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

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.