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When initially proposed, the concept that cancer cells that had undergone epithelial to mesenchymal transition (EMT) also gained stem cell-like qualities was controversial. It seemed counter-intuitive that a cancer cell with the properties of a solid tissue stem cell, which normally resides in a highly specialized niche, would be able to migrate and maintain its stemness. However, Thomas Brabletz and colleagues had noted that cells at the leading edge of an invasive tumour seemed to exhibit these properties and named them migrating tumour stem cells. The molecular details that support this hypothesis are now being unearthed.

Genes that regulate EMT, such as TWIST, SNAI1 and ZEB1, and microRNAs that regulate stem cell characteristics, such as the mir-200c family, can be misregulated in tumour cells. The authors had previously shown that ZEB1 can suppress the expression of miR-200 family members, and others have shown that miR-200 family members can suppress the expression of ZEB1. So how does this feedback loop affect the properties of cancer cells?

ZEB1 is expressed in poorly differentiated colon and pancreatic tumours, and knockdown of ZEB1 using short hairpin RNAs (shRNAs) resulted in reduced tumorigenicity of pancreatic cancer cell line xenografts in orthotopic mouse models. Not only was the incidence of metastases reduced in these models, but the growth of the primary tumour was also reduced. Therefore, the effects of ZEB1 are not confined to migration and invasion as one might expect of an EMT regulator. To investigate whether knockdown of ZEB1 had an effect on the stem cell-like properties of the tumour cells, the cells were cultured in sphere assays, based on the mammosphere culture system, to assess the number of cancer cells with stem cell-like properties. There were fewer spheres formed by pancreatic cancer cells expressing ZEB1 shRNAs than controls, and the number of spheres steadily declined in serial cultures. These data indicate that the loss of ZEB1 reduces the number of cancer stem cells. Does this involve the miR-200 family members and other microRNAs?

Analysis of the NCI60 carcinoma cell line panel indicated that expression of ZEB1 was inversely correlated with expression of miR-200 family members, miR-183 and miR-203, which are known to be involved in the repression of stem cell-promoting mRNAs. Expression of these microRNAs in the poorly differentiated pancreatic cancer cell lines reduced sphere formation in vitro and repressed the translation of genes expressed in stem cells such as KLF4 and BMI1. These findings were extended to human primary pancreatic tumours, of which 25% of differentiated tumours expressed ZEB1 — as did almost all poorly differentiated tumours. Expression of ZEB1 correlated with expression of BMI1, and a lack of ZEB1 expression was associated with a good prognosis in patients in whom complete surgical resection of the pancreatic tumour had been possible.

Therefore, the expression of ZEB1 enables both EMT and stem cell characteristics, through the repression of microRNAs that suppress stem cell-promoting mRNAs. These findings indicate that the ZEB1–miR-200 feedback loop might be a useful target for the suppression of metastasis.

Author: Nicola McCarthy - Copyright © 2010 Nature Publishing Group, a division of MacMillan Publishers Limited; used with permission