Human SP-A 3'-UTR variants mediate differential gene expression in basal levels and in response to dexamethasone

Human surfactant protein A (SP-A) is encoded by two genes (SP-A1, SP-A2), and each is identified with several alleles. SP-A is involved in normal lung function, innate immunity, inflammatory processes, and is regulated by glucocorticoids. We investigated the role of 3'-untranslated region (UTR) of 10 SP-A variants on gene expression using transient transfection of 3'-UTR constructs in the human lung adenocarcinoma cell line NCI-H441. We found: 1) both basal mRNA and protein levels of the reporter gene of SP-A 3'-UTR constructs are significantly (P < 0.01) reduced compared with controls (vector pGL3 and surfactant protein B pGL3) and that differences exist among alleles; and 2) after dexamethasone (Dex) treatment (100 nM for 16 h), mRNA was reduced (31-51%). Seven alleles showed a significant decrease (P < 0.05) in mRNA, and three did not. Reporter activity was also decreased, from 17% (1A(1)) to 38% (1A), with six alleles showing a significant decrease. The data indicate that the 3'-UTR of SP-As play a differential role in SP-A basal expression and in response to Dex. Therefore, a careful consideration of individual use of steroid treatment may be considered.     

Impact of ozone exposure on the phagocytic activity of human surfactant protein A (SP-A) and SP-A variants

Surfactant protein A (SP-A) enhances phagocytosis of Pseudomonas aeruginosa. SP-A1 and SP-A2 encode human (h) SP-A; SP-A2 products enhance phagocytosis more than SP-A1. Oxidation can affect SP-A function. We hypothesized that in vivo and in vitro ozone-induced oxidation of SP-A (as assessed by its carbonylation level) negatively affects its function in phagocytosis (as assessed by bacteria cell association). To test this, we used P. aeruginosa, rat alveolar macrophages (AMs), hSP-As with varying levels of in vivo (natural) oxidation, and ozone-exposed SP-A2 (1A, 1A0) and SP-A1 (6A2, 6A4) variants. SP-A oxidation levels (carbonylation) were measured; AMs were incubated with bacteria in the presence of SP-A, and the phagocytic index was calculated. We found: 1) the phagocytic activity of hSP-A is reduced with increasing levels of in vivo SP-A carbonylation; 2) in vitro ozone exposure of hSP-A decreases its function in a dose-dependent manner as well as its ability to enhance phagocytosis of either gram-negative or gram-positive bacteria; 3) the activity of both SP-A1 and SP-A2 decreases in response to in vitro ozone exposure of proteins with SP-A2 being affected more than SP-A1. We conclude that both in vivo and in vitro oxidative modifications of SP-A by carbonylation reduce its ability to enhance phagocytosis of bacteria and that the activity of SP-A2 is affected more by in vitro ozone-induced oxidation. We speculate that functional differences between SP-A1 and SP-A2 exist in vivo and that the redox status of the lung microenvironment differentially affects function of SP-A1 and SP-A2.     

Exploring the role of 5' alternative splicing and of the 3'-untranslated region of cathepsin B mRNA

The cysteine peptidase cathepsin B is responsible for connective tissue breakdown in several diseases. The pathological expression of cathepsin B may depend on the structure of its mRNA. We investigated the translational efficiency of the cathepsin B mRNA untranslated regions (UTRs) using fusion constructs to green fluorescent protein (GFP) and luciferase. Transfection of fusion constructs with GFP and luciferase containing the full-length 5'-UTR, the variant lacking exon 2, and that lacking exons 2 and 3 into mammalian cells, resulted in modulation of the biosynthetic rate of cathepsin B in a cell-specific manner. Constructs missing these exons were biosynthetically more efficient than the full-length counterpart. Luciferase was cloned upstream of the 3'-UTR, downstream of the 5'-UTR, or sandwiched between the 5'- and the 3'-UTR. The UTRs of cathepsin B downregulated luciferase biosynthesis moderately when present individually, with the 3'-UTR being more efficient than the 5'-UTR, and downregulated it even more when present simultaneously. A truncated cathepsin B-GFP chimeric product derived from the 5'-UTR missing exons 2 and 3 induced cell death. The increased biosynthetic rate and abnormal trafficking of cathepsin B observed in pathologies such as cancer and osteoarthritis may depend on alternative splicing of pre-mRNA.     

Expression of the gene for mitoribosomal protein S12 is controlled in human cells at the levels of transcription, RNA splicing, and translation.

The human gene RPMS12 encodes a protein similar to bacterial ribosomal protein S12 and is proposed to represent the human mitochondrial orthologue. RPMS12 reporter gene expression in cultured human cells supports the idea that the gene product is mitochondrial and is localized to the inner membrane. Human cells contain at least four structurally distinct RPMS12 mRNAs that differ in their 5'-untranslated region (5'-UTR) as a result of alternate splicing and of 5' end heterogeneity. All of them encode the same polypeptide. The full 5'-UTR contains two types of sequence element implicated elsewhere in translational regulation as follows: a short upstream open reading frame and an oligopyrimidine tract similar to that found at the 5' end of mRNAs encoding other growth-regulated proteins, including those of cytosolic ribosomes. The fully spliced (short) mRNA is the predominant form in all cell types studied and is translationally down-regulated in cultured cells in response to serum starvation, even though it lacks both of the putative translational regulatory elements. By contrast, other splice variants containing one or both of these elements are not translationally regulated by growth status but are translated poorly in both growing and non-growing cells. Reporter analysis identified a 26-nucleotide tract of the 5'-UTR of the short mRNA that is essential for translational down-regulation in growth-inhibited cells. Such experiments also confirmed that the 5'-UTR of the longer mRNA variants contains negative regulatory elements for translation. Tissue representation of RPMS12 mRNA is highly variable, following a typical mitochondrial pattern, but the relative levels of the different splice variants are similar in different tissues. These findings indicate a complex, multilevel regulation of RPMS12 gene expression in response to signals mediating growth, tissue specialization, and probably metabolic needs.     

