence and shortened survival

Abstract

Background

While cellular metabolism and acidic waste handling accelerate during breast carcinogenesis, temporal patterns of acid–base regulation and underlying molecular mechanisms responding to the tumour microenvironment remain unclear.

Methods

We explore data from human cohorts and experimentally investigate transgenic mice to evaluate the putative extracellular HCO3-sensor Receptor Protein Tyrosine Phosphatase (RPTP)γ during breast carcinogenesis.

Results

RPTPγ expression declines during human breast carcinogenesis and particularly in high-malignancy grade breast cancer. Low RPTPγ expression associates with poor prognosis in women with Luminal A or Basal-like breast cancer. RPTPγ knockout in mice favours premalignant changes in macroscopically normal breast tissue, accelerates primary breast cancer development, promotes malignant breast cancer histopathologies, and shortens recurrence-free survival. In RPTPγ knockout mice, expression of Na+,HCO3-cotransporter NBCn1—a breast cancer susceptibility protein—is upregulated in normal breast tissue but, contrary to wild-type mice, shows no further increase during breast carcinogenesis. Associated augmentation of Na+,HCO3-cotransport in normal breast tissue from RPTPγ knockout mice elevates steady-state intracellular pH, which has known pro-proliferative effects.

Conclusions

Loss of RPTPγ accelerates cellular net acid extrusion and elevates NBCn1 expression in breast tissue. As these effects precede neoplastic manifestations in histopathology, we propose that RPTPγ-dependent enhancement of Na+,HCO3-cotransport primes breast tissue for cancer development.

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Fig. 1: Ptprg mRNA expression in breast tissue from WT mice is abolished in RPTPγ KO mice (n = 5–7).
Fig. 2: Histopathology of breast tumours and macroscopically normal breast tissue reveals more aggressive characteristics in RPTPγ KO mice.
Fig. 3: Knockout of RPTPγ promotes early breast cancer development and recurrence with no effect on primary tumour growth rate.
Fig. 4: Knockout of RPTPγ amplifies net acid extrusion via Na+,HCO3-cotransport and increases the CO2/HCO3-dependent elevation of steady-state pHi in breast tissue.
Fig. 5: Protein expression of NBCn1, yet not of NHE1, is elevated in normal breast tissue from RPTPγ KO mice.
Fig. 6: The level of PTPRG mRNA, encoding RPTPγ, decreases from normal to malignant breast tissue, in breast cancer tissue of higher malignancy grade, and in breast cancer tissue of more aggressive molecular subtypes.

Data availability

Data generated during this study (Figs. 15) are available from the corresponding author on reasonable request. The datasets analysed in Fig. 6 are publically available as detailed in 'Methods'.

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Acknowledgements

The authors are grateful to Dr. Joseph Schlessinger for generously providing the RPTPγ KO mice. The authors would like to thank Jane Rønn and Viola M. Larsen, Aarhus University, for expert technical assistance.

Funding

The studies were financially supported by the Independent Research Fund Denmark (4183-00258 A and 7025-00050B to EB), the Novo Nordisk Foundation (NNF18OC0053037 to EB) and the Danish Cancer Society (R111-A6862-14-S7 to RS).

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Contributions

RS, TA, ME, NT and NV conducted experiments and analysed data. MB and MT acquired data. PV analysed data. EB conceived of and designed the studies, analysed data and wrote the manuscript. All authors approved the final version.

Corresponding author

Correspondence to Ebbe Boedtkjer.

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Competing interests

EB is an inventor on an issued patent regarding tools targeting NBCn1 in breast cancer (EP-3271402). The remaining authors declare no competing interests.

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All experimental procedures were approved by the Danish Animal Experiments Inspectorate (2014-15-0201-0030). The analysed human data are from previously published and publically available studies.

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Sloth, R.A., Axelsen, T.V., Espejo, M.S. et al. Loss of RPTPγ primes breast tissue for acid extrusion, promotes malignant transformation and results in early tumour recurrence and shortened survival. Br J Cancer 127, 1226–1238 (2022). https://doi.org/10.1038/s41416-022-01911-6

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