Molecular mechanisms of MrgprA3-independent activation of the transient receptor potential ion channels TRPA1 and TRPV1 by chloroquine.

BACKGROUND AND PURPOSE: Itch associates several pathologies and is a common drug-induced side effect. Chloroquine (CQ) was reported to induce itch by activating the Mas-related G protein-coupled receptor MrgprA3 and subsequently TRPA1. In this study we demonstrate that CQ employs at least two MrgprA3-independent mechanisms to activate or sensitize TRPA1 and TRPV1.

EXPERIMENTAL APPROACH: Patch Clamp and calcium-imaging were utilized to examine effects of CQ on TRPA1 and TRPV1 expressed in HEK-293T cells.

KEY RESULTS: In calcium-imaging, CQ induces a concentration-dependent but MrgprA3-independent activation of TRPA1 and TRPV1. While CQ itself inhibits TRPA1 and TRPV1 in patch clamp recordings, co-application of CQ and UVA-light evokes membrane currents through both channels. This effect is inhibited by the reducing agent dithiothreitol (DTT) and reduced on mutants lacking cysteine residues accounting for reactive oxygen species (ROS)-sensitivity. The combination of CQ and UVA-light triggers an accumulation of intracellular ROS, removes fast inactivation of voltage-gated sodium currents and activates TRPV2. On the other hand, CQ is a weak base and induces intracellular alkalosis. Intracellular alkalosis can activate TRPA1 and TRPV1, and CQ applied at alkaline pH-values indeed activates both channels.

CONCLUSION AND IMPLICATIONS: Our data reveal novel pharmacological properties of CQ allowing activation of TRPA1 and TRPV1 via photosensitization as well as intracellular alkalosis. These findings add complexity to the commonly accepted dogma that CQ-induced itch is specifically mediated by MrgprA3 coupling to TRPA1.