As one of the most ordinary malignant tumors, colon cancer does pose an extreme threat to human health . Due to the high incidence and mortality of colon cancer, researchers who have devoted themselves to reducing mortality and improving the quality of life for patients have been seeking new therapeutic methods to inhibit the growth, recurrence, and metastasis of colon cancer . Previously, researchers and our team all have found that Plasmodium infection had antitumor effects on some cancer in some mouse models, but it is not clear whether it has similar influences on colon cancer yet. Therefore, in this study, we investigated the anti-colon cancer effect of Plasmodium infection in mice.
In this study, we have found that Plasmodium infection decelerated the growth of tumors and reduced the size and weight of the tumors in the murine colon cancer model. These results indicated that Plasmodium infection could play an anti-colon cancer role in mice.
Infinite proliferation is one of the important characteristics of tumor cells . Inhibition of cell proliferation could effectively inhibit tumor growth . Ki67 protein, a nuclear antigen related to cell proliferation, is expressed in G1, G2, S, and M period, but no expression in the G0 period . As one of the most reliable indicators for detecting cell proliferation activity of tumor cells, the function of Ki67 protein is closely associated with the process of cell mitosis [37, 38]. In this study, the expression of Ki67 in colon cancer tissues was distinctly decreased after parasite treatment, suggesting that Plasmodium infection could suppress the proliferation of colon cancer cells and further inhibit tumor growth in tumor-bearing mice.
Apoptosis, the most common form of programmed cell death, is one of the keys to maintaining healthy cell homeostasis . Apoptosis resistance is another crucial characteristic of tumor cells . As an important method for prevention and treatment, induction of apoptosis is widely used in antitumor drug research . There are three pathways of apoptosis in mammalian cells, containing the Bcl-2 regulatory mitochondrial pathway, the death receptor pathway, and the endoplasmic reticulum pathway [42, 43, 44]. Mitochondrial apoptosis is the primary form of apoptosis . When mitochondria are damaged, Bax is transported into mitochondria to initiate apoptosis, triggering Cytochrome C releasing in the mitochondria, which further activates the Caspase cascade resulted in apoptosis [46, 47, 48].
In the tumor-being mice infected with P. yoelii, we found the increased proportion of TUNEL-positive cells, the up-regulated expression of pro-apoptotic factors Bax, Caspase-9, and Cleaved Caspase-3, and the down-regulated expression of anti-apoptotic factor Bcl-2 in colon cancer tissues, which indicated Plasmodium infection promoted mitochondria-mediated apoptosis. Since inadequate apoptosis is one of the typical characteristics of cancer, promoting apoptosis will lead to the death of cancer cell. Thus, we have reason to believe that Plasmodium infection promoting mitochondria-mediated apoptosis could inhibit tumor growth through promoting mitochondria-mediated apoptosis and play an anti-colon cancer role in tumor-bearing mice like some anti-tumor drugs [49, 50].
Mitochondrion, a subcellular organelle, which plays a major role in cell energy control and metabolism, is essential for the survival and growth of cells [51, 52]. As regulators involved in the pathways of mitochondrial biogenesis, mitophagy, carcinogenesis, and tumor cell death, including mitochondria-mediated apoptosis, mitochondria may be considered as potential therapeutic targets for cancer . In this study, we found that the mitochondria and nucleus of colon cancer cells were damaged severely in the Plasmodium-treated mice, mainly with the mitochondrial cristae disappearance and vacuolation. The results implicated that the antitumor effect of Plasmodium infection may relate to mitochondria. The maintenance of mitochondrial homeostasis depends on the interaction between mitochondrial biogenesis and mitophagy . PGC-1α protein dominated mitochondrial biogenesis and protected tumor cells from apoptosis [55, 56]. Those studies both in vivo and in vitro certified that down-regulated expression of PGC-1α induced apoptosis through the mitochondrial pathway . Recent study has revealed a potential relationship between mitochondrial biogenesis and apoptosis . In the early stage of apoptosis, mitochondria produced energy to sustain homeostasis, leading to enhanced mitochondrial biogenesis . As the massive production of reactive oxygen species (ROS), mitochondrial energy metabolism was unbalanced, mitochondrial biogenesis was weakened, mitochondrial membrane potential was decreased, Cytochrome C was released, and the Caspase pathway was activated, leading to apoptosis [60, 61]. In this study, we found that the expression of PGC-1α protein in colon cancer cells was reduced after P. yoelii infection. The results suggested that Plasmodium infection could inhibit mitochondrial biogenesis in colon cancer cells. Inhibition of mitochondrial biogenesis could promote apoptosis and inhibit proliferation, which may be one of the mechanisms underlying the antitumor effects of Plasmodium infection.
Autophagy is a process in which intracellular components are degraded into autophagosomes and combined with the lysosomes to form autolysosomes, resulting in the degradation of their encapsulated contents [62, 63]. More and more studies have demonstrated that autophagy is a self-protective mechanism for cells [64, 65]. Mitophagy, a type of mitochondria-specific autophagy, is a self-protective process that dysfunctional mitochondria are degraded selectively . When mitochondria are damaged, PINK1 protein hydrolysis is restrained and expressed steadily on the mitochondrial outer membrane. Then, PINK1 recruits Parkin to the outer membrane of mitochondria and ubiquitinates multiple mitochondrial outer membrane proteins to mediate mitophagy, eventually removing damaged mitochondria [67, 68]. In the study, we found that the number of autolysosomes was decreased and the expression of PINK1/Parkin proteins were declined in colon cancer after Plasmodium infection. The results meant that Plasmodium infection could inhibit mitophagy in colon cancer-bearing mice leading to disruption of mitochondria and mitochondrial dysfunction. Many studies have shown that suppression of autophagy, including mitophagy, could accelerate apoptosis resulting in tumor cell death, ultimately inhibiting tumor growth [69, 70]. Therefore, in colon cancer cells, Plasmodium infection could inhibit mitophagy, thus promoting apoptosis and ultimately inhibiting tumor growth, which might be one of the anti-colon cancer mechanisms of Plasmodium infection.
In this study, although we found that Plasmodium infection might inhibit proliferation and promote apoptosis by controlling mitochondrial biogenesis and mitophagy, it is not clear why Plasmodium infection could inhibit mitochondrial biogenesis and mitophagy. This suppression function may be connected with the subsequent effect of cytokines induced by immune effects after Plasmodium infection, the impact of components or metabolites of Plasmodium itself, or the result of changes in non-coding RNA expression caused by Plasmodium infection. The mechanism of Plasmodium infection controlling mitochondrial biogenesis and mitophagy remains to be further explored.