Annika Hau

The role of let-7 in cell differentiation and cancer

MicroRNAs (miRNAs or miRs) are small noncoding RNAs capable of regulating gene expression at the translational level. Current evidence suggests that a significant portion of the human genome is regulated by microRNAs, and many reports have demonstrated that microRNA expression is deregulated in human cancer. The let-7 family of microRNAs, first discovered in Caenorhabditis elegans, is functionally conserved from worms to humans. The human let-7 family contains 13 members located on nine different chromosomes, and many human cancers have deregulated let-7 expression. A growing body of evidence suggests that restoration of let-7 expression may be a useful therapeutic option in cancers, where its expression has been lost. In this review, we discuss the role of let-7 in normal development and differentiation, and provide an overview of the relationship between deregulated let-7 expression and tumorigenesis. The regulation of let-7 expression, cancer-relevant let-7 targets, and the relationship between let-7 and drug sensitivity are highlighted.

CD95 promotes tumour growth

CD95 (also called Fas and APO-1) is a prototypical death receptor that regulates tissue homeostasis mainly in the immune system through the induction of apoptosis. During cancer progression CD95 is frequently downregulated or cells are rendered apoptosis resistant, raising the possibility that loss of CD95 is part of a mechanism for tumour evasion. However, complete loss of CD95 is rarely seen in human cancers and many cancer cells express large quantities of CD95 and are highly sensitive to CD95-mediated apoptosis in vitro. Furthermore, cancer patients frequently have elevated levels of the physiological ligand for CD95, CD95L. These data raise the possibility that CD95 could actually promote the growth of tumours through its non-apoptotic activities. Here we show that cancer cells in general, regardless of their CD95 apoptosis sensitivity, depend on constitutive activity of CD95, stimulated by cancer-produced CD95L, for optimal growth. Consistently, loss of CD95 in mouse models of ovarian cancer and liver cancer reduces cancer incidence as well as the size of the tumours. The tumorigenic activity of CD95 is mediated by a pathway involving JNK and Jun. These results demonstrate that CD95 has a growth-promoting role during tumorigenesis and indicate that efforts to inhibit its activity rather than to enhance it should be considered during cancer therapy.

Death Induced by CD95 or CD95 Ligand Elimination

CD95 (Fas/APO-1), when bound by its cognate ligand CD95L, induces cells to die by apoptosis. We now show that elimination of CD95 or CD95L results in a form of cell death that is independent of caspase-8, RIPK1/MLKL, and p53, is not inhibited by Bcl-xL expression, and preferentially affects cancer cells. All tumors that formed in mouse models of low-grade serous ovarian cancer or chemically induced liver cancer with tissue-specific deletion of CD95 still expressed CD95, suggesting that cancer cannot form in the absence of CD95. Death induced by CD95R/L elimination (DICE) is characterized by an increase in cell size, production of mitochondrial ROS, and DNA damage. It resembles a necrotic form of mitotic catastrophe. No single drug was found to completely block this form of cell death, and it could also not be blocked by the knockdown of a single gene, making it a promising way to kill cancer cells.

CD95 and CD95L promote and protect cancer stem cells

CD95 (APO-1/Fas) is a death receptor used by immune cells to kill cancer cells through induction of apoptosis. However, the elimination of CD95 or its ligand, CD95L, from cancer cells results in death induced by CD95R/L elimination (DICE), a type of cell death that resembles a necrotic form of mitotic catastrophe suggesting that CD95 protects cancer cells from cell death. We now report that stimulation of CD95 on cancer cells or reducing miR-200c levels increases the number of cancer stem cells (CSCs), which are more sensitive to induction of DICE than non-CSC, while becoming less sensitive to CD95-mediated apoptosis. In contrast, induction of DICE or overexpression of miR-200c reduces the number of CSCs. We demonstrate that CSCs and non-CSCs have differential sensitivities to CD95-mediated apoptosis and DICE, and that killing of cancer cells can be maximized by concomitant induction of both cell death mechanisms.

