Melatonin antagonizes the intrinsic pathway of apoptosis via mitochondrial targeting of Bcl‐2

Abstract

We have recently shown that melatonin antagonizes damage-induced apoptosis by interaction with the MT-1/MT-2 plasma membrane receptors. Here, we show that melatonin interferes with the intrinsic pathway of apoptosis at the mitochondrial level. In response to an apoptogenic stimulus, melatonin allows mitochondrial translocation of the pro-apoptotic protein Bax, but it impairs its activation/dimerization The downstream apoptotic events, i.e. cytochrome c release, caspase 9 and 3 activation and nuclear vesiculation are equally impaired, indicating that melatonin interferes with Bax activation within mitochondria. Interestingly, we found that melatonin induces a strong re-localization of Bcl-2, the main Bax antagonist to mitochondria, suggesting that Bax activation may in fact be antagonized by Bcl-2 at the mitochondrial level. Indeed, we inhibit the melatonin anti-apoptotic effect (i) by silencing Bcl-2 with small interfering RNAs, or with small-molecular inhibitors targeted at the BH3 binding pocket in Bcl-2 (i.e. the one interacting with Bax); and (ii) by inhibiting melatonin-induced Bcl-2 mitochondrial re-localization with the MT1/MT2 receptor antagonist luzindole. This evidence provides a mechanism that may explain how melatonin through interaction with the MT1/MT2 receptors, elicits a pathway that interferes with the Bcl-2 family, thus modulating the cell life/death balance.

Melatonin: A pleiotropic molecule regulating inflammation

“…Alternative mechanisms have been proposed; in U937 monocytes, melatonin reduces apoptosis via intracellular signal transduction stimulated by interaction with MT1/MT2 plasma membrane receptors [23]. However, the majority of reports indicate that the anti-apoptotic of melatonin occurs by acting on the abundance of the two main members of the Bcl-2 family, namely the pro-apoptotic Bax and the anti-apoptotic Bcl-2 proteins [108].…”

“…Melatonin is able to shift the balance towards a cell-protective state [108]. Indeed, many studies have demonstrated that its anti-apoptotic effect involves the down-regulation of Bax [110][111][112][113] and /or the up-regulation of Bcl-2 [110,114,115] that occurs via activation of NF-B [115].…”

“…Oxidative alterations cause Bax disulfide dimerization, and the resulting conformational changes are sufficient for its translocation to mitochondria independently of apoptosis [116]. However, melatonin maintains Bax in a monomeric state within mitochondria [108]. The inhibition of Bax activation may be due to melatonin's anti-oxidant ability, and/or to the simultaneous re-localization of Bcl-2 [108] to mitochondria, thus co-localizing with (and sequestering) Bax.…”

“…However, melatonin maintains Bax in a monomeric state within mitochondria [108]. The inhibition of Bax activation may be due to melatonin's anti-oxidant ability, and/or to the simultaneous re-localization of Bcl-2 [108] to mitochondria, thus co-localizing with (and sequestering) Bax. Indeed, the persistence of 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 -10 -It is interesting to note that melatonin has developed various independent strategies to inhibit the intrinsic pathway of apoptosis at the level of Bax activation, inducing a pro-survival pathway and by this way maintaining viability of cells at the inflammatory site.…”

“…However, upon Bcl-2 silencing, melatonin acquires pro-apoptotic abilities [108], indicating that the very finely regulated changes in Bcl-2 levels during lymphocyte maturation may confer different susceptibility to melatonin, modulating a pro-or anti-apoptotic effect according to the intracellular amount of Bcl-2. M a n u s c r i p t 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 M a n u s c r i p …”