Trefoil Peptides in Airway Epithelium: An Important Addition to the Plethora of Peptides

An increasing number of regulatory peptides have been demonstrated to be expressed in airway epithelium; these modulate various aspects of proliferation, motility and secretory function. These recognized peptides include epidermal growth factor (EGF), transforming growth factor beta (TGF), the insulin-like growth factor family, the fibroblast growth factor family, a few other seemingly unrelated regulatory peptides, such as hepatocyte growth factor, plateletderived growth factor, and various interleukins, interferons and tumor necrosis factor-related proteins. Many of these peptides have roles in other epithelial structures such as intestine and breast, and insights gained in these tissues have been useful in understanding airway function. Over the past twenty years, one family of peptides has been identified as an important regulator of gastric and intestinal epithelial repair and motility. Known as the trefoil factor family (TFF), their abundant expression in both normal and neoplastic tissues has stimulated substantial interest in understanding their regulatory roles in cell motility and mucous secretion. The trefoil family shares a variable number of conserved motifs, usually 39 to 43 residues each, based on a compact three-dimensional trefoil structure of six cysteine residues of almost invariant spacing held together by three pairs of disulfide bonds (1, 2). This structure gives the peptide a characteristic three-loop shape (Figure 1) and provides for exceptional resistance to acid digestion and proteolytic degradation. Other conserved residues have been shown to form a hydrophobic patch on the surface of the trefoil domain which may provide a binding cleft for either a sugar group or an aromatic amino acid side-chain during intermolecular interactions. Three trefoil peptides are currently recognized in humans: TFF1 (also known as pS2) and TFF3 (intestinal trefoil factor), which each have a single trefoil motif and both of which homodimerize, and TFF2 (human spasmolytic polypeptide, hSP), which has two motifs and does not require dimerization for its activity (Williams and Wright, 1997). Each map to the same cluster on chromosome 21 (3, 4). TFF2 is glycosylated via an N-linkage, and TFF2 polymorphisms are known to exist (5). The trefoil motifs in TFF2 form a tight structure compared to the loose structure formed by the dimerization of TFF1; this may lead to differences in binding and function (6). The trefoil peptides localize predominately to the gastrointestinal tract (7). Each of the peptides are found in the intestinal tract within the ductal lumenal cells of Brunner’s glands and in goblet cells near the surface of crypts. Both TFF1 and TFF2 are also found within the stomach in mucous neck cells of the antrum and fundus and in the epithelium of the pancreas and gall bladder. Peptides are packaged by the Golgi apparatus in mucin-producing cells and secreted with mucous into the protective layer above the epithelium. TFF3 also co-localizes with oxytocin-producing cells in the hypothalamus (8) and is found in epithelial cells of the female genitourinary tract (7). TFF1 is expressed in rat hippocampus, cortex, and cerebellum (9), in cultured mouse astrocytes (10) and normal breast ductal epithelium (11). TFF2 has not been reported to be expressed in normal tissues outside the gastrointestinal tract. TFF1 was originally isolated from human breast carcinoma cell lines (12, 13). Pathologic expression of this trefoil peptide since has been demonstrated in a variety of adenocarcinomas, especially those with a mucinous histology, in breast, lung, endometrium, ovary, pancreas, stomach and intestine (summarized in 14). Expression in breast carcinoma is associated with estrogen receptor presence and a favorable clinical outcome (15–17). Expression in lung adenocarcinoma is common, though expression in nonmalignant adenomas, squamous cell carcinoma, and small cell carcinoma is not seen (18). Expression in lung adenocarcinoma, in contrast to breast adenocarcinoma, has been associated with a poorer prognosis (19). The abundance of trefoil peptides in gastrointestinal malignancies is exactly opposite that seen with breast and lung malignancies. TFF1 abundance is decreased in gastric carcinomas and is lost completely in about 50% of these neoplasms (20). A similar decrease in trefoil peptide abundance has been demonstrated in colonic tumors, though complete disappearance is rare (21). The reasons for the wide discrepancies in abundance and prognosis await exploration. The physiological functions of the trefoil peptides are several, and can be divided into two broad categories: mucosal surface protection and repair after injury. Trefoil peptides interact with mucins (22) and regulate mucous viscosity (23), and via these actions may enhance the protective capabilities of the mucosal defense barrier. Both glycosylated and non-glycosylated TFF2 is found in gastric secretions with a diurnal variation, highest at night, suggesting that the maximum cytoprotective effects of trefoil peptides ( Received in original form August 22, 2001 )