Drug Resistance Mediated by Cellular Plasticity
While chemotherapy is the mainstay in our battle against cancer, either intrinsic or extrinsic reasons leads to the cancer cells acquiring the ability of evading the effect of drugs54. One of the main factors for drug resistance or therapy resistance is cellular plasticity. Different pathways as discussed above have been shown to be involved in the phenomenon of cellular plasticity leading resistance towards drugs. An example is up regulation of the p44/MAPK pathway by cytokine TGF-β1, released from the chemo resistant cells and which induces resistance in breast cancer cells. A recent study has linked CD24, a CSC maker to phenotypic switching and thus induction of cellular plasticity to the MAPK pathway. The cellular localization of CD24 is mediated by p38 MAPK pathway activation with phosphorylation leading to Bcl over-expression resulting in the localization of a high-density of CD24 molecules in the cell membrane55 and treatment resistance. Non genomic factors like Estrogen have been shown to affect mediating cellular plasticity in the tumor cells that have a propensity towards undergoing EMT. The resistance is mediated by c-Src activation which in turn mediates activation of cell signaling pathways like MAPK56-. Targeting these factors can yield a therapeutic intervention point to check the treatment resistance mediated by cellular plasticity.
R̥adio-therapy is the use of radiation to curb growth and proliferation of the cancer
cells. Effectiveness of radiotherapy is curtailed by adaptive radioresistance or the
acquisition of resistance by the cells over a time period. In radioresistant breast cancer cell
line the major reason for driving radioresistance was increase in cancer stem cells
(CSCs) alongside elevated EpCAM expression. It was observed that elevated EpCAM
expression conferred resistance towards radiotherapy in breast cancer cell line ZR-75-1 by
enhancing Akt expression which in turn increases the stemness of the cells. Thus, this
pathway act as a driving factor for EpCAM mediated cellular plasticity in the breast cancer
cell line 57.
A commonly upregulated factor CDLYL2 in breast cancer is observed to mediate the cell plasticity phenomenon by regulating miRNA-124 which in turn impacts both STAT3 and NF-κβ pathways58. The over-expression of the CDYL2 lead to changes at morphological and molecular level in the breast cancer cells. Another study by Arnold et al. found radiation to be responsible for conferring radiation-mediated cellular plasticity in breast cancer stem-like cells alongside induction of radioresistance in those cells59. The findings of these researchers confirmed that conferring resistance and also changes in cellular plasticity in the TNBC cell line were mediated by STAT3 via an inflammatory response. The inflammatory response that triggers the cellular pathway is mediated by IL6. This interleukin binds to its own receptors which lead to the activation of Janus family kinase which in phosphorylates and triggers STAT3 pathway. Resistance towards the drug Lapatinib in locally advanced HER2+ breast cancer is mainly mediated by IL-8 that is secreted from tumor macrophages via activation of Src/STAT3/ERK1/2 pathway which in turn mediated EGFR signalling60. This pathway is observed to confer radio-resistance to breast cancer cell lines can be attributed to the activation of signal transduction by activation of the STAT3 pathway by phosphorylation of Tyr705 residue of the signal transducer61. Thus, targeting STAT3 will probably yield inhibition of the phenomenon of drug resistance in breast cancer.
