Proposed Models of Functional Heterogeneity in Cancer and the Effects of Microenvironmental Factors on Cancer Stem Cells: A Literature Review

Introduction: Functional heterogeneity in cancer may result in the metastasis of various types of tumour cells throughout the body. Attempting to explain functional heterogeneity in cancer cells has given rise to two models. The Cancer Stem Cell model proposes that a subset of tumour cells self-replicate and that heterogeneity is a progeny of various cancer stem cells (CSCs). The Clonal Evolution Model proposes heterogeneity as a product of mutations across tumour cells that accumulate and metastasize linearly or branching.  Methods:  Research was conducted through open-access journals and information was compiled surrounding CSC models using the Google Scholar and McMaster Library database search engines. Inclusions were sources that detailed the relationship between both models of functional heterogeneity and microenvironments and treatments. Literature that did not center around tumour microenvironments was not included in this literature review. Results: The two main models of tumour proliferation were explored and related to hypoxic tumour microenvironments. Various markers, etiologic agents and toxins were identified that contribute to tumour progression. Cell signalling and pathways that contribute to major cellular functions were identified, along with possible disruptions and epigenetic changes that lead to tumour and CSC proliferation. Discussion: This study reveals that the tumour microenvironment plays a large role in the proliferation of CSCs.  Although the therapies targeting microenvironments are in early stages of development, focusing on these CSC targeted- therapies may lead to better treatments for cancer or more effective combination therapies. Strengths of the paper include the compilation of major contributing areas to CSC proliferation, whereas limitations encompass the high variability of tumour cells that are not all covered in this review. Conclusion: While no definitively eradicating treatment for CSCs currently exist, the recent developments in cancer research indicate promising new techniques for its management.  Implications: By further studying malignant CSCs, highly effective cancer treatments may result, leading to the advancement of CSC recognition and combination therapy.