Release of GPI-anchored membrane proteins by a cell-associated GPI-specific phospholipase D

Although many glycosylphosphatidylinositol (GPI)-anchored proteins have been observed as soluble forms, the mechanisms by which they are released from the cell surface have not been demonstrated. We show here that a cell-associated GPI-specific phospholipase D (GPI-PLD) releases the GPI-anchored, complement regulatory protein decay-accelerating factor (DAF) from HeLa cells, as well as the basic fibroblast growth factor-binding heparan sulfate proteoglycan from bone marrow stromal cells. DAF found in the HeLa cell culture supernatants contained both [3H]ethanolamine and [3H]inositol, but not [3H]palmitic acid, whereas the soluble heparan sulfate proteoglycan present in bone marrow stromal cell culture supernatants contained [3H]ethanolamine. 125I-labeled GPI-DAF incorporated into the plasma membranes of these two cell types was released in a soluble form lacking the fatty acid GPI-anchor component. GPI-PLD activity was detected in lysates of both HeLa and bone marrow stromal cells. Treatment of HeLa cells with 1,10-phenanthroline, an inhibitor of GPI-PLD, reduced the release of [3H]ethanolamine-DAF by 70%. The hydrolysis of these GPI-anchored molecules is likely to be mediated by an endogenous GPI-PLD because [3H]ethanolamine DAF is constitutively released from HeLa cells maintained in serum-free medium. Furthermore, using PCR, a GPI-PLD mRNA has been identified in cDNA libraries prepared from both cell types. These studies are the first demonstration of the physiologically relevant release of GPI-anchored proteins from cells by a GPI-PLD.     

Mouse decay-accelerating factor: selective and tissue-specific induction by estrogen of the gene encoding the glycosylphosphatidylinositol-anchored form

Neonatal exposure of mice to estrogen (diethylstilbestrol) results in a high incidence (90%) of uterine tumor later in life. In an effort to screen for estrogen-regulated genes in the uterus of the neonatal mouse, we have isolated a murine homologue of the human decay-accelerating factor (DAF), a glycosylphosphatidylinositol (GPI)-anchored membrane glycoprotein and a member of the regulators of complement activation family of proteins that function to prevent autologous complement-mediated tissue damage. The induced mouse DAF cDNA has a 64% sequence identity with the human counterpart at the nucleotide level and a 50% identity in the deduced amino acid sequence. It consists of 390 amino acids and contains four short consensus repeats of internal homology characteristic of human DAF. It also contains a hydrophobic C-terminal that most likely serves as a signal for GPI anchor attachment. Sequence comparison with the recently reported mouse DAF cDNAs confirmed that the estrogen-inducible gene corresponds to the mouse GPI DAF gene. The induction of mouse DAF by estrogen is tissue specific and can be mimicked by the antiestrogen tamoxifen. Furthermore, the regulation of uterine DAF expression by estrogen is limited to the GPI DAF gene. The transmembrane DAF gene is not expressed in the mouse uterus, either with or without estrogen stimulation. These results suggest that the two mouse DAF genes are differentially regulated, and that the GPI-anchored DAF may play important roles in estrogen responses and other physiologic or pathophysiologic processes of the female reproductive system.     

A point mutation of alanine 163 to threonine is responsible for the defective secretion of high molecular weight kininogen by the liver of brown Norway Katholiek rats

To clarify the mechanism of the secretion defect of high molecular weight kininogen (HK) and low molecular weight kininogen (LK) by the liver of Brown Norway (B/N) Katholiek rats causing plasma kininogen deficiency, we cloned cDNAs for HK from cDNA libraries of the livers of B/N Katholiek and B/N Kitasato rats. A point mutation of G to A at nucleotide 487 was found in the cDNA of B/N Katholiek rats by sequence analysis of the cDNAs (including the entire HK-coding region) obtained from both strains. Both B/N Katholiek and B/N Kitasato rat cDNA fragments were introduced into a eukaryotic vector, pRc/CMV, to construct their respective expression plasmid, which was used to transfect COS-1 cells. At 24 h of incubation, the culture medium of COS-1 cells transfected with the B/N Katholiek rat cDNA contained only 10% of the HK antigen that was found in COS-1 cells transfected with the B/N Kitasato rat cDNA. More HK antigen was retained in the former cells. Moreover, cells transfected with B/N Katholiek rat cDNA, in which the A at nucleotide 487 was artificially replaced by G, secreted a significant amount of HK into the medium. These results suggest that a point mutation of G to A at nucleotide 487, which causes a substitution of Ala163 to Thr in the heavy chain of HK and LK, is responsible for the defective secretion of HK and LK by the liver of B/N Katholiek rats.     

Identification of a truncated form of the CC chemokine CK beta-8 demonstrating greatly enhanced biological activity

A new CC chemokine, designated CKbeta-8 or myeloid progenitor inhibitor factor-1, was recently identified in a large scale sequencing effort and was cloned from a human aortic endothelial library. CKbeta-8 cDNA encodes a signal sequence of 21 amino acids, followed by a 99-amino acid predicted mature form. CKbeta-8 was expressed and purified from a baculovirus insect cell expression system, which resulted in the identification of different N-terminal variants of the secreted chemokine. The three major forms (containing amino acids 1-99, 24-99, and 25-99 of the secreted chemokine) showed a large variation in potency. CKbeta-8 activated both monocytes and eosinophils to mobilize intracellular calcium; however, the shortest form of CKbeta-8 (25-99) was >2 orders of magnitude more potent than the longest form. Cross-desensitization experiments in both monocytes and eosinophils suggested that the CCR1 receptor was probably the predominant receptor that mediates this chemokine's physiologic response. However, incomplete desensitization was encountered in both cell systems, suggesting involvement of an additional receptor(s). Interestingly, the short form of CKbeta-8 was the most potent chemotactic chemokine that we have ever evaluated in the monocyte system (EC50 = 54 pM). However, in contrast to its action on monocytes, CKbeta-8 was a very poor chemotactic factor for eosinophils.