Molecular cloning and expression of pulmonary lipid phosphate phosphohydrolases

Pulmonary lipid phosphate phosphohydrolase (LPP) was shown previously to hy drolyze phosphatidic acid and lysophosphatidic acid in purified rat lung pl asma membranes. To better investigate the nature of pulmonary LPP isoforms and their role in the lung, LPPs were cloned by RT-PCR from both adult rat lung and type II cell RNA. The RT-PCR generated LPP1 (849 bp), up to three LPP1 variants, and LPP3 (936 bp) cDNAs. The three LPP1 variants include LPP 1a (852 bp) and two novel isoforms, LPP1b (697 bp) and LPP1c (1004 bp). The pulmonary LPP1 and LPP3 isoforms are essentially identical to the previous ly cloned rat liver and intestinal LPPs, respectively, and the LPP1a isofor m has 80% sequence identity to the human homolog. The LPP2 isoform was not detected in lung by RT-PCR. Northern analyses revealed that the mRNAs for L PP1 and LPP3 increase in fetal rat lung in late gestation to day 1 after bi rth. These mRNAs decrease somewhat during the neonatal period but increase slightly during postnatal development. Expression of LPP1, LPP1a, and LPP3 cDNAs in HEK 293 cells established that they encode functional LPP. In cont rast, the novel isoforms LPP1b and LPP1c contain frameshifts that would res ult in premature termination, producing putative catalytically inactive pol ypeptides of 30 and 76 amino acids, respectively. Further investigation of the LPP1b isoform revealed that it was present across a variety of tissues, although at lower levels than LPP1/1a. Transient mammalian expression of L PP1b failed to increase phosphatidate phosphohydrolase activity in HEK 293 cells.