The Niche

A recent Cell paper by Priyush Gupta, Rob Weinberg, Eric Lander and other researchers from the Broad and MIT reports a potential way to kill the cancer cells that really matter.

Here is A screen for cancer killers from NatureNews (quotes from University of Toronto’s John Dick, others)
Here is New screening for more potent cancer drugs in the New York Times. (quotes from Stanford’s Mike Clarke, others)

Cancer stem cells are cells capable of growing malignant tumours anew, and there is a surfeit of controversy about whether this is an elite subpopulation or the majority of cells in the tumour. (See Cancer stem cells, becoming common)
The researchers manipulated immortalized cancer stem cells and were able to sort out a subpopulation that resembles cancer stem cells, then they were able to identify drug that selectively kills the cancer stem cells. That’s a big deal, as a growing body of evidence indicates that the cells best able to cause a tumour to regrow are also particularly able to resist cancer drugs. (See Cancer stem cells resemble healthy ones, resist chemotherapy)
Two questions come to mind:
1) How well do these cells represent cancer stem cells? (See Careful assays for cancer stem cells )
2) Will compounds that kill cancer stem cells also kill healthy stem cells? (See How breast cancer resists treatment )

Here’s a nice summary from Jane Visvader, a breast cancer stem cell expert at the The Walter and Eliza Hall Institute of Medical Research.
This is an elegant demonstration of the power of using high throughput screening to target resistant cancer cell subsets. The authors have shown that they can specifically target mesenchymal-like cells in breast tumors, found to be resistant to a standard chemotherapeutic agent (Paclitaxel), using salinomycin.

Here’s this from Piyush Gupta, which addresses whether the cells act like bona fide cancer stem cells

The gist: Cancer stem cells have been difficult to study because they
cannot be maintained as pure populations in culture. Passage through
an EMT has been recently reported (by the Weinberg groups) to induce a
significant increase in the proportion of stem-like epithelial cells.
We show that passage through an EMT also confers increased drug
resistance to cells. Using genetic vectors to induce an EMT, it was
possible to induce a stable increase in the proportion of stem-like
cells. We then performed a chemical screen to find compounds that were
specifically toxic to cells that had passed through an EMT. Compounds
that were identified in this way were then tested the identified
compounds on cancer cell lines that we had not genetically
manipulated, to determine if they acted on bona fide cancer stem cells.

Why the study is of interest:
Cancer stem cells are resistant to many forms of death-inducing
insults. This has suggested that it may be difficult to find therapies
that specifically target CSCs. Our study shows that it is possible to
find agents that selectively kill cancer stem cells and provides a
general method for doing so.

Caveats and further experiments:
Further study will be needed to determine if the chemical we
identified, salinomycin, can be used in patients. In such studies, the
long-term effects on normal stem cell biology will also need to be
evaluated.

Posted by Monya Baker on August 14, 2009
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Two papers out the first week in August probe the molecular machinery behind cells that fuel breast cancer.

Researchers led by Jenny Chang at Baylor College of Medicine in Houston found a characteristic signature in a group of tumour cells remaining in a patient after treatment for breast cancers.1 These resistant cells were enriched whether patients underwent chemotherapy or were given drugs that block the action of tumour-promoting sex hormones, suggesting that there is a particular population of cells that can resist treatment and cause the tumour to regrow. Chang found that the gene expression in this subpopulation overlapped with two other sorts of cells: those that readily perpetuate tumours and those that allow cells to self-renew over long periods in culture. What’s more, these resistant cells may also be responsible for the spread of cancer outside the breast. Cells with this gene expression are particularly dominant in a relatively uncommon cancer called “claudin-low,” which displays characteristics of undergoing a process called the epithelial–mesenchymal transition (EMT), implicated in the spread of a malignancy. Similarly, cells remaining in tumours that had been treated with an estrogen-blocking drug showed elevated expression of mesenchymal markers. Homing in on unique characteristics of these cells may, says Chang, allow researchers to identify drugs that could eliminate these cells.

