Abstract
In this Commentary, the operational definition of cancer stem cells or cancer initiating cells includes the ability of certain cells, found in a heterogeneous mixture of cells within a tumor, which are able to sustain growth of that tumor. However, that concept of cancer stem cells does not resolve the age-old controversy of two opposing hypotheses of the origin of the cancer, namely the stem cell hypothesis versus the de-differentiation or re-programming hypothesis. Moreover, this cancer stem concept has to take into account classic experimental observations, techniques, and concepts, such as the multi-stage, multi-mechanism process of carcinogenesis; roles of mutagenic, cytotoxic and epigenetic mechanisms; the important differences between errors of DNA repair and errors of DNA replication in forming mutations; biomarkers of known characteristics of normal adult organ-specific stem cells and of cancer stem cells; and the characteristics of epigenetic mechanisms involved in the carcinogenic process. In addition, vague and misleading terms, such as carcinogens, immortal and normal cells have to be clarified in the context of current scientific facts. The ultimate integration of all of these historic factors to provide a current understanding of the origin and characteristics of a cancer stem cell, which is required for a rational strategy for prevention and therapy for cancer, does not follow a linear path. Lastly, it will be speculated that there exists evidence of two distinct types of cancer stem cells, one that has its origin in an organ-specific adult stem cell that is ‘initiated’ in the stem cell stage, expressing the Oct4A gene and not expressing any connexin gene or having functional gap junctional intercellular communication (GJIC). The other cancer stem cell is derived from a stem cell that is initiated early after the Oct4A gene is suppressed and the connexin gene is expressed, which starts early differentiation, but it is blocked from terminal differentiation.
Highlights
How Some Historic Experimental Findings and Hypotheses of Cancer Shaped Today’s Concept of Cancer Stem Cells. These three introductory quotes embody much of my concern in this Commentary as it concerns the concept of cancer stem cells
Evolution selected for the stem cells to be able to be maintained in a low oxygen micro-environment, have various anti-oxidant systems to protect against free radicals, and to have both a nuclear membrane to act as another barrier to free radical production of the mitochondria of its neighboring differentiated daughters [57,58] and DNA repair enzymes [59]
In this Commentary, an examination of the origin of the cancer stem cell concept was developed by viewing historical experimental observations, techniques and concepts that have led to the operational concept that these cancer stem cells are those responsible for sustaining the growth of any tumor
Summary
How Some Historic Experimental Findings and Hypotheses of Cancer Shaped Today’s Concept of Cancer Stem Cells. The new concept emerged suggesting that carcinogenesis was a multistep, multi-mechanism process, consisting of an initiation of a single “normal” cell to become “immortal”, followed by a promotion event over a long period of regular exposures at a threshold level to clonally amplify this single “initiated” cell into a benign tumor, which transitioned to become an invasive and metastatic cell by the progression process Another important concept that added to this classic understanding of this initiation/promotion/progression process was that, even though all the cells in the tumor appeared to be heterogeneous in terms of their genotypes and phenotypes, they were derived from a single common “normal” ancestor [19,20]. Potter conceptualized the stem cell hypothesis as: “Oncogeny as partially-blocked ontogeny” [1]
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