*5.3. Anti-Apoptosis*

Under normal conditions, the homeostasis between apoptosis and proliferation of intestinal epithelial cells regulates the normal morphological structure and physiological function of the intestinal tract [98]. However, pathological factors, such as intestinal flora disorder, may induce local inflammation and subsequently, the infiltration of immune cells, such as leukocytes, that can be easily activated by microbial products causing the overproduction of RNS and ROS, which finally causes abnormal apoptosis among intestinal cells [34,99,100]. Although mechanisms underlying apoptosis are complicated, it has been considered that apoptosis is mainly mediated by two ways [101]. On the one hand, pro-apoptotic factors such as ROS may change mitochondrial permeability and induce the release of second mitochondria-derived activator of caspases (SMACs) into the cytoplasm to bind and inactivate the inhibitor of apoptosis proteins (IAPs) like Bcl-2 [102,103], which inhibit the activation of caspase and contribute to protecting intestinal epithelial cells from apoptosis [104]. On the other hand, increased mitochondrial permeability can also cause the release of cytochrome c (Cyto C), the inducer of apoptotic protease activating factor-1 (Apaf-1), through the mitochondrial apoptosis-induced channel (MAC), which is generally suppressed by Bcl-2 family proteins [105], to induce the production of caspase 9 and caspase 3 and promote apoptosis [105].

The effects of C3G on apoptosis are various in different cell models. It has been reported that C3G can potentially inhibit human colon cancer cell proliferation through promoting apoptosis and suppressing angiogenesis [106,107]. But in other normal cases, C3G showed the protective effects on gastrointestinal cells, as well as endothelial cells, and obviously inhibited apoptosis by regulating apoptosis associated proteins, such as reducing the cytoplasmatic levels in Bax [108] and inhibiting the expression of caspase-8 [109], caspase-9 [108], and caspase-3 [108,109] to attenuate gastrointestinal damage. The impacts of C3G-Ms on apoptosis are similar to C3G, and multiple studies have suggested that PCA [35,36,100] and FA [110,111] may act against apoptosis in various models, although other studies revealed that C3G-Ms (PCA, FA) might promote apoptosis of colorectal adenocarcinoma cells [112]. The mechanism of C3G-Ms against apoptosis is still unclear, but a recent study has shown that in addition to the direct quenching of ROS, PCA may inhibit the expression of pro-apoptotic Bax in mitochondria and subsequently, increase the ratio of Bcl-2/Bax to reduce the production of caspase 8, caspase 9, and caspase 3 in injured gastrointestinal mucosa (Figure 5) [36].

**Figure 5.** Potential mechanisms of C3G&C3G-Ms against apoptosis in intestinal epithelial cells. Intestinal flora disorder can induce the overproduction of pro-apoptotic factors such as ROS to increase mitochondrial permeability and cause the release of SMACs to bind and inactivate IAPs, such as Bcl-2. Since IAPs inhibit the activation of MAC and caspase to inhibit apoptosis, the inactivation of IAPs will induce the release of Cyto C through MAC, and subsequently induce the expression of Apaf-1 and caspase to cause apoptosis. C3G and its metabolites PCA can directly quench ROS and activate IAPs to inhibit the release of Cyto C and expression of caspases. Apaf-1, apoptotic protease activating factor-1; Cyto C, cytochrome C; C3G, cyanidin 3-glucoside; IAPs, inhibitor of apoptosis proteins; MAC, mitochondrial apoptosis-induced channel; PCA, protocatechuic acid; ROS, reactive oxygen species; SMACs, second mitochondria-derived activator of caspases.
