Release of mucins from cultured airway surface epithelial cells can be stimulated by extracellular ATP via a P2-purinergic receptor-mediated mechanism (K. C. Kim and B. C. Lee. 1991. Br. J. Pharmacol. 103:1053-1056). In this report, we studied the mechanism by which extracellular ATP induces the mucin release. We found that: (1) ATP increased both mucin release and generation of inositol phosphates in a dose-dependent fashion, and their dose-effect relationships were almost superimposed; (2) the increases in both mucin release and the phosphatidylinositol phosphate (PI) turnover by extracellular ATP were partially, but almost equally, blocked by the pretreatment with pertussis toxin (42% for mucin release and 44% for PI turnover). We conclude that in cultured airway goblet cells extracellular ATP stimulates mucin release by a signal transduction mechanism, which seems to involve coupling of ATP-activated P2 purinoceptors with phospholipase C, at least in part, via pertussis toxin-sensitive GTP-binding proteins. This may be an important finding in understanding the regulation of mucin release by airway goblet cells, since a number of agents present in the airway could influence this signal transduction pathway and subsequently modulate the mucin secretion.
Three highly homologous cDNAs encoding human pregnancy-specific beta 1-glycoprotein (SP1) were isolated from a human placental cDNA library. These cDNAs share greater than 90% nucleotide homology in their coding sequences, and greater than 79% of the encoded amino acids are homologous. Proteins encoded by these cDNAs are very similar to members of the carcinoembryonic antigen family and contain repeating domains, conserved disulfide bridges, and beta-sheet structure typical of the immunoglobulin gene superfamily. However, the high degree of sequence homology and relatively lesser degree of glycosylation among the SP1 proteins suggest that they exist as a unique family instead of being members of the CEA family. Both soluble and potentially membrane-bound forms of SP1 proteins were present in the placenta. Northern blot analysis using specific probes confirmed the expression of multiple mRNA species in human term placenta.
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