Characterization of a human surfactant protein A1 (SP-A1) gene-specific antibody; SP-A1 content variation among individuals of varying age and pulmonary health

The human surfactant protein A (SP-A) locus consists of two functional genes (SP-A1, SP-A2) with gene-specific products exhibiting qualitative and quantitative differences. The aim here was twofold: 1) generate SP-A1 gene-specific antibody, and 2) use this to assess gene-specific SP-A content in the bronchoalveolar lavage fluid (BALF). An SP-A1-specific polyclonal antibody (hSP-A1_Ab(68-88)_Col) was raised in chicken, and its specificity was determined by immunoblot and ELISA using mammalian Chinese hamster ovary (CHO) cell-expressed SP-A1 and SP-A2 variants and by immunofluorescence with stably transfected CHO cell lines expressing SP-A1 or SP-A2 variants. SP-A1 content was evaluated according to age and lung status. A gradual decrease (P < 0.05) in SP-A1/SP-A ratio was observed in healthy subjects (HS) with increased age, although no significant change was observed in total SP-A content among age groups. Total SP-A and SP-A1 content differed significantly between alveolar proteinosis (AP) patients and HS, with no significant difference observed in SP-A1/SP-A ratio between AP and HS. The cystic fibrosis (CF) ratio was significantly higher compared with AP, HS, and noncystic fibrosis (NCF), even though SP-A1 and total SP-A were decreased in CF compared with most of the other groups. The ratio was higher in culture-positive vs. culture-negative samples from CF and NCF (P = 0.031). A trend of an increased ratio was observed in culture-positive CF (0.590 +/- 0.10) compared with culture-positive NCF (0.368 +/- 0.085). In summary, we developed and characterized an SP-A1 gene-specific antibody and used it to identify gene-specific SP-A content in BALFs as a function of age and lung health.     

SP-A1 and SP-A2 variants differentially enhance association of Pseudomonas aeruginosa with rat alveolar macrophages

Chronic airway inflammation caused by Pseudomonas aeruginosa is an important feature of cystic fibrosis (CF). Surfactant protein A (SP-A) enhances phagocytosis of P. aeruginosa. Two genes, SP-A1 and SP-A2, encode human SP-A. We hypothesized that genetically determined differences in the activity of SP-A1 and SP-A2 gene products exist. To test this, we studied association of a nonmucoid P. aeruginosa strain (ATCC 39018) with rat alveolar macrophages in the presence or absence of insect cell-expressed human SP-A variants. We used two trios, each consisting of SP-A1, SP-A2, and their coexpressed SP-A1/SP-A2 variants. We tested the 6A(2) and 6A(4) alleles (for SP-A1), the 1A(0) and 1A alleles (for SP-A2), and their respective coexpressed SP-A1/SP-A2 gene products. After incubation of alveolar macrophages with P. aeruginosa in the presence of the SP-A variants at 37 degrees C for 1 h, the cell association of bacteria was assessed by light microscopy analysis. We found 1) depending on SP-A concentration and variant, SP-A2 variants significantly increased the cell association more than the SP-A1 variants (the phagocytic index for SP-A1 was approximately 52-95% of the SP-A2 activity); 2) coexpressed variants at certain concentrations were more active than single gene products; and 3) the phagocytic index for SP-A variants was approximately 18-41% of the human SP-A from bronchoalveolar lavage. We conclude that human SP-A variants in vitro enhance association of P. aeruginosa with rat alveolar macrophages differentially and in a concentration-dependent manner, with SP-A2 variants having a higher activity compared with SP-A1 variants.     

Post-transcriptional regulation in higher eukaryotes: the role of the reporter gene in controlling expression

We have investigated whether reporter genes influence cytoplasmic regulation of gene expression in tobacco and Chinese hamster ovary (CHO) cells. Two genes, uidA encoding beta-glucuronidase (GUS) from Escherichia coli and Luc, encoding firefly luciferase (LUC), were used to analyze the ability of a cap, polyadenylated tail, and the 5'- and 3'-untranslated regions (UTR) from tobacco mosaic virus (TMV) to regulate expression. The regulation associated with the 5' cap structure and the TMV 5'-UTR, both of which enhance translational efficiency, was reporter gene-independent. The poly(A) tail and the TMV 3'-UTR, which is functionally equivalent to a poly(A) tail, increase translational efficiency as well as mRNA stability. The regulation associated with these 3' ends was highly reporter gene-dependent; their effect on GUS expression was almost an order of magnitude greater than that on LUC expression. In tobacco, the tenfold reporter gene effect on poly(A) tail or TMV 3'-UTR function could not be explained by a differential impact on mRNA stability; GUS and LUC mRNA half-life increased only twofold when either the poly(A) tail or TMV 3'-UTR was present. In CHO cells, however, GUS mRNA was stabilized to a greater extent by a poly(A) tail or the TMV 3'-UTR than was LUC mRNA.