pH-sensing in skin tumors: methods to study the involvement of GPCRs, acid-sensing ion channels and transient receptor potential vanilloid channels.

Solid tumors exhibit an inversed pH-gradient with increased intracellular pH (pHi ) and decreased extracellular pH (pHe ). This inside-out pH-gradient is generated via sodium/hydrogen antiporter 1, vacuolar-type H+ ATPases, monocarboxylate transporters, (bi)carbonate (co)transporters, and carboanhydrases. Our knowledge on how pHe -signals are sensed and what the respective receptors induce inside cells is scarce. Some pH-sensitive receptors (GPR4, GPR65/TDAG8, GPR68/OGR1, GPR132/G2A, possibly GPR31 and GPR151) and ion channels (acid-sensing ion channels ASICs, transient receptor potential vanilloid receptors TRPVs) transduce signals inside cells. As little is known on the expression and function of these pH-sensors, we used immunostainings to study tissue samples from common and rare skin cancers. Our current and future work is directed towards investigating the impact of all the pH-sensing receptors in different skin tumors using cell culture techniques with selective knockdown/knockout (siRNA/CRISPR-Cas9). To study cell migration and proliferation, novel impedance-based wound healing assays have been developed and are used. The field of pH-sensing in tumors and wounds holds great promise for the development of pH-targeting therapies, either against pH-regulators or -sensors to inhibit cell proliferation and migration.