3. Cancer stem cells and cell size
Tumors generally contain multiple clones, in which differentially sized tumor cells can be easily observed. It seems reasonable to speculate, a priori, that a certain population of cells in tumors with certain sizes might be endowed with particular characteristics to promote survival and longevity. In other words, can cell size be used as a determinant of CSCs vs. non-CSCs? Very few studies by far have been conducted to prospectively address this interesting question. A group recently generated a liver-derived progenitor cell (LDPC) line, RA1, by overexpressing the simian virus 40 (SV40) large T antigen (TAg) in primary LDPCs [49]. Interestingly, following transformation, LDPCs decreased in size significantly and the propagating cells measured 1 μm in diameter compared with the 10 μm size of the parental LDPCs. These small cells multiplied continuously and, after passage 36, they started to increase in size and reached a maximum size of 10–12 μm by passage 42. The authors speculated [49] that forced cell cycle entry by TAg might have been the trigger for the “reprogramming” of cells causing a change in their cell size, possibly via the process of ‘de-differentiation’, a feature observed in other stem cells and CSCs. To date, RA1 cells are the smallest mammalian cells to be reported in the literature.
Bortolomai et al. investigated cancer stem/tumor initiating cell characteristics in the human epidermoid carcinoma cell line, A431, via growth as non-adherent spheres in specific media and ALDH enzymatic activity [50]. Spheres manifested increased stem-cell like properties including holoclone formation, high ALDH activity (the ALDH-positive fraction increased from 46% in adherent cultures to 65% in spheres), and a transient induction of stem cell markers such as Nanog, Nestin and Oct4. When compared to parental cells, spheres were greatly enriched in a podoplanin-positive subpopulation characterized by small cell sizes and the ability to propagate tumors in nude mice at a lower cell dose [50].
In contrast, Srivastava et al. interrogated the DAOY medulloblastoma cell line with respect to the relationship between cell size and stem-like potential and observed opposing results [51]. They purified SP/non-SP DAOY cells, which were also sorted separately for viability, cell size, cell cycle status, and proliferative capacity evaluation. The SP, non-SP, CD133+, and CD133− fractions were all capable of reconstituting the original parental DAOY population. However, SP cells, which have been shown to enrich for CSCs in many tumor systems [47], differed from the non-SP cells with respect to increased cell size, decreased S-phase, and slightly decreased proliferative capacity. Another example of stem-like cancer cells with increased cell size is polyploid giant cancer cells (PGCCs) that are frequently found in human solid tumors. These cells are large atypical cancer cells with multiple copies of DNAs and have been recently been studied in human ovarian cancer cell lines and primary ovarian cancer [52]. Of interest, these PGCCs are highly resistant to oxygen deprivation, express normal and CSC markers, divide asymmetrically and cycle slowly, and, surprisingly, can differentiate into adipose, cartilage and bone cells. A single PGCC can form cancer cell spheroids in vitro and generate tumors in immunodeficient mice, which manifest a mesenchymal phenotype with increased expression of CSC markers CD44 and CD133 and become more resistant to treatment with cisplatin [52].
Our laboratory, in the past 10 years, has been meticulously dissecting prostate cancer cell heterogeneity. Using cell surface markers, SP, holoclone and sphere formation, as well as tumor transplantation and serial transplantation assays, we have provided strong evidence for the presence of CSCs in long-term cultured prostate cancer cell lines and xenografts as well as in primary patient tumors [53], [54], [55], [56], [57], [58], [59], [60] and [61]. We have recently made attempts to determine a correlation between CSCs and cell size in the most aggressive, fully undifferentiated prostate cancer cells PC3. PC3 cells completely lack differentiation markers such as androgen receptor (AR) and prostate-specific antigen (PSA). Virtually 100% of PC3 cells express commonly used CSC surface markers such as CD44 and integrin α2β1; consequently, these markers would not differentiate between tumorigenic CSCs vs. non-CSCs. We have shown that PC3 holoclones harbor long-term self-renewing tumor-propagating cells [57].
To address whether cell size is able to provide tumorigenic stratification in PC3 cells, we first utilized FSC-based FACS sorting (Figure 1A) to fractionate PC3 into, relatively, large and small sized populations and then implanted 100 and 1,000 cells, respectively, subcutaneously, in NOD/SCID mice. This experiment revealed a tendency of small cells being more tumorigenic manifested by more and larger tumors regenerated (Figure 2A). We then employed size-sieving approach by using nylon mesh of different pore sizes (Figure 1B) to separate PC3 cells into two cell populations varying in the cell sizes, i.e., small (< 10 μm) and large (≧ 20 or 30 μm) (Figure 2B-E; Figure 3A-B) followed by clonal (i.e., 2D) and clonogenic (i.e., 3D) assays as well as in vivo tumor regeneration. In two independent experiments, small PC3 cells demonstrated higher clonal capacity than large PC3 cells (Figure 2C; Figure 3B). Importantly, two separate tumor experiments again revealed the trend of small PC3 cells being more tumorigenic (Figure 2D-E). Similar studies in another AR-/PSA- prostate cancer cell line IGR also revealed that small IGR cells displayed higher clonal (Figure 3C) and clonogenic (Figure 3D-E) capacities than corresponding large cells.