Just a few days later, researchers led by Michael Clarke of Stanford University tried a different approach to identify regulatory pathways in breast cancer stem cells. This work showed that these stem cells are governed by the same regulatory pathways as their healthy counterparts, mammary stem cells. The researchers were able to isolate cancer stem cells from human samples and perform analysis on small quantities of cells.2 This identified some three dozen microRNAs differentially expressed between tumourigenic cells and other cells in the tumour. The researchers also compared tumourigenic cells with normal mouse and human mammary stem/progenitor cells as well as embryonal carcinoma cells. Three clusters of microRNA were downregulated in all these cell types, one of which was miRNA-200c, previously found to regulate the epithelial–mesenchymal transition. Elevating levels of this microRNA suppressed self-renewal and encouraged differentiation in both normal and cancerous breast stem cells. Further work showed that this microRNA controls levels of a well-known protein called BMI1, which also regulates stem cells of the blood and brain. Thus, it seems, stem cell functions such as self-renewal, proliferation and EMT all seem to be governed by similar mechanisms—not just in healthy and disease tissue, but across different types of tissues.

Continue reading "Breast cancer stem cells resemble healthy stem cells, resist chemotherapy" »

Posted by Monya Baker on August 10, 2009
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Hi everyone, We've got this Cell Stem Cell article slated to go up as a story in a couple Thursdays, but the press release is really making the rounds. So here's the reported story by Asher Mullard.

The origins of cancer stem cells have long been elusive and controversial. While some researchers think that they are stem cells gone awry, others suspect that cancer stem cells arise from differentiated cells that have been reprogrammed. In support of the reprogramming theory, a tissue-culture study suggests that mutant mouse embryonic fibroblast (MEF) cells that lack retinoblastoma proteins are reprogrammed when they are cultured in suspension, and come to resemble cancer stem cells.

The retinoblastoma 1 (RB1) pathway is crucial for detecting cell-cell contact and regulating cell cycle arrest. Consequently, whereas cultured MEFs normally grow in a monolayer, MEFs lacking RB1 grow into mounds of cells that eventually detach and form colonies of free-floating ‘spheres’. The RB1 pathway is also frequently lost in cancers. These and other observations set Douglas Dean of Brown Cancer Center in Louisville, USA, and his colleagues wondering whether spheres had a role in reprogramming differentiated cells into cancer stem cells.

Reporting in Cell Stem Cell, Dean and his colleagues show that when RB1–/– cells are forced to grow in spheres for 2 weeks, a new cellular morphology emerges[1]. Moreover, some cells from 2 week old spheres, but not from younger spheres or from the original monolayer, persistently express embryonic stem cell genes, including Oct4 and Nanog as well as cancer stem cell markers. “It’s like a switch has been flipped,” says Dean. Thus, he argues that spheres might enable reprogramming of cancer stem cells. “There may be a technique down the road for producing an inducible pluripotent stem cell from a fibroblast using this technique,” he adds excitedly.

But Thea Tlsty, a pathologist at University of California San Francisco, is not convinced for two reasons. In the 2 weeks it took for the new morphology and characteristics to arise, the culture setup might have selected stem-cell-like cells that were already present in the population, rather than enabled de novo reprogramming. And although Dean took precautions to minimize this possibility, he acknowledges it has not yet been definitively ruled out.

And Tlsty is also uncertain about the claim that cells from aged spheres even have the characteristics of cancer stem cells. When Dean transplanted sphere-reared cells into mice, he observed tumour formation, supporting the notion that these cells have cancer stem cell capabilities. Yet this evidence isn’t conclusive, explains Tlsty. “A cancer stem cell phenotype is usually accompanied by an invasive phenotype, and while I saw a growth in vivo, I did not see an examination of invasive phenotypes.”

