Cancer is a devastating disease that has claimed many lives. Natural bioactive agents from plants are gaining wide attention for their anticancer activities. Several studies have found that natural plant-based bioactive compounds can enhance the efficacy of chemotherapy, and in some cases ameliorate some of the side-effects of drugs used as chemotherapeutic agents. In this paper, we have reviewed the literature on the anticancer effects of four plant-based bioactive compounds namely, curcumin, myricetin, geraniin and tocotrienols (T3) to provide an overview on some of the key findings that are related to this effect. The molecular mechanisms through which the active compounds may exert their anticancer properties in cell and animal-based studies also discussed.

Gamma‐tocotrienol (γT3) is an analogue of vitamin E with beneficial effects on the immune system, including immune‐modulatory properties. This study reports the immune‐modulatory effects of daily supplementation of γT3 on host T helper (Th) and T regulatory cell (Treg) populations in a syngeneic mouse model of breast cancer. Female BALB/c mice were fed with either γT3 or vehicle (soy oil) for 2 weeks via oral gavage before they were inoculated with syngeneic 4T1 mouse mammary cancer cells (4T1 cells). Supplementation continued until the mice were euthanized. Mice (n = 6) were euthanized at specified time‐points for various analysis (blood leucocyte, cytokine production and immunohistochemistry). Tumour volume was measured once every 7 days. Gene expression studies were carried out on tumour‐specific T lymphocytes isolated from splenic cultures. Supplementation with γT3 increased CD4+ (p < 0.05), CD8+ (p < 0.05) T‐cells and natural killer cells (p < 0.05) but suppressed Treg cells (p < 0.05) in peripheral blood when compared to animals fed with the vehicle. Higher interferon (IFN)‐γ and lower transforming growth factor (TGF)‐ꞵ levels were noted in the γT3 fed mice. Immunohistochemistry findings revealed higher infiltration of CD4+ cells, increased expression of interleukin‐12 receptor‐beta‐2 (IL‐12ꞵ2R), interleukin (IL)‐24 and reduced expression of cells that express the forkhead box P3 (FoxP3) in tumours from the γT3‐fed animals. Gene expression studies showed the down‐regulation of seven prominent genes in splenic CD4+ T cells isolated from γT3‐fed mice. Supplementation with γT3 from palm oil‐induced T cell‐dependent cell‐mediated immune responses and suppressed T cells in the tumour microenvironment in a syngeneic mouse model of breast cancer.

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