4. Melatonin and mitophagy regulation in mammalian cells
Myriad studies have delineated the preventive role of melatonin against
atherosclerosis. To begin with, our group for the first time, reported
that melatonin administration impedes activation of NLRP3 inflammasome
and consequently ceases secretion of inflammatory factors from
macrophages in atherosclerotic lesions in a murine model of
atherosclerosis (Ma et al., 2018). The underlying mechanism seems to be
related to melatonin-mediated activation of mitophagy via a SIRT3-FOXO3
(forkhead box O3)-PRKN signaling cascade, resulting in ROS scavenging
and inhibition of ROS-activated NLRP3 inflammasomes (Ma et al., 2018).
Similar results were noted from examination of melatonin in subarachnoid
hemorrhage (Cao et al., 2017). This study revealed that melatonin
treatment significantly upregulates autophagy-associated molecules such
as ATG5 and MAP1LC3A, and mitophagy molecules such as PRKN and PINK1 in
subarachnoid hemorrhage, resulting in ROS scavenging, remarkable
inactivation of the NLRP3 inflammasomes, and attenuation of cytokine
secretion (Cao et al., 2017). Furthermore, examination of a rat model of
radiculopathy showed that melatonin alleviates apoptosis and NLRP3
inflammasome activation through instigating PRKN-dependent mitophagy
(Xie et al., 2021).
Conversely, melatonin was shown to induce OPA1 (OPA1 mitochondrial
dynamin like GTPase)-mediated mitophagy and mitochondrial fusion via
AMP-activated protein kinase (AMPK) under ischemia-reperfusion (I/R)
injury in vivo and in vitro , prompting the role of
AMPK-OPA1 axis as a new paradigm for melatonin-evoked mitophagy
induction (Zhang et al., 2019). Likewise, melatonin significantly
attenuates calcium deposition in vascular smooth muscle cells in a
pattern dependent on mitophagy via AMPK-OPA1 axis (Chen, Zhou, Yang,
Liu, Wu & Sha, 2020). Besides, melatonin downregulates cleaved CASP3
(caspase 3) and RUNX2 (RUNX family transcription factor 2), upregulates
MAP1LC3B and MFN2 (mitofusin 2), reduces mitochondrial superoxide, and
activates mitophagy via the AMPK-OPA1 signaling axis (Chen, Zhou, Yang,
Liu, Wu & Sha, 2020).
In addition, melatonin treatment reactivates mitophagy and boosts
mitochondrial function via upregulation of HSPA1L (heat shock protein
family A [Hsp70] member 1 like) in senescent mesenchymal stem cells.
Mechanistically, HSPA1L forms a complex with cellular PRNP (prion
protein) then recruits PRNP to the mitochondria. Afterward, the
HSPA1L-PRNP complex binds to COX4I1 (cytochrome c oxidase subunit 4I1),
resulting in elevated mitochondrial membrane potential, induced
antioxidant enzymes, and mitophagy, validating the role of the
HSPA1L-PRNP-COX4I1 axis in melatonin-induced mitophagy induction (Lee,
Yoon, Song, Noh & Lee, 2020). Likewise, supplementation of human
mesenchymal stem cells with melatonin, causes HSPA1L upregulation, and,
enhances HSPA1L-mediated recruitment of PRKN to mitochondria, to favor
mitophagy and cell survival (Yoon, Kim, Lee & Lee, 2019). Therefore,
the HSPA1L-PRKN axis is modulated by melatonin and promotes mitophagy.
A number of studies have noted PINK1 and PRKN modulation by melatonin.
Melatonin supplementation culminates in upregulation of PINK1, PRKN,
PPARGC1A (PPARG coactivator 1 alpha), NRF1 (nuclear respiratory factor
1), and TFAM (transcription factor A, mitochondrial) proteins with a
role in mitophagy and mitochondrial integrity in rats with liver
fibrosis (Kang, Hong & Lee, 2016). Melatonin-mediated upregulation of
PRKN is also observed in nucleus pulposus cells in a time- and
dose-dependent manner (Chen et al., 2019). In addition, melatonin
enhances PRKN mitochondrial translocation via inhibition of MST1
(macrophage stimulating 1) phosphorylation, compromises mitophagy in
diabetic cardiomyopathy (Wang et al., 2018).
