The anti-cancer initiation action of CLA can be attributed to the DNA damage leading to mutagenesis. The results from testing the chemoprotective effects on animal cell lines in the bladder (TSU-Pr1) showed that CLA decreased DNA synthesis and induced apoptosis in TSU-Pr1 cells. CLA was considered to act as an inhibitor of DNA synthesis [81]. Some studies originally believed that the anti-carcinogenic effects of CLA were due to its antioxidant activity. They reported the antioxidant activity of CLA by tracking the inhibition of CLA on the chelation of iron by β-hydroxy derivatives (Fenton reaction - catalysis of hydrogen peroxide) [14,15,82]. In a more recent study, Yu et al. demonstrated the protective effect of CLA against free scavenging radicals [83]. Another hypothesis suggested that the anti-carcinogenic activity of CLA was due to its antioxidant properties, promoting the in situ defense mechanism against free radical attacks on the cell membrane [84]. The supplementation of β-carotene in combination with CLA may be cytotoxic to human malignant melanoma and breast cancer cells in vitro [85]. CLA performed in vitro antioxidant activity by protecting the cells from peroxidation [83,85]. These results did not give any strong evidence and therefore it has not been conclusively established whether CLA works as an antioxidant. Further study is needed to resolve the relationship between the anti-cancer effects and antioxidant activity of CLA.

The anti-promotional activity of CLA against cancer has been evaluated in several studies. The results suggested few potential mechanisms between CLA and animal/cell studies are likely to be involved: alternation of eicosanoid metabolism (COX-2, 5-LOX), induction of apoptosis (bcl-2), cell proliferation (c-myc), effects of PPAR. Belury et al. suggested that CLA affects the composition of the cell membrane, which, in turn alters eicosanoid synthesis [86]. A few studies also suggested that CLA acts as a preventive agent on eicosanoid metabolism which has a focus on tumor induction [87]. The reduction of eicosanoid was also monitored by the inhibition of the glycoprotein enzyme COX, responsible for the synthesis of prostaglandins and thromboxanes from arachidonic acid. Urquhart et al. investigated the role of CLA in the regulation of eicosanoid synthesis. They used selective inhibitor (indomethacin) for COX-1 and incubated it on human saphenous vein endothelial cells (HSVEC) with CLA (50:50, mixture of c9, t11 and t10, c12-CLA). The addition of CLA to HSVEC cells resulted in inhibition of eicosanoid production, suggesting that the cancer-preventive role of CLA in carcinogenesis was mediated through the inhibition of COX-1. The CLA mixture proved its beneficial anti-inflammatory effects that contributed to its anti-carcinogenic properties [87].

Previous studies in this report have documented the anti-proliferative properties of CLA. Tumorigenesis can be reduced by inhibiting cell growth or by increasing apoptosis. CLA has been demonstrated to stimulate the accumulation of proteins such as p27, p53 and p21, which have an additional role in tumor suppression [29,88,89]. Previously, CLA was presented to induce several tumors present in mammary, breast, colon and prostate tissues. The biochemical and molecular mechanisms to describe its anti-tumor effects have been suggested, but at the cellular level, exclusion of cancerous cells was manifested through a process of programmed cell death, called apoptosis [23,58,90,91]. Apoptosis depends on the activity of the Bcl-2 regulator proteins which are believed to work as an apoptosis “suppressor gene” [92]. In mitochondria, the release of cytochrome c to the cytosol is partially controlled by members of the Bcl-2 family. They inhibit the onset of apoptosis by blockage of the protein-conducting channels. They select and maintain the long-living cells in the G0 phase of the cell cycle [93]. Previously, we reported that Bcl-2 expression was decreased in various tumor lesions of animals fed with CLA [47,90]. It was showed that CLA inhibited the proliferation of colon and prostate cancer cells through the induction of apoptosis, attributing to its ability to down-regulate ErbB3 signaling and PI3/Akt pathway [47,48]. Furthermore, another study demonstrated that CLA targeted Bcl-2 by triggering apoptosis of p53 mutant mammary tumor cells [94]. As follows, the anti-proliferative effects of CLA appeared to be linked to the induction of apoptosis. In particular, we proposed a schematic model of the mechanism on CLA against carcinogenesis. It has been previously reported that CLA induced lipid metabolism and activated a group of nuclear transcription factors (PPARn). We predicted a model of peroxisome proliferation induced by CLA which resulted in apoptosis. The model was inspired by CLA inhibition in hepatic tumor cell lines (Figure 2). More details on the effects of each individual molecule can be found in the experimental studies [65,66,67,68].