Angiotensin Type 2 Receptors: Painful, or Not?

Abstract

Pain in response to various types of acute injury can be a protective stimulus to prevent the organism from using the injured part and allow tissue repair and healing. On the other hand, neuropathic pain, defined as 'pain caused by a lesion or disease of the somatosensory nervous system', is a debilitating pathology. The TRPA1 neurons in the Dorsal Root Ganglion (DRG) respond to reactive oxygen species (ROS) and induce pain. In acute nerve injury and inflammation, macrophages infiltrating the site of injury undergo an oxidative burst, and generate ROS that promote tissue repair and induce pain via TRPA1. The latter discourages using the injured limb, with a lack of movement helping wound healing. In chronic inflammation caused by diabetes, cancer etc., ROS levels increase systemically and modulate TRPA1 neuronal functions and cause debilitating neuropathic pain. It is important to distinguish between drug targets that elicit protective vs. debilitating pain when developing effective drugs for neuropathic pain. In this context, the connection of the Angiotensin type 2 receptor (AT₂R) to neuropathic pain presents an interesting dilemma. Several lines of evidence show that AT₂R activation promotes anti-inflammatory and anti-nociceptive signaling, tissue repair, and suppresses ROS in chronic inflammatory models. Conversely, some studies suggest that AT₂R antagonists are anti-nociceptive and therefore AT₂R is a drug target for neuropathic pain. However, AT₂R expression in nociceptive neurons is lacking, indicating that neuronal AT₂R is not involved in neuropathic pain. It is also important to consider that Novartis terminated their phase II clinical trial (EMPHENE) to validate that AT₂R antagonist EMA401 mitigates post-herpetic neuralgia. This trial, conducted in Australia, United Kingdom, and a number of European and Asian countries in 2019, was discontinued due to pre-clinical drug toxicity data. Moreover, early data from the trial did not show statistically significant positive outcomes. These facts suggest that may AT₂R not be the proper drug target for neuropathic pain in humans and its inhibition can be harmful.

Keywords: TRPA1; acute nerve injury; angiotensin type 2 receptor; angiotensin type 2 receptors agonism; neuropathic pain; reactive oxygen species.

FIGURE 1

Role of AT₂R in various disease conditions. In the vast majority of situations (blue arrows), AT₂R agonist-induced activation or tissue over-expression of AT₂R has been shown to exert protective actions in a host of disease conditions, particularly those with a strong inflammatory component. With regard to pain (gray shaded box), there are conflicting opinions, with some studies concluding that activation of AT₂R exerts anti-nociceptive actions, while others conclude that AT₂R antagonists produce relief from neuropathic pain.

FIGURE 2

Molecular mechanisms AT₂R-mediated nociception (A). Spared Nerve Injury (experimental injury) in mice activates tissue RAS, and increases Ang II. Ang II () binds to AT₂R () on macrophages. At the same time macrophages are attracted to the injury site by macrophage chemoattractants (MC). Those macrophages that interact with MC are directed to injured tissue. (): TRPA1 Nociceptor in DRG. (B). Macrophages with AT₂R () at the injured site bind Ang II () and undergo respiratory burst (RB), an innate immune response that is an integral part of removal of any pathogens, and activates macrophage erythropoietin signaling to promote acute inflammation resolution. According to Shepherd et al., reactive oxygen/nitrogen species (ROS/RNS []) generated during this process activates TRPA1 nociceptors () in DRG and cause pain. Since AT₂R is a reparative/wound healing molecule, its involvement in activating respiratory burst of macrophages, an essential mechanism for inflammation resolution, and thereby indirectly activating TRPA1 nociceptors is part of a protective pain mechanism rather than debilitating chronic neuropathic pain.