Actions of betacellulin in the gastrointestinal tract

This work employed a transgenic mouse model to investigate the
functions and effects elicited by the peptide growth factor
betacellulin (BTC), an EGFR ligand, on the physiology and pathology
of the gastrointestinal tract. BTC, a 32-kDa glycosylated protein,
was initially purified from a mouse pancreatic β-cell carcinoma
cell line. In addition to the EGFR, BTC is also able to directly
bind and activate the related ERBB4 receptor. BTC is expressed in a
wide variety of tissues, particularly in the pancreatic β-cells,
stomach, small intestine, lung, kidney and uterus. Three different
transgenic mouse lines (L2, L4, and L5) overexpressing BTC under
the control of a ubiquitous promoter were established by pronuclear
DNA microinjection. Southern blot analysis revealed different
integration sites for each line. RT-PCR, Northern and Western blot
analysis revealed strong expression of the transgene in the lung,
heart, brain and pancreas, with detectable levels in other tissues
like stomach, intestine, muscle, bones, liver, thymus, spleen,
kidneys, adrenal glands, ovaries, brain and eyes of transgenic
mice. BTC-transgenic mice showed stunted growth and significantly
reduced relative pancreas and carcass weights. In contrast, the
absolute and relative weights of eyes, lung, stomach, intestine and
spleen were significantly increased. BTC-transgenic mice exhibit a
remarkable, age-dependent hyperplasia of the gastric epithelium.
The lesions were characterized by a tumor-like hyperplasia of
foveolar epithelium with large cystic formations and a severe
depletion of the preexisting body of the mucosa, thus resembling
some aspects of human gastric tumors found in patients with
Ménétrier disease where TGFA, another ligand of the EGFR, plays an
important role. BTC-transgenic animals showed hypoalbuminemia (due
to protein loss across the mucosa) and an increase in the gastric
pH (as a consequence of parietal cell depletion), while gastrin
levels were not altered. The hyperplastic lesions originated
exclusively in the lesser curvature of the stomach and were
histologically detectable for the first time at the age of 4 weeks.
Another important aspect of the gastric lesions was the pronounced
infiltration with mononuclear cells. Gastric inflammation is
regularly observed during infection with Helicobacter pylori in
humans, which can lead to development of stomach cancer or
Ménétrier disease. In conclusion, our study is the first to reveal
an association between gastric hyperplastic lesions and
overabundance of an EGFR ligand but TGFA. BTC-transgenic mice
showed a significant reduction in the length of small and large
intestine while the weight of theses tissues was significant
increased. The villi were narrower and longer as compared with
control littermates. No pathological alterations such as fibrosis,
metaplastic or neoplastic lesions could be found in the intestinal
mucosa of transgenic animals, but the cell proliferation index was
increased. It was further demonstrated that the BTC effects in the
intestine are EGFR-dependent and that BTC overproduction increases
the multiplicity of intestinal adenomas in Apc+/Min mice. A
distinct expression pattern of ERBB1 and ERBB4 could be responsible
for the significantly reduced pancreas weight in BTC-transgenic
mice: ERBB1 is expressed predominantly in the islets, while ERBB4
expression is nearly absent in the endocrine tissue but is high in
the exocrine part. It can be postulated that the reduced exocrine
pancreas is mediated by the ERBB4 receptor and not by the EGFR.
This concept is supported by the maintenance of the pancreas weight
reduction in BTC-transgenic mice in an EGFR-deficient background.
To evaluate the functional consequence of BTC excess in the
pancreatic tissue, we submitted transgenic mice to
caerulein-induced acute pancreatitis. The serum markers indicative
of pancreatitis were significant lower in the transgenic mice as
compared to their control littermates, an indication for a weaker
pancreatitis in this group. Further, BTC-transgenic animals
developed less tissue inflammation, pancreatic edema and acinar
necrosis. Interestingly, BTC-transgenic mice showed significantly
higher levels of apoptosis during pancreatitis as compared to their
littermates. Further studies will be necessary to investigate
whether a shift in death response from necrosis to apoptosis is the
mechanism resulting in a protection against caerulein-induced
pancreatitis. If confirmed, this finding may have important
therapeutic implications.