The invasion of primary tumors into distant organs such as the lungs and liver is considered the principal cause of mortality in patients afflicted with colorectal cancer [26]. Unfortunately, a vast majority of patients with colorectal cancer are diagnosed at the advanced stages of the disease, especially when the initial tumors invaded other organs. This phenomenon reduces the survival rate of patients by 10%. Alternatively, all therapeutic options, including chemotherapy, surgery, and radiotherapy, can remarkably affect the quality of life of patients. It has been shown that both radiotherapy and chemotherapy have serious adverse effects on the human and mainly emerged as gastrointestinal toxicity, namely mucositis, enteritis, diarrhea, nausea, and vomiting [27]. Therefore, it seems prophylactic strategies and alternative treatments are needed to prevent the emergence of colorectal cancer.
In the present study, the impact of two probiotic Lactobacillus sp., namely L. acidophilus and L.rhamnosus, on the inhibition of the growth and metastasis of Caco2 cancer cells was determined through analyzing the cell invasion assay in-vitro and the expression of MMP-9 and MMP-12 genes. Studies indicated that the proteolytic activity of the MMP-9 enzyme contributes to the digestion of the extracellular matrix in the colon, facilitating the cell invasion process during metastasis [28–31]. Conversely, evidence showed that the expression of MMP-12, which is a metalloelastase enzyme, has inhibitory effects on the growth and proliferation of colorectal cancer cells, and it is associated with increased survival of patients with colorectal cancer [32].
We showed that LAS and LRS had a significant effect on the viability of Caco2 cells compared with those treated with the MRS medium (control). Also, our results revealed that LAS and LRS containing secreted bioactive compounds significantly reduced the invasion of metastatic colon cancer cells in-vitro (Figure 3), as shown by decreased expression of MMP-9 (Figure 4A). Furthermore, LAS increased the expression levels of MMP-12 (Figure 4B), while LRS had no effect on MMP-12 expression. To the best of our knowledge, the current research is the first study reporting that released bioactive compounds from L. acidophilus and L. rhamnosus can regulate the expression levels of MMP-9 and MMP-12 genes in Caco-2 cells, suggesting their potential role in the inhibition of colon cancer cell invasion.
It is now known that MMPs have detrimental roles in metastasis of colon cancer, promoting the invasion of primary tumors through the digestion of collagen in the ECM [28,29]. It has been shown that the MMP-9 enzyme has proteolytic activity, participating in reconstruction and breakdown of the ECM, a phenomenon observed in the invasion and metastasis of colorectal cancer. The MMP-9 protein is capable of regulating the tumor microenvironment and increasing the levels of vascular endothelial growth factor (VEGF), which is involved in the angiogenesis process [33]. Also, MMP-9 effectively contributes to the formation of early metastatic niches [34]. A number of preclinical analyses demonstrated that the selective inhibition of MMP-9 is able to decrease tumor proliferation and metastasis rates in colorectal cancer. It can also induce programmed cell death in pancreatic cancer cells [35,36]. Escamilla and colleagues showed that cell-free supernatants extracted from probiotic Lactobacillus rhamnosus GG led to a marked reduction in the growth and invasion of HCT-116 cells, thereby diminishing the expression and activity of MMP-9 [20].
On the other hand, a large body of evidence indicates that the inhibition of MMP-12 has deleterious effects on the treatment course of cancer [37,38]. While elevated expression of MMP-12 has been reported in patients diagnosed with CRC, its expression level has been higher in patients with no liver metastasis compared with those with liver metastasis [39]. Besides, the expression of MMP-12 is able to lower the expression rate of VEGF and increase the expression of angiostatin, which is an endogenous inhibitor of the angiogenesis process [40]. Consistent with these statements, a number of investigations demonstrated that the expression of MMP-12 is associated with increased overall survival of patients and reduced tumor growth [41,42]. The degree of MMP-12 expression has been conversely attributed to the metastasis process of primary colon cells [39]. Our findings showed that the effect of LAS on the cell invasion and proliferation was more pronounced than that of LRS. This may be due to the amplifying role of LAS in the expression of MMP-12. In addition to the anti-proliferative, pro-apoptotic, and anti-metastatic effects of lactobacilli [43–45], our results showed that L. acidophilus and L. rhamnosus also have remarkable anti-proliferative and anti-metastatic effects on colorectal cancer cells.
A number of studies demonstrated the relationship between a diet enriched with Lactobacillus and reduced risk of colon cancer [46]. Several studies, in vitro and in vivo, revealed that probiotics modulate cancer cells, like Caco2 cell line, proliferation and apoptosis [47–49]. Furthermore, it is now known that probiotics have various properties, including radio-protective, antioxidant, and antagonistic activity, as well as toxin neutralization. They are able to improve the intestinal microbial environment and immune system response [50]. Thus, they could be used as an alternative therapy instead of using invasive treatments, such as radiotherapy and chemotherapy.