The secret life of quiescent cancer stem cells.

Quiescence represents a survival strategy adopted by cancer stem cells (CSCs) to resist harsh environmental conditions and cytotoxic insults. Recent studies show that quiescent CSCs are found in a low metabolic state in which they are exquisitely dependent on Bcl-2 or Bcl-XL, indicating new potential routes of clinical intervention.

Stem cells in both normal tissues and tumors can adopt a proliferative or a quiescent phenotype. In normal tissues, cycling stem cells (where present) usually contribute to homeostatic tissue maintenance whereas dormant stem cells are primarily devoted to tissue repair upon injury. In a twisted parallel of the situation in normal tissues, actively dividing cancer stem cells (CSCs) drive tumor progression whereas quiescent cancer stem cells (qCSCs) () resist cytotoxic insults and repopulate the tumor after chemotherapy. qCSCs are generally rare, but they can become the prevalent CSC population after cytotoxic treatments that kill proliferating CSCs.

Figure 1.

Lung cancer stem cells. The figure shows a small sphere of lung cancer stem cells containing one quiescent cancer stem cell (CSC) positive for the lipophilic dye PKH26 (red). Lung CSC spheroids were stained with PKH26 and cultured for 2 weeks, and then fixed, counterstained with DAPI (blue), and photographed with an Olympus FV1000 confocal microscope (60× magnification, 3× zoom).

Several recent studies have focused on qCSCs in tumors of different origin, achieving important insights into their regulatory mechanisms and potential weak points. Among the most relevant discoveries in this field over the last years was the demonstration, first in leukemias and then in solid tumors, that qCSCs are highly dependent on Bcl-2 family members for their survival and metabolic functions.

To understand the peculiar dependency of qCSCs on Bcl-2/Bcl-XL, it is necessary to trace the function of these proteins back to their molecular structure, which is related to pore-forming domains of bacterial toxins and reveals the ability to regulate transport across intracellular membranes. In particular, Bcl-2 and Bcl-XL were found to facilitate mitochondrial ATP/ADP exchange, thus improving oxidative phosphorylation and mitochondrial respiration. This function is essential to prevent apoptosis in cells with a low metabolic activity as a result of perturbed mitochondrial respiration, such as cells deprived of growth factors or those harboring defects in mitochondrial respiratory chains. In particular, Bcl-XL has been previously shown to prevent the death of metabolically arrested cells by sustaining ATP/ADP exchange and mitochondrial respiration.

Interestingly, leukemic qCSCs were recently found to be characterized by a low rate of energy metabolism and to strongly rely on mitochondrial oxidative phosphorylation. This “low energy” state likely enables qCSCs to persist in harsh situations such as low oxygen levels and nutrient scarcity, 2 conditions that are frequently encountered by cancer cells. However, when mitochondrial respiration is inhibited qCSCs are unable to utilize glycolysis, in contrast to normal stem cells that efficiently use glycolysis for energy homeostasis. The tight link of leukemic qCSCs to mitochondrial respiration is dependent on elevated expression levels of Bcl-2, which stimulates coupled respiration thus maintaining qCSCs in a state of “hyperventilation” that protects them from cell death. Besides stimulating mitochondrial respiratory functions, the high expression of Bcl-2/Bcl-XL typical of qCSCs probably contributes to the resistance of these cells to chemotherapy, thus providing a double survival advantage.

However, the link between qCSCs and mitochondrial respiration could turn into a deadly embrace, representing the Achilles’ heel in the powerful survival strategy of this CSC subpopulation. In line with this hypothesis, it has recently been demonstrated that quiescent colon cancer cells residing in metabolically compromised environments can be efficiently targeted by a small-molecule inhibitor of mitochondrial respiration, VLX600. This molecule strongly shifts energy production from oxidative phosphorylation to glycolysis, a conversion that would be tolerated by glycolysis-proficient cells but causes bioenergetic catastrophe and death of qCSCs as a result of either an intrinsic inability to rely on glycolysis or the difficulty in finding glucose in metabolically compromised areas.

Despite their common dependency on mitochondrial respiration, an important difference has emerged between qCSCs of solid and hematologic tumors. In fact, the survival of leukemic qCSCs has been shown to depend primarily on Bcl-2, whereas qCSCs in solid tumors seem to be prevalently dependent on Bcl-XL. We have shown that lung cancer stem cells can express both Bcl-2 and Bcl-XL, but lose viability and clonogenic capacity only upon Bcl-XL inhibition. Moreover, treatment with the small molecule Bcl-2/Bcl-XL inhibitor ABT-737 preferentially eliminates quiescent lung CSCs (whereas actively proliferating CSCs are relatively more sensitive to chemotherapeutic agents) and decreases lung CSC content in tumor xenografts. In colorectal tumors, it has recently been shown that CSCs in toto do not express Bcl-2 and are exquisitely sensitive to Bcl-XL inhibition, which increases mitochondrial priming and sensitizes CSCs toward chemotherapy-induced death. Together, these studies indicate the exciting possibility of targeting therapy-resistant CSCs by exploiting their unique metabolic features. Based on these premises, future clinical trials will be needed to evaluate the specific activity of Bcl-2/Bcl-XL inhibitors in cancer patients, with a specific focus on CSC-related endpoints.