CD95 is part of a let-7/p53/miR-34 regulatory network.

The death receptor CD95 (APO-1/Fas) mediates apoptosis induction upon ligation by its cognate ligand CD95L. Two types of CD95 signaling pathways have been identified, which are characterized by the absence (Type I) or presence (Type II) of mitochondrial involvement. Micro(mi)RNAs are small noncoding RNAs that negatively regulate gene expression. They are important regulators of differentiation processes and are found frequently deregulated in many human cancers. We recently showed that Type I cells express less of the differentiation marker miRNA let-7 and, hence, likely represent more advanced tumor cells than the let-7 high expressing Type II cells. We have now identified miR-34a as a selective marker for cells that are sensitive to CD95-mediated apoptosis. Both CD95 and miR-34a are p53 target genes, and consequently, both the sensitivity of cancer cells to CD95-mediated apoptosis and the ability to respond to p53 mediated DNA genotoxic stress are linked. Interestingly, while miR-34a was found to positively correlate with the ability of cells to respond to genotoxic stress, let-7 was negatively correlated. The expression level of CD95 inversely correlated with the expression of let-7 suggesting regulation of let-7 expression by CD95. To test a link between p53 and miR-34a, we altered the expression of CD95. This affected the ability of cells to activate p53 and to regulate miR-34a. Our data point to a novel regulatory network comprising p53, CD95, let-7, and miR-34a that affects cancer cell survival, differentiation, and sensitivity to apoptotic signals. The possible relevance of this regulatory network for cancer stem cells is discussed.

Erratum: CD95 promotes tumour growth (Nature (2010) 465 (492-496))

This corrects the article DOI: 10.1038/nature09075

Corrigendum: CD95 promotes tumour growth

This corrects the article DOI: 10.1038/nature09075

Erratum: Corrigendum: CD95 promotes tumour growth

This corrects the article DOI: 10.1038/nature09075

Erratum: CD95 promotes tumour growth (Nature (2010) 465 (492-496) DOI: 10.1038/nature09075)

This corrects the article DOI: 10.1038/nature09075

Abstract 4847: CD95/Fas protects cancer cells from cell death

CD95 (also called Fas and APO-1) is a prototypical death receptor that regulates tissue homeostasis mainly in the immune system through the induction of apoptosis. During cancer progression CD95 is frequently downregulated or cells are rendered apoptosis resistant, raising the possibility that loss of CD95 is part of a mechanism for tumor evasion. However, complete loss of CD95 is rarely seen in human cancer and many cancer cells express large quantities of CD95 and are highly sensitive to CD95-mediated apoptosis in vitro. Furthermore, cancer patients frequently have raised levels of the physiological ligand for CD95, CD95L. These data raise the possibility that CD95 could actually promote the growth of tumors through its non-apoptotic activities. We have recently shown that cancer cells in general, regardless of their CD95 apoptosis sensitivity, depend on constitutive activity of CD95, stimulated by cancer-produced CD95L, for optimal growth (1). Consistently, loss of CD95 in mouse models of ovarian cancer and liver cancer reduces cancer incidence as well as the size of the tumors. We have now generated inducible CD95 and CD95L knock down cell lines to characterize the molecular mechanisms of the dependency of cancer cells on CD95. In all cancer cells knocking down CD95 or CD95L slows down growth and in many cells causes cell cycle arrest and even cell death. In CT26L cells knock down of CD95L causes all cells to die within 2 days. This form of cell death cannot be inhibited by inhibitors of apoptosis or necroptosis and has features of mitotic catastrophe. The data suggest that CD95 maintains viability of cancer cells by preventing DNA damage. 1. Chen et al (2010) CD95 promotes tumor growth. Nature, 465, 492-496. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4847. doi:1538-7445.AM2012-4847