Not only does a hyperactivated Wnt pathway and the resultant dysregulation of the proteolytic degradation of beta-catenin lead to tumorigenesis, but it is also strongly linked to the development of drug resistance seen in many invasive cancers. Conventional chemotherapy affects cancer cells by exposing them to cytotoxic agents that inhibit cell proliferation and induce cell death in rapidly-proliferating cells while radiotherapy involves the utilization of ionizing radiation to tumor cells, damaging their DNA and causing cell death as a consequence. CSCs are generally known to be more resistant to chemo and radiation therapies as compared to the non-stem cell populations in the tumor, allowing them to induce a cancer relapse even after such treatments. Given that the Wnt signaling pathway plays an important role in maintaining cancer stem cells, it could be seen as a major mechanism underlying the development of therapeutic resistance in many cancer types. For example, in a study using carboplatin-resistant in-vivo patient-derived xenografts as well as isogenic triple-negative breast cancer (TNBC) cell-line models, it was found that there was an increased activation of the Wnt/beta-catenin pathway which was correlated with the expression of stem cell markers in both the models studied. Moreover, this study also proved that the inhibition of the Wnt signaling pathway in these models resulted in their resensitization to carboplatin from their resistant states – thus highlighting a potential mechanism for overcoming chemotherapeutic resistance, especially in breast cancer62. Similarly, another study found that a protein called ST8SIA1 and its corresponding mRNA was upregulated in chemotherapy-resistant TNBC patients and that the inhibition of the ST8SIA1 mRNA led to the increase in the efficacy of chemotherapy through the suppression of the Wnt/beta-catenin pathway – thus linking the signaling pathway to the development of chemotherapeutic resistance, particularly in breast cancer63. In addition, it has also been found that chemotherapy and radiation can upregulate the Wnt/beta-catenin pathway even in non-cancer stem cell populations - thus upregulating the genes that protect the cells from cell cycle arrests and apoptosis as well as enhancing DNA damage repair which reverses the effects of chemotherapeutic drugs64.  Immune evasion by ER+ breast cancer cells has also been shown to be mediated by the Wnt signaling pathway. For example, a recent study used microarray-based gene expression studies to identify the overexpression of a microRNA – miR-18a – involved in the suppression of immune-related pathways in ER+ breast cancer cell lines. The function of this particular miRNA was also found to be linked to the Wnt pathway as the inhibition of the signaling pathway in miR-18a over-expressed cells resulted in the restoration of antigen presentation properties65. Additionally, another study showed that the activation of the Wnt/bata-catenin pathway led to the absence of a T-cell gene expression signature which thereby caused a lack of T-cell infiltration in human metastatic melanoma samples. Furthermore, the Wnt pathway is also shown to have a role in the suppression of tumor cell-intrinsic cytokines which inhibits the recruitment of CD103+ dendritic cells and the activation of the CD8+ T-cells64.
The Hh connection to cancer was first unearthed when a mutation in the Ptc gene was linked to BCC, also known as Gorlin’s syndrome. Following this discovery, the Ptc mutation has been found in other Gorlin’s syndrome-related tumors such as meningioma, medulloblastoma and even other cancers like breast cancer and transitional cell carcinoma. From a functional perspective, the mutation of the Ptc gene leads to the inactivation of the normal Ptc protein and causes the hyperactivation of the Hh pathway due to the enhanced function of Smo, thereby leading to the overexpression of the Hh target genes. Additional mutations of the Smo gene as well as the overexpression of the hh gene have also been identified in vivo and in vitro models.
One of the initial theories to explain the phenomenon of drug resistance in breast cancer cites breast cancer stem cells (BCSCs), which are thought to be inherently resistant to cytotoxic and targeted therapies. Recent research has been able to find new evidence that links Notch1 and Notch4 in the development of drug-resistant behavior exhibited by BCSCs. A study using a mammary tumor mouse model also showed that cells showed a temporal increase in expression of the Notch ligand, DII1, as the tumor progressed from early to late stages. Moreover, these DII1+ cells also exhibit chemotherapeutic resistant gene signatures and the consequent blocking of the ligand sensitized the cells to chemotherapy as well as stopping tumor growth and metastasis66. Moreover, the small interfering RNA (siRNA)- mediated knockdown of Notch1 in ALDH+ cells, resulted in an inhibition of their growth and increased apoptosis and tumor growth in mice models67.
The molecular mechanisms that contribute to the phenomenon of resistance remain to be elucidated completely. Yet research on the same has paved the way for targeting various crucial factors that have been seen to play a role in conferring cellular plasticity and therapy resistance in breast cancer cells. These factors can thus act as potential biomarkers for targeted therapy and new treatment modalities can be developed on the same.
(Figure 1): Cell intrinsic pathways involved in cancer cell plasticity.