Stromal Indian hedgehog signaling is required for intestinal adenoma formation in mice

Indian hedgehog (IHH) is an epithelial-derived signal in the intestinal stroma, inducing factors that restrict epithelial proliferation and suppress activation of the immune system. In addition to these rapid effects of IHH signaling, IHH is required to maintain a stromal phenotype in which myofibroblasts and smooth muscle cells predominate. We investigated the role of IHH signaling during development of intestinal neoplasia in mice.

Glioma-associated oncogene (Gli1)-CreERT2 and Patched (Ptch)-lacZ reporter mice were crossed with Apc(Min) mice to generate Gli1CreERT2-Rosa26-ZSGreen-Apc(Min) and Ptch-lacZ-Apc(Min) mice, which were used to identify hedgehog-responsive cells. Cyp1a1Cre-Apc (Apc(HET)) mice, which develop adenomas after administration of β-naphthoflavone, were crossed with mice with conditional disruption of Ihh in the small intestine epithelium. Apc(Min) mice were crossed with mice in which sonic hedgehog (SHH) was overexpressed specifically in the intestinal epithelium. Intestinal tissues were collected and analyzed histologically and by immunohistochemistry and quantitative reverse-transcription polymerase chain reaction. We also analyzed levels of IHH messenger RNA and expression of IHH gene targets in intestinal tissues from patients with familial adenomatous polyposis (n = 18) or sessile serrated adenomas (n = 15) and normal colonic tissue from control patients (n = 12).

Expression of IHH messenger RNA and its targets were increased in intestinal adenomas from patients and mice compared with control colon tissues. In mice, IHH signaling was exclusively paracrine, from the epithelium to the stroma. Loss of IHH from Apc(HET) mice almost completely blocked adenoma development, and overexpression of SHH increased the number and size of adenomas that developed. Loss of IHH from Apc(HET) mice changed the composition of the adenoma stroma; cells that expressed α-smooth muscle actin or desmin were lost, along with expression of cyclooxygenase-2, and the number of vimentin-positive cells increased.

Apc mutant epithelial cells secrete IHH to maintain an intestinal stromal phenotype that is required for adenoma development in mice.