The primary growth-inhibitory effects of resveratrol are mediated via both p53-dependent and p53-independent upregulation of p21WAF1 and downregulation of key cell cycle activators. A number of studies have demonstrated that resveratrol-induced growth arrest is followed by apoptotic cell death and that it directly interferes with cell survival by the modulation of apoptotic and survival pathway genes. Apoptosis is regulated by a complex network of pro-apoptotic and anti-apoptotic proteins. The apoptotic signals can be initiated by external stimuli/ligands and by cellular stress caused by gamma/UV radiation and cytotoxic drugs, leading to altered mitochondrial permeability. As a consequence of alterations in mitochondria permeability pore transition, release of cytochrome
c into the cytoplasm occurs where it can bind to and induce conformational change of APAF-1, resulting in the formation of the “apoptosome” complexes. Apoptosomes recruit and activate caspase 9, which in turn activates the effector caspases 3, 6, and 7. Caspase activation is tightly regulated by the inhibitor of apoptosis proteins (IAP), which include NAIP, cIAP1, cIAP2, XIAP, and survivin. IAP binds to caspases and antagonizes their activity. SMAC, the second mitochondria-derived activator of caspase, also known as DIABLO, is released into the cytoplasm and binds and neutralizes the IAPs; this restores caspase activity and induces apoptosis [
32]. Survivin prevents Smac/DIABLO release from mitochondria in certain cancer cells [
33]. Mitochondrial release of cytochrome C is further regulated by Bcl2 family proteins. Bcl2-like anti-apoptotic proteins reside in the outer mitochondrial membrane and inhibit cytochrome C release, while proapoptotic Bax, BID, and BIM translocate to mitochondria to facilitate apoptosis. p53 induces a number of mitochondria-mediated apoptotic genes, such as Bax, Noxa, PUMA, and BID, and represses the anti-apoptotic Bcl2 and CIAPs. Induction of phosphatase and tensin homolog deleted on chromosome-10 (PTEN), APAF-1, and p53-regulated apoptosis-inducing protein-1 (p53AIP1) may also contribute to apoptosis [
34]. Resveratrol-mediated apoptosis has been associated with p53 activation in various human cancer cells [
35,
36]. It induces the expression of pro-apoptotic Bax, Bak, PUMA, Noxa, and Bim, and inhibits the expression of anti-apoptotic Bcl2, Bcl-XL, and Mcl-1, directly affecting the mitochondrial death pathway [
36]. Consequently, Bcl2 overexpression has been shown to suppress resveratrol-induced caspase-3 activation and apoptosis [
37]. In thyroid cancer cells, resveratrol-induced apoptosis has also been associated with the accumulation of p53 (ser15) phosphorylation and non-steroidal anti-inflammatory (NSAID) drug-activated gene-1(NAG-1) with pro-apoptotic and anti-tumorigenic activities. Interestingly, the p53 binding sites within the promoter region of NAG-1 have been shown to play a pivotal role in controlling the effects of resveratrol on NAG-1 expression [
38].
Resveratrol-mediated apoptosis also occurs via the death receptor Fas/CD95/APO-1 [
39]. Fas/CD95/APO-1/DR2 and TRAILR are death receptor family members, that activate a death-signaling cascade after binding to corresponding ligands [
40,
41]. The cytoplasmic domain (death domain, DD) of Fas interacts with adaptor proteins, such as Fas-associated via death domain (FADD) which in turn recruits procaspase 8. Active caspase 8 can directly cleave caspase 3 and FLICE-inhibitory protein (FLIP) is known to inhibit caspase-8 activation [
42,
43]. Amplification of the death signal can also occur in certain cells via the engagement of the mitochondrial pathway of caspase activation. Proteolytically cleaved, active procaspase-8 cleaves BH3 interacting domain death agonist (BID), resulting in tBID (truncated BID), which can inactivate Bcl2. Inhibition of Bcl2 in the mitochondrial membrane releases cytochrome C, which binds to APAF1 and caspase-9 which can activate procaspase-3. Resveratrol can redistribute FAS/CD95 into lipid rafts in a ligand-independent way, enhancing the efficacy of signaling by FAS/CD95 and other death receptors in colon cancer cells [
44]. It also induces the redistribution of FAS/CD95 and other death receptors in lipid rafts, and sensitizes cells to death receptor agonists [
45]. A similar synergy has been reported between resveratrol and TRAIL [
46]. Resveratrol enhanced TRAIL-induced apoptosis through G1 cell cycle arrest and survivin depletion [
47]. Furthermore, resveratrol was shown to overcome the chemoresistance of human multiple myeloma cells and potentiated the apoptotic effects of bortezomib and thalidomide through the suppression of NF-κB and STAT3, which in turn lead to the downregulation of anti-apoptotic genes, cyclin D1, cIAP-2, XIAP, survivin, Bcl-2, Bcl-xL, Bfl-1/A1, TRAF2, and Akt and increased Bax/caspase-3-associated apoptosis [
48]. In MCF7 human breast cancer cells, the phosphorylation of Akt was significantly reduced which is followed by decreased pro-caspase-9 activation [
49]. In LNCaP and PC-3 prostate cancer cells, resveratrol can inhibit AR (androgen receptor)-, and ER (estrogen receptor) alpha-dependent PI3K phosphoinositide-3-kinase activities, respectively [
50].
The sphingomyelin pathway responds to diverse environmental stresses such as UV radiation, heat shock, oxidative stress, cytokines (TNF-α and IL-lβ), and anti-cancer drugs and is involved in inflammation, cell cycle arrest, apoptosis, and stress [
51,
52]. Ceramide is the second messenger in this pathway, generated either by the hydrolysis of sphingomyelin or by
de novo synthesis. Ceramide induces apoptosis through c-Jun N-terminal kinase (JNK) and stress-activated protein kinase (SAPK) and promotes the dephosphorylation of Bcl2, Bax, and Bad. It also activates caspase-3. In addition, ceramide regulates cathepsin-D, binding directly to PLA2 (phospholipase-A2), and may induce apoptosis [
52]. The anti-proliferative effects of resveratrol correlate with a dose-dependent increase in
de novo ceramide biosynthesis and subsequent inhibition of c-MYC and ODC in colon cancer cells [
53]. Androgen-independent human prostate cancer cells, DU145, are resistant to ionizing radiation-induced cell death, but become sensitized to apoptosis with prior resveratrol treatment due to ceramide accumulation [
54]. In metastatic breast cancer cells, ceramide mediates the anti-cancer effects of resveratrol [
55]. Resveratrol also promotes the accumulation of mature cathepsin-D [
56].