“To clear up my confusion, I would want them to start with single differentiated somatic cell,” says Tlsty. “And I would want to see some evidence of invasion potential.”

Dean readily notes another limitation of his study. “This is a cell culture based set of studies,” he says. “What we’re doing is trying to put forward a hypothesis that we and others can test in real tumours.”

Related articles
John Dick: Careful assays for cancer stem cells
http://www.nature.com/stemcells//2009/0903/090326/full/stemcells.2009.47.html

Cancer stem cells, becoming common
http://www.nature.com/stemcells/2008/0812/081203/full/stemcells.2008.153.html

Cancer stem cell sightings and slightings
http://www.nature.com/stemcells/2007/0709/070927/full/stemcells.2007.93.html

References
Liu, Y. et al. Mouse fibroblasts lacking RB1 function form spheres and undergo reprogramming to a cancer stem cell phenotype. Cell Stem Cell 4, 336–347 (2009).

Posted by Monya Baker on April 03, 2009
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Here are a couple papers just out in Nature. One, from Mike Clarke of Stanford, shows how human breast cancers resist treatment. I talked to him about how the paper came to be. The other, from Tannishtha Reyes of Duke, finds an alternative pathway to attack resistant leukemia in mice.

How breast cancer resists treatment
Self-renewing blood and leukaemia cells need hedgehog

Also, this week, a lovely feature about how getting interoperability between computational biologists and stem cell biologists.

Plus, a way to make mesenchymal turn into bone via physical stimulus.
Nanotubes guide mesenchymal stem cells toward becoming bone

Posted by Monya Baker on February 06, 2009
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Here's a sneak peek at just one of the research hihglights that will appear on Nature Reports Stem Cells over the next few days.

Are ruddy cheeks a sign of health or a symptom of sickness? New work from Mickie Bhatia and colleagues at McMaster University suggests that, when it comes to embryonic stem cells, the very qualities researchers use to pick out a robust cell line may in fact be bestowed by precancerous transformations. “Current measurements are not capable of distinguishing the difference between great stem cells and cancer stem cells in vitro,” says Bhatia.

Continue reading "Robust embryonic stem cells may harbor precancerous surprises" »

Posted by Monya Baker on January 05, 2009
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Why do I seem to be working harder and harder, but covering less and less? The vast majority of my readers, I’m sure, found the answer long before I did. In the eighteen months since Nature Reports Stem Cells has gone live, stem-cell happening have increased appreciably, inexorably, exhaustingly.

With so much content, we need filters more than ever, and on the cancer stem cell front, I just want to give a big thank-you to Alex Bersenev for his post over on Hematopoiesis, who summarized, organized, and emphasized important threads of my and others’ reporting on cancer stem cells. Another nice collection (including links to some open-access reviews) is on Jim Till’s cancer stem cell blog.

Also, and I meant to put this up long, long ago, his coverage of the ASH meeting focuses on aging and cancer. Check that out here.

Posted by Monya Baker on December 29, 2008
Categories: Blood stem cells, Cancer | Permalink | Comments (0) | TrackBacks (0)

Nature recently published a paper by Sean Morrison and others finding that melanoma stem cells are not rare and that standard assays to identify tumorigenic cells fail to detect a large portion of them. This prompted two letters describing an earlier study by David Taussig and others which found that the antibodies used to detect the leukemogenic cells first identified by John Dick changed their behaviour. Another letter pointed out the role that the extracellular matrix plays in shielding transplanted cells from the immune response, and suggested that this could provide insight in developing immune-based therapies to cancer.

Here, we publish that correspondence along with replies from David Taussig, which describes evidence for that cancer stem cell hypothesis, including his own evidence that leukemia-initiating cells are less than 1 in 100 cells. Finally, a reply by John Dick and colleagues says that the effects described by Taussig do not apply to a key leukemogenic cell marker and goes on to describe misconceptions about the cancer stem cell model.

Read those letters below. Here are links to NPG's research and other articles on cancer stem cells.

