Cloning and expression of rat lung acidic Ca2+-independent PLA2 and its organ distribution

A clone for a rat acidic Ca2+-independent phospholipase A(2) (aiPLA(2)) was isolated from a cDNA library prepared from rat granular pneumocytes with a probe based on the human aiPLA(2) sequence (T. S. Kim, C. S. Sundaresh, S. I. Feinstein, C. Dodia, W. R. Skach, M. K. Jain, T. Nagase, N. Seki, K. Ishikawa, N. Nomura, and A. B. Fisher. J. Biol. Chem. 272: 2542-2550, 1997). In addition, a consensus sequence for mouse aiPLA(2) was constructed from several mouse cDNA clones in the GenBank and dbEST databases. Each sequence codes for a 224-amino acid protein with 88% identity of the amino acids among the three species and conservation of a putative lipase motif (GDSWG). Translation of mRNA produced from the rat clone in a wheat germ system resulted in expression of PLA(2) activity with properties similar to those of the human enzyme, i.e., acidic pH optimum and Ca2+ independence. The localization of aiPLA(2) in rat tissues was studied with the human cDNA probe, polyclonal and monoclonal antibodies, and aiPLA(2) activity. aiPLA(2) is present in the lung as evidenced by high levels of mRNA and protein expression and by enzymatic activity that is inhibited by anti-PLA(2) antibody and by the transition state analog 1-hexadecyl-3-trifluoroethylglycero-sn-2-phosphomethanol (MJ33). Immunocytochemistry showed the presence of aiPLA(2) in alveolar type II cells, alveolar macrophages, and bronchiolar epithelium. In the brain, heart, liver, kidney, spleen, and intestine, aiPLA(2) mRNA content was <50% of that in the lung, immunoreactive protein was not detectable, and enzymatic activity was not inhibited by MJ33 or aiPLA(2) antibody. These results show marked enrichment of aiPLA(2) in the lung compared with the other organs and suggest translational control of aiPLA(2) expression.