Differences in the translation efficiency and mRNA stability mediated by 5'-UTR splice variants of human SP-A1 and SP-A2 genes.

Surfactant protein A (SP-A) plays an important role in host defense, modulation of inflammatory processes, and surfactant-related functions of the lung. The human SP-A (hSP-A) locus consists of two functional genes, SP-A1 and SP-A2. Several hSP-A 5'-untranslated region (UTR) splice variants for each gene have been characterized and shown to be translated in vitro and in vivo. In this report, we investigated the role of hSP-A 5'-UTR splice variants on SP-A production and molecular mechanisms involved. We used in vitro transient expression of hSP-A 5'-UTR constructs containing luciferase as the reporter gene and quantitative real-time PCR to study hSP-A 5'-UTR-mediated gene expression. We found that 1) the four (A'D', ABD, AB'D', and A'CD') 5'-UTR splice variants under study enhanced gene expression, by increasing luciferase activity from 2.5- to 19.5-fold and luciferase mRNA from 4.3- to 8.8-fold compared with the control vector that lacked hSP-A 5'-UTR; 2) all four 5'-UTR splice variants studied regulated mRNA stability. The ABD variant exhibited the lowest rate of mRNA decay compared with the other three constructs (A'D', AB'D', and A'CD'). These three constructs also exhibited significantly lower rate of mRNA decay compared with the control vector; 3) based on the indexes of translational efficiency (luciferase activity/mRNA), ABD and AB'D' exhibited higher translational efficiency compared with the control vector, whereas the translational efficiency of each A'D' and A'CD' was lower than that of the control vector. These findings indicate that the hSP-A 5'-UTR splice variants play an important role in both SP-A translation and mRNA stability.