Continue reading "Cancer stem cells: controversies and misconceptions" »

Posted by Monya Baker on December 18, 2008
Categories: Blood stem cells, Cancer | Permalink | Comments (0) | TrackBacks (1)

Given the excitement generated bythe recent Nature Paper, here's a list of cancer stem cell articles published by Nature Reports Stem Cells.
Also check out Nature's web focus on cancer stem cells.

Cancer stem cells, becoming common
Tumour cells that can initiate a new tumour are not so rare as previously thought, putting the concept of the 'cancer stem cell' under the spotlight again.
Published online: 03 December 2008; doi:10.1038/stemcells.2008.153
Full Text

Two of a kind
Cancer stem cells use an embryonic stem cell-like transcriptional program to induce and maintain tumours
Published online: 11 September 2008; doi:10.1038/stemcells.2008.126
Full Text

Stem cell meeting 2008: in with the old, in with the new
Although excitement around advances in reprogramming somatic cells shows no signs of abating, new ideas regarding the field are surfacing.
Published online: 17 July 2008; doi:10.1038/stemcells.2008.108
Full Text

A mutant Methuselah for blood-making progenitors
Cells that regenerate blood increase tenfold in mutant mice
Published online: 24 April 2008; doi:10.1038/stemcells.2008.73
Full Text

Cancer and embryonic stem cells share genetic fingerprints
At least two modules of genes promote stemness
Published online: 17 April 2008; doi:10.1038/stemcells.2008.62
Full Text

Skin cancer needs beta-catenin
Without beta-catenin cancer stem cells no longer support tumours
Published online: 03 April 2008; doi:10.1038/stemcells.2008.57
Full Text

Cancer and stem cells: Beckman conference
Cancer cells emerge when checkpoints fail
Published online: 13 March 2008; doi:10.1038/stemcells.2008.47
Full Text

Stuck on youth
Some cancer stem cells retain an embryonic pathway
Published online: 24 January 2008; doi:10.1038/stemcells.2008.23
Full Text

MicroRNA reins in tumor-initiating cells
A microRNA that silences two oncogenes is quiet in cancer stem cells
Published online: 03 January 2008; doi:10.1038/stemcells.2007.137
Full Text

Leukemia might elbow out blood makers
A new mouse model helps researchers study human cancer cells
Published online: 01 November 2007; doi:10.1038/stemcells.2007.112
Full Text

Cancer stem cell sightings and slightings
Experts debate the rarity and relevance of 'tumour-initiating cells'
Published online: 27 September 2007; doi:10.1038/stemcells.2007.93
Full Text

Stem cells not all to blame
Cells driving tumor growth may not be all that rare
Published online: 16 August 2007; doi:10.1038/stemcells.2007.75
Full Text

Stem cell meeting 2007: Routes and roadblocks on the way to cures
While the basic side of stem cell research is prospering, several talks on translating research to therapy were sobering reminders of the challenges ahead.
Published online: 12 July 2007; doi:10.1038/stemcells.2007.52
Full Text

What's the relationship between stem cells and tumors?
Like some stem cells, cancer cells can grow without pause. Some cancers use stem cells' tricks to do this, and so some cancer researchers study stem cells.
Published online: 14 June 2007; doi:10.1038/stemcells.2007.25
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The space race
Defective stem cells physically compete for space in the niche of the Drosophila ovary
Published online: 24 January 2008; doi:10.1038/stemcells.2008.24
Full Text

More reprogramming tips
A gene used to reprogram differentiated cells blocks microRNA processing
Published online: 06 March 2008; doi:10.1038/stemcells.2008.43
Full Text

Pituitary stem cells found using a general stem cell marker
Genetic approaches identify a distinct, postnatal stem cell population
Published online: 15 May 2008; doi:10.1038/stemcells.2008.77
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What comes after iPS?
Though applications of reprogrammed cells will be valuable, the questions they engender will be just as important
Published online: 03 April 2008; doi:10.1038/stemcells.2008.54
Full Text

