PCSC Regulation by miRNAs
Tumorigenesis resembles abnormal embryogenesis. Like developing organs, tumors comprise a spectrum of phenotypically distinct cells and tumor cell heterogeneity may underlie fundamental differences in tumor responses to clinical therapeutics. Recent evidence suggests that, like developing organs that contain stem cells that can both self-renew and differentiate, tumors may harbor stem cell-like cells with enhanced renewal and tumor-initiating capacities. Although the CSC concept is still debated and the true molecular nature and significance of putative CSCs remain to be fully elucidated, it is undeniable that malignant tumors are immortal at the population level. One of the key unanswered biological questions of therapeutic implication is how cancer cells in general and CSCs in particular are regulated at the molecular level with respect to their self-perpetuating (immortal) tumorigenic potential, i.e., self-renewal properties.
Remarkably, malignant cells share a similar gene expression profile with embryonic SC (ESCs). ESCs express a group of core regulators of self-renewal and pluripotency including Nanog (also called Nanog1), a homeodomain transcription factor located on chromosome 12. Our lab has recently provided convincing evidence that NanogP8, a retrotransposed Nanog homolog localized on chromosome 15, is ‘reactivated’ in somatic cancer cells and plays a critically important role in tumor development (Jeter et al, 2009).
Specifically, our results have revealed that:
- PCa cells preferentially express the mRNA of a retrotransposed Nanog pseudogene called NanogP8. The NanogP8 protein is identical to Nanog protein except for one amino acid difference;
- Nanog-positive cells are increased in Gleason 6-8 prostate tumors compared to the matched benign tissues and, NanogP8 mRNA and Nanog protein are enriched in the CD44+ and SP cells;
- Nanog knockdown inhibits tumor development of prostate, breast, and colon cancer cells; and
- the tumor-inhibitory effects of Nanog knockdown are associated with inhibition of clonogenicity and self-renewal of tumor cells. The results suggest that Nanog functions as a positive regulator of tumorigenic PCa cells (Jeter et al, 2009). Emerging data from others begin to corroborate our findings.
Do NanogP8-expressing (prostate) cancer cells possess CSC properties and is NanogP8 overexpression SUFFICIENT to promote tumor development? We recently addressed these two critical questions by establishing the NanogP8-GFP (NP8-GFP) promoter tracking and doxycycline (dox) inducible NanogP8-expressing systems. Our results (Jeter et al, 2011) have demonstrated that:
- NP8-GFP+ PCa cells exhibit CSC characteristics such as enhanced clonal growth and tumor regenerative capacity.
- Nanog1-GFP+ PCa cells are undetectable, consistent with the Nanog1 mRNA generally not expressed in human somatic cancer cells.
- Dox-inducible NanogP8 or Nanog1 overexpression has been successfully established in LNCaP, Du145, and MCF7 cells and the induced Nanog protein localizes to the nucleus and binds to known Nanog1 targets such as c-Myc, Oct-3/4, E-cadherin, FGF-4, Gli-1, and HoxC13.
- Nanog induction promotes drug-resistance in MCF-7 cells and survival and sphere formation of LNCaP cells in androgen-deficient conditions.
- Most important, Nanog (especially NanogP8) induction promotes tumor regeneration in Du145 cells, and, remarkably, castration-resistant tumor development in LNCaP cells.
- These pro-tumorigenic effects of NanogP8 are associated with key molecular changes, including an upregulation of molecules such as CXCR4, IGFBP5, CD133 and ALDH1.
- Comparisons of Nanog1 versus NanogP8 mechanisms of action have revealed both shared and, significantly, distinct functions of the two molecules.
Intriguingly, we have found that transgenic NanogP8 expression in the epidermal compartment significantly affects animal development and surprisingly inhibits tumor development in a chemical carcinogenesis model. Another focus is to elucidate the epigenetic and biochemical mechanisms of action for NanogP8-regulated tumorigenesis. Along this latter line, we are also exploring the global epigenetic landscape of PCSCs.