How do Differentiated Tumor Cells Support Cancer Stem Cells?
Differentiated colorectal cells produce stem cell factor (SCF) to activate KIT signaling in tumor-initiating cells, researchers report in the September issue of Gastroenterology. This pathway maintains the stem-like features of these tumor cells, predominantly under conditions of hypoxia.
Colorectal tumors are organized hierarchically and their growth is mediated by an undifferentiated population of stem-like cells. Differentiated epithelial cells are an important component of the niche that maintains normal stem cells in the gut, such as the Paneth cells in the small intestine and, possibly, CD24+ cells in the large intestine.
Szabolcs Fatrai et al investigated whether these differentiated also support colorectal cancer stem cells.
They cultured undifferentiated colonospheres from human colon tumors, used them to generate stably differentiated cell lines, and collected and identified secreted factors. They then analyzed how these secreted factors affected expression of genes involved in stemness, differentiation, and the epithelial to mesenchymal transition (EMT). Studies have associated EMT in colorectal cancer cells with tumor progression and metastasis.
Fatrai et al found that the more-differentiated cells from various tumors, or medium conditioned by them, increased the colony-forming capacity of colonosphere cells derived from the same tumors, and also induced the EMT.
In their screen of growth factors and cytokines secreted by the differentiated cells, they identified SCF as one that increased the colony-forming capacity of colonospheres, with similar potency to that of the conditioned medium from the differentiated cells. SCF also prevented loss of clonogenic potential under differentiation-inducing conditions.
SCF supported colony formation by cells that expressed KIT (CD117). KIT a receptor tyrosine kinase expressed in specialized goblet cells that contribute to the stem cell niche in the colon crypt base in mice. It is expressed by several human colon cancer cell lines.
An article by Evan Chen et al, in the September issue of Gastroenterology, reported that KIT is expressed by a subset of human colon tumors, and that knockdown of KIT reduced proliferation of colon cancer cell lines and growth of xenograft tumors in mice.
Fatrai et al found that inhibitors of KIT, such as imatinib or ISCK03, or a neutralizing antibody against SCF, reduced expression of genes that regulate stemness and the ability of colonsphere cells to undergo the EMT. The inhibitors also reduced the cells’ clonogenic potential and ability to form xenograft tumors in mice.
Bioinformatic and immunohistochemical analyses revealed a correlation between expression of KIT- and hypoxia-related genes in colon tumors. Expression was highest in relapse-prone, mesenchymal-type tumors.
Hypoxia maintains embryonic, hematopoietic, and neural stem cell populations, and studies have found hypoxia and hypoxia-inducible factor (HIF) to regulate cancer stem cells. Hypoxia also promotes tumor recurrence after surgery to remove liver metastases.
The authors therefore placed the colonospheres under hypoxic conditions. They found that hypoxia increased the colony-forming potential of cultured cells, as well as expression of KIT and the stem cell marker OLFM4. Hypoxia also increased the ability of these cells to form xenograft tumors in mice. ISCK03 or the SCF-neutralizing antibody blocked their hypoxia-induced colony-forming potential. Expression of HIF1A and HIF2A correlated with expression of KIT in samples of aggressive metastasis-prone tumors from patients.
Fatrai et al propose that these SCF-producing differentiated tumor cells help support the maintenance of KIT-positive stem-like cells in hypoxic, peri-necrotic areas of tumors.
In an editorial that accompanies the article, Yatrik M. Shah and Gijs R. van den Brink write that because KIT signaling maintains colorectal tumor-initiating cells, drugs that target SCF signaling via this receptor might be used in combination with conventional chemoradiation therapy.
They say that it is important to target this pathway because it regulates the EMT and mesenchymal features of colorectal tumors, which are important for progression. Imatinib might be tested in early-phase trials of patients with KIT-positive colorectal tumors—especially those of the mesenchymal cell-like subclass.