Posted by Monya Baker on December 03, 2008
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This one is by freelancer Simone Alves. We'll have it up on Nature Reports next Thursday.
PTEN (phosphatase and tensin homolog) is an important tumour suppressor in humans, and its disruption contributes to a wide variety of cancers. The fact that it appears to negatively regulate the PI3K/Akt pathway, which controls cellular proliferation and differentiation, implies that it is important in adult tissue homeostasis; however, until now, its role in this process had not been fully explored. A study published this week in Disease Models & Mechanisms, from Alejandro Sánchez Alvarado at the University of Utah, in Salt Lake City, describes a new planarian model system to study the role of this tumour suppressor in tissue homeostasis. The team shows that the homolog's function in stem cell regulation is highly conserved.

Planarians are small, nonparasitic flatworms that have a remarkable capacity for regeneration. They contain a large population of undifferentiated cells called neoblasts, which are considered adult stem cells because they constantly self-renew and produce differentiating daughter cells. Early in his career, Sánchez Alvarado used the simplicity of planarians to his advantage. "He almost single-handedly turned planarians into a system for studying stem cell biology," says Sean Morrison, director of the University of Michigan Center for Stem Cell Biology, in Ann Arbor.

The Utah team found that planarians had two homologues of the human phosphatase and tensin homolog (PTEN) gene and, using RNA interference, discovered that planarians could survive with just one functioning gene. However, when both genes were knocked down, the team noticed cancer-like outgrowths on the flatworms, which proved to be lethal. This was a surprise to the researchers. "We got the mammalian phenotype, not the invertebrate phenotype that we expected," says Sánchez Alvarado.

The loss of PTEN function caused general tissue disorganisation, with neoblasts overproliferating and adult cells losing signs of differentiation, which are both hallmarks of mammalian cancers. Normal regeneration and wound-healing processes were also lost, and expression of the oncoprotein Akt increased. What particularly interested Sánchez Alvarado was that a drug called rapamycin, which inhibits the mammalian target of rapamycin (mTOR) protein kinase in mammals, was able to largely sidestep the effects of the PTEN double knockdown in planarians without affecting normal physiological neoblast proliferation. He suggests that planarians may, in the future, be used to screen other potential cancer drugs.

"The discovery that PTEN and TOR regulate stem cell function and regeneration in planaria demonstrate that these remarkable animals use evolutionarily conserved mechanisms that are surprisingly similar to the mechanisms we observe in mammalian stem cells," says Morrison. Indeed, Sánchez Alvarado is excited at the idea that planarians represent a system in which to model aspects of biology that have so far been inaccessible using other invertebrates. He hopes to go on to identify how functionally conserved other tumour suppressors might be. "These results further support the value of studies in planarians to discover regulatory mechanisms that could provide new insights into how the human body works and what goes wrong in the context of disease," Morrison adds.

References
1. Oviedo, N. et al. Planarian PTEN homologs regulate stem cells and regeneration through TOR signalling. Dis. Models Mech . doi:10.1242/dmm.000117 (published online 2008).

Posted by Monya Baker on September 18, 2008
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Below is a summary of a couple Cell Stem Cell papers that offer map-fragments to one of stem cells Holy Grails: culturing the cells that give rise to blood. This could lead to more broadly applicable alternatives to treatments that now use cord blood or bone marrow transplants. This will become a formal highlight next week.

Also, an article published yesterday in Nature Medicine shows how embryonic stem cells can be used to evauluate mutations implicated in breast cancer.

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Developing a way to reliably produce hematopoietic stem cells (HSCs) is a bloody tough problem. Unlike most tissues, cells of the hematopoietic system emerge from several embryonic sites and then circulate through the body. This mobility has perplexed researchers who hope that mimicking the in vivo environment will help them culture HSCs. Now though, two British research teams report complementary techniques for isolating HSCs in Cell Stem Cell. These could form the lifeblood of techniques creating easier alternatives to bone marrow transplantation.

