Interplay between microenvironmental acidosis and Ca2+ signals in Pancreatic Adenocarcinoma (PDAC) progression

Pancreatic ductal adenocarcinoma (PDAC) is among the leading causes of death by cancer in the world. PDAC is a cancer affecting the exocrine pancreas and represents a challenging problem due to the lack of screening tests and late-stage diagnosis. According to the Global Cancer Observatory, Pancreatic tumors are located in the 7th place from an incidence point of view in Europe, with 140.116 new cases, representing 3,2% of all European cancers, with 3% of incidence in men, and 3.4% in women. PDAC poor prognosis and therapeutic failure is due to several factors among which the peculiar tumor microenvironment physico-chemical properties are highly relevant. Indeed, Pancreatic cancer cells are embedded in a highly hypoxic, desmoplastic, and acidic stroma that isolates the tumor from drugs and infiltrating immune cells. In particular, one of the most significant players is the acidic tumor microenvironment which drives PDAC progression by selecting for more aggressive cell phenotypes although the underlying mechanism is still not clear. In this context, Ca2+ channels represent good target candidates due to their ability to integrate signals from the TME. Ca2+ channels are indeed pH and hypoxia sensors and alterations in Ca2+ homeostasis and expression in cancer progression have been extensively reported. This work aims to understand the effect of the acidic microenvironment and its role in promoting a more aggressive cell cancer phenotype, and how the extracellular pH is able to modulate the calcium signaling and activate specific signal transduction pathways. To this end, I contributed to the characterization of 4 different PDAC cell models based on short- or long-time exposure to the acidic microenvironment: control cells, kept at pHe 7.4, 4 days selection at pHe 6.6, 30 days of selection at pHe 6.6, and 30 days of selection and then recovered for two weeks in physiological pHe 7.4. Interestingly, I demonstrated that long-term acidic selection selects for more aggressive PDAC cell phenotypes by promoting cell proliferation, adhesion, invasion, and as well as epithelial to mesenchymal transition. In the second part of my thesis, I investigated the interplay between acidosis and Ca2+ signals by analyzing the effect of the acidic selection on spontaneously induced Ca2+ oscillations. The data clearly show that tumor acidosis selection modulates Ca2+ oscillations by enhancing the frequencies and amplitude properties. A deeper analysis of the molecular nature responsible for the oscillating behaviors pointed out a crucial role for Store-operated Ca2+ entry (SOCE). Finally, we correlate the aggressive invasion activity observed in long-term acidic selected cells with SOCE activity by pharmacologically inhibiting ORAI-1 channel. Our results shed new light on the role of PDAC TME acidosis in the selection of aggressive cancer cell phenotype underlying a critical role for Ca2+ signals.