Current view: intestinal stem cells and signaling

CaSR stimulates secretion of Wnt5a from colonic myofibroblasts to stimulate CDX2 and sucrase-isomaltase using Ror2 on intestinal epithelia

To understand whether extracellular calcium-sensing receptor (CaSR) expression on colonic myofibroblast cells (18Co) contributed to epithelial homeostasis, we activated the CaSR with 5 mM Ca(2+), screened by RT-PCR Wnt family members, and measured their secretion. Transcripts for Wnt 1, 2, 2b, 3a, 4, and 7a were either absent or unchanged whereas Wnt3 decreased and Wnt5a increased. We assessed Wnt5a secretion by Western blot. High Ca(2+) (5 mM) substantially increased Wnt5a secretion; small interfering RNA (siRNA) against the CaSR reduced this to constitutive amounts. Expression of Wnt5a plasmid but not Wnt1 or Wnt3a increased caudal homeodomain factor CDX2 transcripts and protein in HT-29 adenocarcinoma cells. Wnt5a increased activity of a sucrase-isomaltase (SI) promoter in Caco-2BBE cells. Wnt5a protein stimulation of CDX2 transcripts and protein and SI reporter were increased by overexpression of wild-type Ror2, a Wnt5a receptor, and reduced with siRNA against Ror2. CaSR activation of HT-29 cells increased Ror2 protein expression. Ror2 protein was expressed in mouse jejunum from crypt base to villus tip and in the colon on surface epithelia. Our results show that activation of a G protein-coupled receptor, the CaSR, stimulates secretion of Wnt5a from myofibroblasts. Stimulation of epithelia by the CaSR increased the expression of a receptor for Wnt5a, the tyrosine kinase Ror2, suggesting existence of a unique paracrine relationship for CDX2 homoeostasis in the intestine and revealing new contributions of CaSR-activated myofibroblasts to intestinal stem cell niche microenvironments.

Human colon cancer stem cells: a new paradigm in gastrointestinal oncology

For the past half century, oncologists have had systemic drugs available, agents that are able to induce tumor responses in patients with colorectal cancer. However, in cases of advanced colorectal cancer, these regimens are almost never curative. The recently introduced concept that cancer stem cells (SCs) drive tumor growth suggests a reason for these therapeutic failures--current chemotherapeutics target rapidly dividing cells but cancer SCs divide only slowly, and, they are relatively resistant to cytotoxic systemic therapies. It also suggests a solution--development of therapeutics that target cancer SCs. However, there is a paucity of information about the mechanisms by which SC populations are maintained and about the mechanisms by which tumor SCs are involved in colon cancer development. In this article, we discuss these mechanisms and recent developments in the identification and isolation of colon cancer SCs using new SC markers. We then discuss the role of SCs in homeostasis of normal colonic epithelium, and mechanisms by which dysregulation of crypt mechanisms can lead to initiation and progression of colon cancer. Our hypothesis, which has received recent experimental support, is that the mechanism that links abnormalities at the gene level (eg, APC mutations) and abnormalities at the tissue level (eg, proliferative shift, dysplasia, carcinoma) from cancer initiation to metastasis is SC overpopulation. Finally, we discuss the concept that symmetric cancer SC division is an essential mechanism that drives tumor growth, and that development of a new generation of therapeutics that target colon cancer SCs by inhibiting symmetric SC division holds promise for truly curative approaches for patients with advanced colorectal cancers.