Drug resistance, a reversible state of cancer cells?

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(Stem Cells, Dec 2012) Researchers suggest that resistance to drug treatments may be a normal part of a cancer stem cell’s life.

One of cancer’s deadliest features is the development of drug resistance. Scientists have known for a long time that all it takes for an untreatable cancer to grow back is time and a small number of cancer cells that can withstand the drug treatment. More specifically, and in keeping with current theories about cancer progression, cancer recurrence could result from a small number of drug-resistant (or CSCs) that can survive a treatment and regrow the tumor. A recent Stem Cells article by Shinta Kobayashi and a team of collaborators from several Japanese pharmaceutical companies led by Tatsumi Yamazaki reports intriguing findings on how colon CSCs become resistant to drugs (1).

To support the cancer stem cell hypothesis, scientists have been trying to discover new CSC-specific markers for different cancers. In the case of colon cancer, a gene named Lgr5 had been a good CSC marker candidate, because it is specifically expressed in healthy colon stem cells and it is thought that CSCs share some features with the healthy stem cells from the corresponding tissue. Therefore, Yamazaki’s team decided to investigate Lgr5 further.

Firstly, to detect the expression of Lgr5 in tissues, the researchers raised against Lgr5, which were notoriously unavailable until now. With this tool in hand, the researchers observed that cells from colon cancers that express the Lgr5 gene (Lgr5+) could regrow a tumor when injected into mice much more frequently than cells from the same cancer that do not express the gene (Lgr5-). The observation that Lgr5- cells could form tumors at all may have seemed to undermine the usefulness of Lgr5 as a good CSC marker. But this story was about to get much more interesting.

When the authors established Lgr5+ cell lines from cancer samples, they sought to determine if these cells could develop drug-resistance. By exposing Lgr5+ cells in culture to a drug called irinotecan, about half of the cells stopped dividing but survived the treatment. Most notably, these drug resistant cells had lost expression of Lgr5, even though they maintained expression of other known colon CSC markers. Contrarily, when the researchers removed irinotecan from the culture media and re-plated the Lgr5- survivors, the cells recovered Lgr5 expression, proliferative capacity and tumorigenic potential in mice.

Kobayashi and colleagues could also see this drug-induced interconversion of CSCs in vivo; when Lgr5+ cells were injected into mice treated with irinotecan, there was no tumor growth and the cells quickly lost Lgr5 expression. Stopping the irinotecan treatment, however, allowed the cells to recover Lgr5 expression and form tumors indistinguishable from those obtained with Lgr5+ cells that were never exposed to the drug.

Previous studies had already shown that healthy colons have a pool of slowly Lgr5- stem cells that can replace faster cycling Lgr5+ stem cells that were lost from the intestinal . Using their antibody on cancer samples, Yamazaki’s team observed both Lgr5+ and Lgr5- stem cells that appeared to be randomly scattered throughout , raising the interesting possibility that drug-sensitive and resistant cells coexist prior to the use of chemotherapy.

What’s next? One of the most pressing needs may be to find out if Lgr5 expression is simply a surrogate marker for a given CSC regulatory state (drug-sensitive vs. -resistant), or part of the cellular mechanism related to drug resistance. In another intriguing finding, the researchers report that if primary cultures from the cancer samples were denied attachment to a substrate and grown as , only a few of the cells expressed Lgr5 and could initiate tumor growth; but if the cells were grown as on a solid surface, virtually all of them expressed Lgr5 and showed a much greater tumorigenic capacity in mice.

One thing is for sure: Yamazaki’s research team is anything but short of options.

(1) Kobayashi S et al. LGR5-Positive Colon Cancer Stem Cells Interconvert with Drug-Resistant LGR5-Negative cells and are Capable of Tumor Reconstitution. Stem Cells, 2012 Dec;30(12):2631-44. doi: 10.1002/stem.1257.

 

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