Prostaglandins (PGs), bioactive lipid molecules produced by cyclooxygenase enzymes (COX-1 and COX-2), have diverse biological activities, including growth-promoting actions on gastrointestinal mucosa. They are also implicated in the growth of colonic polyps and cancers. However, the precise mechanisms of these trophic actions of PGs remain unclear. As activation of the epidermal growth factor receptor (EGFR) triggers mitogenic signaling in gastrointestinal mucosa, and its expression is also upregulated in colonic cancers and most neoplasms, we investigated whether PGs transactivate EGFR. Here we provide evidence that prostaglandin E2 (PGE2) rapidly phosphorylates EGFR and triggers the extracellular signal-regulated kinase 2 (ERK2)--mitogenic signaling pathway in normal gastric epithelial (RGM1) and colon cancer (Caco-2, LoVo and HT-29) cell lines. Inactivation of EGFR kinase with selective inhibitors significantly reduces PGE2-induced ERK2 activation, c-fos mRNA expression and cell proliferation. Inhibition of matrix metalloproteinases (MMPs), transforming growth factor-alpha (TGF-alpha) or c-Src blocked PGE2-mediated EGFR transactivation and downstream signaling indicating that PGE2-induced EGFR transactivation involves signaling transduced via TGF-alpha, an EGFR ligand, likely released by c-Src-activated MMP(s). Our findings that PGE2 transactivates EGFR reveal a previously unknown mechanism by which PGE2 mediates trophic actions resulting in gastric and intestinal hypertrophy as well as growth of colonic polyps and cancers.

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The A4 or 13 protein is a peptide that constitutes the major protein component of senile plaques in Alzheimer disease. The A4/13 protein is derived from a larger, transmembrane amyloid precursor protein (APP). The putative abnormal processing events leading to amyloid accumulation are largely unknown. Here we report that a 42-residue synthetic peptide, 181-42, corresponding to one of the longer forms of the A4/13 protein, accumulates in cultured human skin fibroblasts and is stable for at least 3 days. The A4/13 protein is derived from a larger transmembrane protein, the amyloid precursor protein (APP), which is expressed as multiple different mRNA splicing products (3-5).Accumulation of the A4/1B protein is apparently the result of abnormal processing, since the extracellular domain of APP is normally cleaved at residue 16 within the A4/13 region (6, 7). The fact that normal processing precludes A4/13 accumulation suggests that abnormal processing may be the initial step leading to amyloid deposition. Using synthetic peptide analogs of the A4/(3 protein, we have defined some of its intrinsic biochemical and physical properties that are related to its assembly into amyloid-like fibrils and its ability to aggregate (8). We found that assembly into amyloid-like fibrils and aggregation in SDS/polyacrylamide gels are separate and distinct properties of the amyloid peptides. Low pH (pH 3.5-6.5) and high concentrations of peptide are important for promoting assembly of the peptides into amyloid-like fibrils (8). The length of the hydrophobic C terminus (-42 residues) and a high concentration of peptide are critical for the ability of the (31 2 peptide to self-aggregate into multiple discrete bands in SDS/polyacrylamide gels. These intrinsic factors may be important for amyloid deposition in vivo because the acid environment of endosomes and lysosomes and their ability to concentrate solutes would promote amyloid fibril formation and peptide aggregation, which could compromise the ability of the cell to degrade the A4/13 protein.To explore whether cells are able to degrade the A4//3 protein, we examined uptake and degradation of three peptides that corresponded to residues 1-28 (extracellular portion) (p13-28), FRHDSGYEVHHQKLVFFAEDVGSNKGAIIGLMVG-GVVIA) were synthesized by fluoren-9-ylmethoxycarbonyl chemistry using a continuous-flow semiautomatic instrument, purified by reverse-phase HPLC, and characterized by sequencing and electrospray mass spectrometry (8). Na1251 was obtained from Amersham; chloroglycouracil (lodo-Gen) from Pierce, Bio-Gel P-2 from Bio-Rad, fetal bovine serum from GIBCO, Dulbecco's modified Eagle's medium (DMEM) from Irvine Scientific, and Percoll from Pharmacia. Analytical-grade solvents and other reagents were from various commercial sources (Sigma; Fisher Scientific).Peptide Iodination. Aliquots (10 pg) of each peptide in 40 jul of 1 M Tris (pH 7.4) were radioiodinated to a specific activity of 50,000-150,000 cpm per ng in the presence of 50 u.g of Iodo-Gen at 0C for 20 min (8). Free ...

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