BACKGROUND AND AIM - The epidermal growth factor (EGF) peptide family includes six closely-related proteins, all of which bind to the EGF receptor. In the intestinal epithelium, transforming growth factor alpha (TGFalpha) appears to be the most physiological ligand for the EGF receptor. The present studies were designed to examine the effect of TGFalpha overexpression on duodenal epithelial proliferation using a metallothioneine-inducible promoter/enhancer transgenic mouse (MT-TGFalpha). The MT-TGFalpha mouse model was further studied to determine the in vivo effect of unregulated TGFalpha production on the physiological proliferative responses to fasting and refeeding.
METHODS - MT-TGFalpha mice were given 25 mM oral ZnSO4 to induce transgene expression and were studied 1 to 2 months later. Duodenal histology was analyzed morphometrically in well-oriented transverse sections. The vincristine metaphase-arrest technique was used to assess proliferation in duodenal crypts. Immunohistochemical staining and in situ hybridization were used to assess transgenic TGFalpha protein and mRNA expression, respectively.
RESULTS - Normally fed MT-TGFalpha mice had deeper crypts (0.12 vs. 0.08 mm), longer villi (0.66 vs. 0.54 mm), and greater luminal diameters (2.65 vs. 2.19 mm) compared to controls (P<0.05 for all three dimensions). The crypt cell mitotic index in normally fed transgenic mice was 1.5 fold greater than the index in normally fed controls (20+/-2 vs. 35+/-4 mitoses per crypt; P <0.05). Fasting and refeeding MT-TGFalpha mice resulted in no significant change in their high baseline rate of crypt proliferation, while proliferation in control mice rose from a lower baseline during fasting to a level with refeeding that approximated rates in MT-TGFalpha mice. Transgenic TGFalpha protein and mRNA were localized to the villus epithelial compartment with little or no evidence of mRNA or protein expression in the crypt epithelium.
CONCLUSION - Overproduction of TGFalpha in the mouse duodenal epithelium results in a pronounced increase in crypt epithelial cell proliferation and a resulting increase in the dimension of the crypt/villus unit. This appears to be mediated through a paracrine and/or juxtacrine effect on crypt cells by TGFalpha produced in the villus epithelium. Fasting and refeeding experiments suggest that TGFalpha may also play a role in the proliferative response to refeeding or that the full potential for proliferation is realized by TGFalpha overexpression alone. Collectively, these studies suggest that TGFalpha is a physiological autocrine and paracrine regulator of small intestinal epithelial proliferation.