miR-221 and miR-222 are two oncomiRs that are consistently up-regulated in breast cancer tissues (24). Activation of β-catenin by miR-221 and miR-222 promotes estrogen-independent growth of cancer cells, whereas Wnt-inhibitory factor-1 (WIF1) is suppressed by these miRNAs (25). WIF1 acts as a tumor suppressor gene in Wnt/β-catenin signaling pathway (26). Curcumin inhibits breast cancer cell proliferation by altering expression of signaling proteins, including Ras, phosphatidylinositol-3-kinase (PI3K), protein kinase B (Akt), mammalian target of rapamycin (mTOR) and Wnt/β-catenin pathway (15). Therefore, targeting oncogenes and tumor suppressor genes is one of the most effective methods in the treatment of cancers. The present study investigated the possible anti-cancer effects of nano curcumin (CC-CUR) against different types of breast cancer cell lines by analyzing the expression level of miR-221, miR-222 and β-catenin as oncogene and WIF1 as a tumor suppressor gene. The results of cell viability assay which showed more toxicity of CC-CUR nanocomposite at lower concentrations than CUR reveals that in lower doses, CC-CUR has higher toxicity than CUR on breast cancer cells and to achieve more toxicity, higher doses of parent curcumin is needed which can be explained by the fact that the nanocurcumin is more soluble in addition to higher cellular uptake.
MCF-7 cell line is a Luminal A subtype expressing estrogen receptor (ER) and progesterone receptor (PR). This cell line is considered non-invasive and non-tumorigenic in vivo unless supplemented with estrogen, meaning they rely on estrogens for growth (27). MDA-MB-231 is a basal type hormone-independent TNBC that is thought to be more aggressive and invasive than other breast cancer subtypes (28) .Also, SKBR3 cells represent basal-like BC that are estrogen-independent (her2+, ER/PR−) (29).
The results of previous studies showed that the expression of miR-221 and miR-222 was increased in breast cancer tissues compared with normal breast tissues (10, 30). In addition, the level of miR-221 and miR-222 were associated with the advanced clinical stage and different tumor types (31). miR-221 and miR-222 represses multiple negative regulators of the Wnt/β-catenin signaling pathway, including WIF1, SFRP2, DKK2, and AXIN2, to activate the pathway. Notably, the patient survival is inversely correlated with miR-221 and miR-222 expression level whereas it is positively correlated with that of WIF1, DKK2, SFRP2, and AXIN2 genes (13).
Our results indicated that not only the level of miR-221 and miR-222 genes are different in different types of intact breast cancers, but also the response rate of these genes to treatment is different. In our study the expression level of miR-221 and miR-222 reached to the lowest level in MCF7 cells compared to MDA-MB-231 and SKBR3 cells after treating with curcumin. Therefore, we can conclude that the effects of curcumin on the expression of miR-221 and miR-222 is more prominent in hormone dependent cancerous cells and it may lead to the hypothesis that these cells are more sensitive with better responding to certain treatments.
Many researches indicated that curcumin could acts as an anti-cancer agent in breast cancer cell line due to the regulation of different pathways such as miR-221 and miR-222 and Wnt/β-catenin (32, 33). In this study we observed different effects of curcumin on different cell lines. Evaluating the effects of parent curcumin (CUR) and its nanocomposite form (CC-CUR) on three different cells lines showed that CC-CUR down-regulated the expression of miR-221 and miR-222 in MCF-7 cells, although its effect on the Wnt/β-catenin pathway was more prominent. CUR also modulated these pathways but the effect of the CC-CUR was more significant.
Liu et al, reported that Wnt/β-catenin signaling is hyperactivated in TNBC, promoting the invasive potential of TNBC. Inhibiting miR-221/222 expression in a TNBC cell line (MDA-MB-231) suppressed its proliferation, viability, epithelial-to-mesenchymal transition, and migration. Whereas expression of miR-221/222 in a non-TNBC line (MCF-7) promotes all the aforementioned cancer characteristics (13). However, the effects of our treatment on Wnt/β-catenin pathway on both MDA-MB-231 and MCF-7 cells was similar and CC-CUR effect was significantly better than CUR. While the expression of miR-221 and miR-222 was not satisfactory in MDA-MB-231 cells, especially in the CUR group, as the level of these genes increased compared to other groups. The CC-CUR also could not decrease the expression level of miR genes in MDA-MB-231 cells and its level was equal to the control group.
The results in SKBR3 cells were quite unexpected as the expression level of miR genes in the CC-CUR group increased considerably. Treatment of SKBR3 cells with CC-CUR significantly increased the level of WIF1 gene expression without decreasing the level of β-catenin. On the other hand, CUR did not change the level of miR genes compared to the control group, while it significantly decreased β-catenin expression. Kim et al claimed that the expression levels of miR-221 and miR-222 were associated with the aggressive characteristics of breast cancer subtypes and their expression level vary in breast cancers according to tumor characteristics (34). Generally, the gene expression values in our study indicated that curcumin in its parent or composite forms affects cancerous cell death through the Wnt/β-catenin pathway and the significant point is that this effect is not basically by modulating the expression of miR-221 and miR-222 genes and shows the effect of other paths involved in this process.
WIF1 has been shown to be down-regulated in various human cancers, including breast cancer, and has been regarded as a tumor suppressor gene (26). Rubin et al. demonstrated that loss of WIF1 could trigger Wnt/β-catenin signaling and thereby contributes to tumor invasion and metastasis (35). Therefore, up-regulation of this gene can be a promising strategy in controlling tumor proliferation and metastasis and our results indicated that in all three cell lines the level of WIF1 significantly increased after using CC-CUR.
Previous studies demonstrated that up-regulated miR-221/222 may simultaneously suppress multiple inhibitors of Wnt/β-catenin signaling pathway such as WIF1, supporting the idea that miRNA represents a potent activator of the pathway (13). While our results indicating the importance of other genes in activation of Wnt/β-catenin pathways. Therefore, it remains to be clarified whether miR-221 and miR-222 regulate other signaling pathways to promote oncogenesis or other genes regulating Wnt/β-catenin pathway.