The poor survival and high recurrence rates in patients with HNSCC demand a re-assessment of the pathobiology of these cancers. Here, we showed that head and neck cancer stem cells reside in perivascular niches. Notably, we observed that endothelial cell-secreted factors have a major impact on the self-renewal and survival of cancer stem cells. These data suggest that therapeutic targeting of the tumor endothelium may reduce the rate of head and neck tumor recurrence and metastasis by decreasing the proportion of cancer stem cells.
Prince and colleagues (2007) reported that CD44+ cells from primary HNSCC exhibit a cancer stem-cell phenotype, and are capable of initiating tumors at low numbers (9
). ALDH1 has been recently described as a putative marker for CSC in head and neck tumors (26
). Here, we demonstrate that the combination of ALDH1 and CD44 selects a sub-population of cells with properties of cancer stem cells than if used as single markers. One thousand ALDH+CD44+ cells were capable of initiating tumors much more efficiently than 10,000 ALDH−CD44− cells. ALDH+CD44+ cells could also be transplanted serially and generated secondary xenografts, evidencing the self-renewal nature of these cells. It is noteworthy that although control ALDH−CD44− cells formed a few primary xenografts (2 out of 15), we did not observe any tumor being formed from the serial transplantation experiments with these cells. The two tumors generated from ALDH−CD44− cells could be due to the existence of a few progenitor cells with the capability of tumor initiation or possible inaccuracy of FACS sorting. Moreover, the histology of the xenografts obtained from the ALDH+CD44+ cells resembled that of primary tumors. These findings confirmed that the xenografts were of human epithelial origin and supported the hypothesis that ALDH+CD44+ cells have a behavior that is consistent with the behavior of cancer stem cells.
We observed that ALDH-positive cells are found primarily in the basal layer of the oral epithelium, where stem cells of the skin have been traditionally found (14
). In contrast, in the HNSCC the ALDH-positive cells have a more disperse localization within the tumor microenvironment. Of note, the ALDH-positive cells were consistently localized within close proximity of blood vessels. The close association of cancer stem cells and blood vessels has earlier been documented in the nervous system and that these vascular niches helped in the maintenance of stem cells and cancer stem cells (17
). However, such association has not been reported yet for head and neck tumors.
The cancer stem cells are believed to escape current therapies like radiation and chemotherapy and possibly lead to recurrences in various cancers. Thereby, identifying and targeting cancer stem cells or their niches might be a novel therapeutic strategy in the clinic (34
). However, to be able to target the cancer stem cells or their niches, we have to understand its pathobiology, identify their niches and their effects on the cancer stem cells. Endothelial cells have been implicated in the self-renewal and survival of neural cancer stem cells (20
). Studies in hematopoietic stem cells suggest that the vascular niche can promote cell survival signals (18
), which could make them resistant to chemotherapies. Other groups have studied the effects of endothelial cell survival and self-renewal on cancer stem cells in neural tumors (20
). Anti-angiogenic agents (e.g. bevacizumab) have been shown to mediate a depletion in the cancer stem cells in models of gliomas and medulloblastomas. Here, we used a unique experimental approach to selectively eliminate tumor-associated endothelial cells and evaluate the effect on the stem cell compartment. Unlike previous experimental strategies that were based on anti-angiogenic drugs, the approach used here eliminates the risk of a direct effect of the drug on the viability or stemness of the tumor stem cells. We observed that selective ablation of tumor-associated blood vessels is sufficient to decrease the proportion of head and neck tumor stem cells within 4 days, while no changes were observed in tumor volume in the same time period.
Our work demonstrates that endothelial cells initiate signaling events that enhance the survival and self-renewal of stem cells in head and neck tumors. In addition, the data presented here supports the concept that head and neck cancer indeed follows the cancer stem cell hypothesis, since implantation of few cells consistently gives rise to tumors that can be serially passaged in vivo. Collectively, these data suggest that therapeutic strategies that include anti-angiogenic agents might have the benefit of reducing the proportion of cancer stem cells in head and neck tumors. These results might translate into lower recurrence rates and better survival of head and neck cancer patients.