Alexander Medvinsky and his colleagues at the University of Edinburgh went straight to the heart of HSC development — the aorta-gonad-mesonephros (AGM) region, where the first multipotent HSCs are thought to arise in the embryo. By dissociating and reconstructing the mouse AGM's three-dimensional structure, they developed a new method to expand and track the development of HSCs, and tracked down a population of cells containing markers (VE-cadherin and CD45) generally found on separate types of cells1.

"Medvinsky characterized a cell that's wearing two hats: It's an endothelial cell and a blood cell," says M. William Lensch, of Children's Hospital Boston and the Harvard Medical School. "When you see a cell like this, it lends credibility that they develop directly from the vasculature," rather than from elsewhere in the developing embryo.

Medvinsky’s population of cells contained one marker associated with the inner lining of blood vessels and another specific to blood cells themselves, and he thought that these cells might constitute “pre-HSC” progenitors capable of acquiring stem cell function. Indeed, when he injected the VE-cadherin+CD45+ cells in irradiated mice, they rapidly developed into a large pool of definitive HSCs that that restored hematopoiesis. Since these cells mature within the AGM microenvironment, this constitutes an active niche that drives the specification of fully mature HSCs, says Medvinsky.

Rather than trying to isolate HSCs directly, a team led by Majlinda Lako of Newcastle University developed a relatively efficient way of coaxing hematopoietic differentiation from human embryonic stem cells (hESCs). They co-cultured hESCs with AGM-derived stromal cell lines, and found that hematopoietic activity increased at least 31-fold compared to previous co-culturing methods2. They then injected the induced-hESCs into the femurs of immunocompromised mice, and found substantially greater engraftment efficiencies than previously reported — up to 16% for cells co-cultured with the best cell line. Finally, Lako’s group screened around 40 signaling molecules for positive enhancers of hematopoietic differentiation, and flagged the transcription factors TGF-β1 and TGF-β3 as the most efficient inducers of hematopoiesis.

Together, the studies show that nascent cells must mature within the proper context to become definitive HSCs, regardless of whether you start with pre-HSCs or hESCs, says Hanna Mikkola, of the University of California, Los Angeles. “The message from both papers is you really need to have the correct embryonic environment for functional maturation in culture.”

The question of how that maturation occurs remains unanswered. The TGF family members identified by Lako are probably involved, Mikkola says, but she doubts these factors tell the whole story. Lako agrees. As a follow-up, Lako's group is currently sifting through a library of other candidate factors, including calcium signaling molecules and insulin-like growth factors, for other key regulators of HSC development.

Lako’s results are impressive, says Medvinsky. But he thinks that co-culturing hESCs with his VE-cadherin+CD45+ cells could be even more successful. “With our system we might be able to produce a better outcome.” Lako, however, suspects her stromal cell lines may already contain some of Medvinsky’s “niche” factors. “It’s very likely that we’re using the same signals to induce our human ES cells,” she says. In either case, both authors recognize that more work will be needed to nail down the molecular cues before fully transplantable HSCs can be cultured. "Because you can do this all in a lab dish now, you have the ability to really focus on what the molecules are," notes Lensch. These two studies now inject new blood into achieving that goal.

References
1. Taoudi, S. et al. Extensive hematopoietic stem cell generation in the AGM region via maturation of VE-cadherin+CD45+ pre-definitive HSCs. Cell Stem Cell 3: 99–108 (July 2008).

2. Ledran, M.H. et al. Efficient hematopoietic differentiation of human embryonic stem cells on stromal cells derived from hematopoietic niches. Cell Stem Cell 3: 85–98 (July 2008).

Author affiliation
Elie Dolgin is a Canadian science writer currently residing in Milwaukee, Wisconsin.

Posted by Monya Baker on July 07, 2008
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