Melatonin signaling in mitochondria extends beyond neurons and neuroprotection: Implications for angiogenesis and cardio/gastroprotection

We read with interest the paper of Suofu et al. (1), demonstrating that neuronal mitochondria produce melatonin, which upon binding to its melatonin type 1 (MT₁) receptor on the mitochondrial membrane (MM) inhibits cytochrome c release, caspase activation, and apoptosis. We commend the authors on their thorough investigation but wish to point out some major questions: (i) Is the presence of the MT₁ receptor in mitochondria limited to the neuronal cells or may it also apply to other cells, for example, endothelial cells that are components of blood vessels and are critical for delivery of oxygen and nutrients to all tissues, vessels’ regeneration, and angiogenesis? (ii) Is the MT₂ receptor expressed on mitochondria and what is its role?

Melatonin is abundantly expressed beyond the neural system; for example, in gastrointestinal tissues, including the stomach, where it has been shown to protect gastric mucosa against stress, ischemia, and nonsteroidal antiinflammatory drug-induced injury, and to accelerate ulcer healing (2–6). We recently demonstrated that both MT₁ and MT₂ are expressed on the MM of gastric endothelial cells (GECs); MT₁ was expressed in MM while MT₂ was expressed in the nucleus and MM (Fig. 1). However, in contrast to the study of Suofu et al. (1), we found that treatment of GECs with exogenous melatonin (10 µM) increased by 1.7-fold (P < 0.001) MM potential, which drives ATP synthesis (7). Moreover, treatment of GECs with exogenous melatonin increased the MM expression of MT₁ and MT₂ by 2.5- and 1.6-fold, respectively, and increased in vitro angiogenesis (new blood vessel formation) by 1.4-fold (P < 0.001). It should be noted that the angiogenic response to melatonin was independent of either cell proliferation or apoptosis, leading us to speculate that, at least in GECs, melatonin signaling via mitochondria extends beyond cellular protection. Although, we have not yet determined the relative contributions of MT₁ vs. MT₂ in the angiogenic response of GECs to melatonin signaling, our finding that exogenous melatonin increased the MM expression of both MT₂ and MT₁ is at least indicative that both receptors are involved. Suofu et al. (1) did not assess the relative MM expression levels of MT₁ vs. MT₂, but rather used the MT₂ selective inhibitor, 4P-PDOT, as evidence that melatonin signaling in neuronal mitochondria is mediated exclusively through MM-expressed MT₁. Nevertheless, luzindole, which did prevent melatonin from blocking Ca²⁺-mediated cytochrome c release, has a much greater affinity for MT₂ vs. MT₁. Moreover, the possibility that MT₂ plays a “decoy” role, akin to Flt1 in VEGF signaling, cannot be excluded. In such a scenario, MT₂ may play a more subtle role in regulating (e.g., dampening) melatonin signaling. On the other hand, the role of MT₂ in mitochondrial signaling by melatonin may be restricted to certain cell types (e.g., endothelial cells but not to brain neurons). The work of Suofo et al. (1) demonstrates novel melatonin signaling in neuronal mitochondria. Our work uncovered that this mechanism operates in endothelial cells that are major components of all blood vessels. We further suggest that melatonin signaling in mitochondria may also be applicable to other physiological processes, such as angiogenesis, gastroprotection, cardioprotection, and aging (3–6, 8, 9).

Fig. 1.

Localization of melatonin receptors MT₁ and MT₂ in MM of gastric endothelial cells (GECs). (A) Staining of mitochondria using MitoTracker, a fluorescent dye that stains mitochondria (red) in a manner dependent on mitochondrial membrane potential. (B) Immunofluorescence staining for MT₁ in GECs (green). (C) Overlay of MitoTracker and MT₁ immunostaining images showing localization of MT₁ in MM as yellow/orange staining (arrows). (D) Staining of mitochondria using MitoTracker (red). (E) Immunofluorescence staining for MT₂ in GECs (green); please note strong MT2 expression in the nucleus (N). (F) Overlay of MitoTracker and MT₂ immunostaining images showing localization of MT₂ in MM as yellow/orange staining (arrows).