Background & Aims
Progastrin stimulates colonic mucosal proliferation and carcinogenesis through the cholecystokinin 2 receptor (CCK2R)—partly by increasing numbers of colonic progenitor cells. However, little is known about the mechanisms by which progastrin stimulates colonic cell proliferation. We investigated the role of bone morphogenetic proteins (BMPs) in progastrin induction of colonic cell proliferation via CCK2R.
We performed microarray analysis to compare changes in gene expression in the colonic mucosa of mice that express a human progastrin transgene (hGAS), gastrin knockout (GAS−/−) mice, and C57BL/6 mice (controls); the effects of progastrin were also determined on in vitro colonic crypt cultures from cholecystokinin 2 receptor knockout (CCK2R−/−) and wild-type mice. Human colorectal and gastric cancer cells that expressed CCK2R were incubated with progastrin or Bmp2 protein; levels of β-arrestin-1 and -2 (ARRB1 and ARRB2) were knocked down using small interfering RNAs. Cells were analyzed for progastrin binding, proliferation, changes in gene expression, and symmetric cell division.
The BMP pathway was downregulated in the colons of hGAS mice, compared with controls. Progastrin suppressed transcription of Bmp2 through a pathway that required CCK2R and was mediated by ARRB1 and ARRB2. In mouse colonic epithelial cells, downregulation of Bmp2 led to decreased phosphorylation of Smads1/5/8 and suppression of Id4. In human gastric and colorectal cancer cell lines, CCK2R was necessary and sufficient for progastrin binding and induction of proliferation; these effects were blocked when cells were incubated with recombinant Bmp2. Incubation with progastrin increased the number of CD44+, bromodeoxyuridine+, and NUMB+ cells, indicating an increase in symmetric divisions of putative cancer stem cells.
Progastrin stimulates proliferation in colons of mice and cultured human cells via CCK2R- and ARRB1- and 2-dependent suppression of Bmp2 signaling. This process promotes symmetric cell division.