Preclinical models have suggested that cancer stem cells play a role in tumor recurrence and metastasis following adjuvant therapy, and Max S. Wicha, MD, and his research team are deciphering the mechanisms by which this might happen. A true understanding of cancer stem cells will have important therapeutic implications, according to Dr. Wicha, who is Director of the University of Michigan Comprehensive Cancer Center, Ann Arbor, and Distinguished Professor of Oncology.
In brief, the research to date on cancer stem cells has led to four pivotal findings poised to impact cancer care: (1) tumor regression is an inadequate endpoint for treatment response, (2) cancer stem cells may be resistant to therapy, (3) effective therapies should target cancer stem cells while sparing normal stem cells, and (4) genes in the cancer stem cell self-renewal pathway may provide new therapeutic targets.
MicroRNAs and Stem Cells
MicroRNAs play important roles in normal cellular differentiation and oncogenesis, and work from Dr. Wicha’s lab has demonstrated they can regulate the transition among cancer stem cell states.
His team defined a set of micro-RNAs that are over- or underexpressed in stem cells, one of which they considered particularly interesting: micro-RNA-93 (mir-93), which is located in the MCM7 gene and is believed to function as a tumor suppressor. They found that low mir-93 expression is associated with tumor-initiating capacity, while its overexpression diminishes the presence of cancer stem cells.
Using a series of breast cancer cell lines representing different stages of differentiation and mouse xenograft models, they demonstrated that mir-93 modulates the fate of breast cancer stem cells by regulating their proliferation and differentiation states. Enforced expression of mir-93 completely blocks tumor development and metastases by reducing breast cancer stem cells.
Targeting Stem Cells in the Adjuvant Setting
Additional experiments showed that the overexpression of mir-93 inhibits the growth of established tumor xenografts, a finding that is clinically applicable as it suggests the effects of targeting stem cells can be “more dramatic” in the adjuvant as compared to advanced disease setting, he said.
To simulate the adjuvant setting, Dr. Wicha and colleagues induced mir-93 and/or administered docetaxel immediately after tumor cell implantation. Although tumors grew after 4 to 5 weeks in control animals, no tumor growth was observed following mir-93 induction and/or 8 weeks of docetaxel treatments in the experimental groups. After 10 additional weeks of follow-up, tumors developed in all mice who received docetaxel alone but in none who received mir-93 induction, whether or not they received docetaxel.
They extended these observations to primary breast tumors by examining the effect of mir-93 induction on three primary breast xenografts directly established from patients’ tumors. Again, induction of mir-93 upon cell implantation completely prevented tumor growth, this time in a human model.
Taking the experiment a step further, they established that mir-93 overexpression inhibited cell invasion and metastasis when occurring in the adjuvant setting, but in advanced disease the scenario was different: mir-93 had only a modest effect in reducing the growth of established tumors. Resistance to cytotoxic chemotherapy—in this experiment, docetaxel—was also abrogated in the presence of mir-93.
“If you just give chemotherapy, you suppress the growth of a tumor, but when you stop treatment—as we do in the clinic—the tumor returns. If you knock out stem cells, even transiently, and then stop treatment, the animals are cured,” Dr. Wicha said. “Together these studies suggest that mir-93 regulates the cancer stem cell population and that this population mediates tumor growth following adjuvant therapy.”
The models predict that the efficacy of stem cell–targeting agents should be most pronounced in the adjuvant setting, where tumor growth from micrometastasis is dependent on stem cell self-renewal, Dr. Wicha said.
Why Cytotoxic Chemotherapy Fails
In their experiments with docetaxel, Dr. Wicha and colleagues found that while the taxane killed cancer cells, the dying cells actually stimulated the production of cancer stem cells through the release of interleukins (ILs).
“We found that a potent regulator of stem cells is a series of cytokines, in particular, IL-6 and IL-8,” he said. IL-8 binds to the cytokine receptor CXCR1, and this stimulates self-renewal of stem cells through the Wnt pathway. “That’s why we get an increase in cancer stem cells with cytotoxic chemotherapy,” he explained.
Repertaxin is an inhibitor of CXCR1 that potently blocks the chemotherapy-induced increase in cancer stem cells. A phase I clinical trial is evaluating the strategy of adding repertaxin to paclitaxel, and early results suggest relative lack of toxicity.
The IL-6 inhibitor tocilizumab (Actemra) also appears promising as a means of reversing resistance to trastuzumab (Herceptin) in the HER2-overexpressing population. HER2 is believed to be an important driver of breast cancer stem cells, he pointed out.
Drawing upon the fact that deletion of PTEN, a tumor-suppressor gene, is a common genetic mutation in breast cancer, the researchers modeled PTEN deletion in HER2-overexpressing cells and found that cells became resistant to trastuzumab by activating an inflammatory loop that involves IL-6. Blockade of the IL-6 pathway with drugs such as tocilizumab can completely reverse this resistance, he said.
“With trastuzumab, you actually stimulate the growth of HER2-positive PTEN-deleted tumors, but you can completely reverse this if you combine trastuzumab and tocilizumab,” he said. “Tocilizumab overcomes de novo and acquired trastuzumab resistance.”
These two inhibitors are just the beginning of new therapeutics aiming to put the cap on stem cells by targeting probably half a dozen pathways that are stem cell regulators, he predicted.
“At our cancer center in Michigan, we now have nine clinical trials underway with inhibitors of notch, hedgehog, Wnt, HER2/AKT, and the cytokines CXCR1 and IL-6 receptors,” he said. “Over the next few years, we will see many more trials targeting stem cells.” ■
Disclosure: Dr. Wicha has financial holdings with is a scientific advisor for OncoMed Pharmaceuticals, is a scientific advisor for Verastem, Paganini, and MedImmune, and receives research support from Dompe Pharmaceuticals and MedImmune.