Furthermore, ample evidence has shown that melatonin modulates NFE2L2
(nuclear factor, erythroid 2 like 2), HAS3 (hyaluronan synthase 3), and
MTOR (mechanistic target of rapamycin kinase) complex 1 (MTORC1) to
induce mitophagy. Melatonin upregulates NFE2L2 to induce
NFE2L2-dependent mitophagy to heal brain injury in a murine model of
subarachnoid hemorrhage (Sun, Yang, Li & Hang, 2018). Also, melatonin
activates HAS3 and associated mitophagy, in a neuroblastoma N2a cell
line (Lee et al., 2019). Furthermore, melatonin activates mitophagy
through MTORC1 modulation, which in turn, ceases inflammation by
suppressing IL1B (interleukin 1 beta) secretion upon immunopathology of
traumatic brain injury (Lin et al., 2016).
Melatonin also modulates other signaling pathways to activate mitophagy.
Interestingly, our group revealed that melatonin backs up the CGAS
(cyclic GMP-AMP synthase)-STING1 (stimulator of interferon response
cGAMP interactor 1)-TBK1 (TANK binding kinase 1) signaling pathway,
leading to mitophagy reactivation involving ALDH2 (aldehyde
dehydrogenase 2 family member) activation in APP- (amyloid beta
precursor protein) and PSEN1 (presenilin 1)-mutant mice (Wang et al.,
2020). Our study suggested the role of melatonin in rescuing myopathic
changes in the heart via reinstating mitophagy. Moreover, melatonin
therapy inhibits mitochondrial fission and boosts mitophagy through
inhibiting NR4A1 (nuclear receptor subfamily 4 group A member 1)-PRKDC
(protein kinase, DNA-activated, catalytic subunit)-TP53 (tumor protein
p53) signaling pathway in nonalcoholic fatty liver disease (Zhou et al.,
2018).
Despite mitophagy activation, ample studies have revealed other
mechanisms of melatonin regarding mitophagy regulation in mammalian
cells. It was suggested that melatonin maintains mild induction of
mitophagy through preventing excessive mitophagy induction. This notion
was observed in a murine model of microvascular I/R injury, where
melatonin activates PRKAA1 (protein kinase AMP-activated catalytic
subunit alpha 1) to inhibit DNM1L-based mitochondrial fission.
Subsequently, the VDAC1-HK2 (hexokinase 2) interaction is recovered,
which results in inhibition of the mitochondrial permeability transition
pore opening, and attenuation of PINK1-PRKN-dependent mitophagy (Zhou et
al., 2017). It is perceived that melatonin-evoked
PRKAA1-DNM1L-VDAC1-HK2-mitochondrial permeability transition pore
signaling pathway is a cytoprotective mechanism that blunts excessive
mitophagy-evoked cell death ensuing microvascular I/R damage.
More cell signaling pathways are reported to be modulated by melatonin
to govern mitophagy including the MT2A (metallothionein 2A)-SIRT3-FOXO3
pathway, which inhibits mitophagy in H9c2 cells (Wu, Yang, Gao, Wang &
Ma, 2020), and MAPK8 (mitogen-activated protein kinase 8)-PRKN pathway,
which is negatively modulated by melatonin, and causes a cessation of
excessive mitophagy induction in human HeLa cells (Chen, Liu, Li & Gao,
2018). In addition, melatonin maintains mild levels of
mitophagy-associated proteins such as PRKN, BECN1, SIRT3, FOXO3, and
BNIP3L, thus keeping mitophagy in check (Wu, Yang, Gao, Wang & Ma,
2020). Melatonin can also upregulate SIRT1 (sirtuin 1), as a result
suppressing excessive PINK1-PRKN mitophagy (Yi, Zheng, Zhu, Cai, Sun &
Zhou, 2020). Collectively, these data suggest that melatonin regulates
mitophagy by two dogmas: (i) induction of a low-level activation and
(ii) prevention of excessive activation, both of which, can be
beneficial to cell homeostasis and inflammation suppression.