The major APN transcript of the alveolar type II epithelial cell originates from a unique upstream promoter region

Reverse transcription polymerase chain reaction (RT-PCR) analysis of proteolytic enzymes in cultures of human respiratory epithelial cells

BACKGROUND

Pancreatic proteolytic digestive enzymes are a major extracellular barrier to the sucessful systemic delivery of biopharmaceuticals via the oral route, whereas in health in the lungs these powerful proteases are virtually absent from the extracellular fluids. Despite this, the absorption of some (but not all) natural peptides and proteins from the lungs may be poor, and one has to acknowledge that information on the activity and spatial distribution of proteolytic enzymes in the human lung is scarce. Here, we investigated expression patterns of a series of proteolytic enzymes in several human respiratory cell types on mRNA level in an attempt to better understand the fate of inhaled biopharmaceuticals.

METHODS

The mRNA expression of proteolytic enzymes (i.e., carboxypeptidases: CPA1, CPA2, CPB, CPM; gamma-glutamyltransferases: GGT1, GGT2; angiotensin-converting enzymes: ACE, ACE2; aminopeptidases: APA, APB, APN, APP1, APP2, APP3; endopeptidases: 24.11 (neprilysin), 24.15 (thimet oligopeptidase), 24.18 (meprin A); enteropeptidase; trypsin 1, trypsin 2; neutrophilic elastase; dipeptidyl peptidase 4; gamma-glutamylhydrolase) was investigated by semiquantitative RT-PCR in human bronchial (hBEpC, Calu-3, 16HBE14o-) and alveolar (A549) epithelial cells, respectively. Gastrointestinal Caco-2 cells were used as comparison.

RESULTS

Obvious differences were observed in proteinases' expression pattern between the investigated cell types. Although considered to be of bronchial epithelial phenotype, neither Calu-3 nor 16HBE14o- cells matched the mRNA expression pattern of hBEpC in primary culture. Of all investigated cell lines, Caco-2 expresses the highest number of proteases and peptidases.

CONCLUSIONS

Although mRNA expression does not necessarily signify enzyme functionality, our results provide the first comprehensive analysis of peptidase and protease expression and distribution in human lung epithelial cells and are the basis for further investigations.

Stromal-mediated down-regulation of CD13 in bone marrow cells originating from acute myeloid leukemia patients

The metallopeptidase CD13 is expressed on normal myeloid cells of monocytic and granulocytic origin and on the surface of leukemic blasts in most acute myeloid leukemias (AML). To study the mechanisms regulating lineage restricted CD13 expression in AML we determined normalised CD13 mRNA levels in bone marrow cells and peripheral blood cells of 27 AML patients. Cells of bone marrow origin had lower levels of normalised CD13 mRNA than cells of peripheral blood origin, even though fluorescence intensity and fraction of cells expressing CD13 on the surface was unchanged. In particular, AML patients with very low levels of normalised CD13 mRNA in bone marrow cells showed an increase in CD13 mRNA expression in peripheral blood. To evaluate the effects of bone marrow microenvironment on CD13 mRNA expression, we cultured leukemic myeloid cells with and without murine stromal cells. Bone marrow cells with high and low CD13 surface expression that entered the stromal layers all down-regulated CD13 mRNA expression as compared to cells in suspension above. For peripheral blood cells within stromal layers, CD13 mRNA expression was diminished in only 3 out of 6 cases. The ambiguous effect of stromal cells on peripheral blood cells may illustrate a differentiation-dependent response towards stroma. We determined the polyadenylation status of CD13 mRNA for 9 bone marrow aspirates and 7 peripheral blood samples. Polyadenylation was diminished in bone marrow cells from AML patients with low levels of normalised CD13 mRNA, raising the possibility of involvement of mRNA instability in regulation of CD13 mRNA expression in this subgroup of patients.

Single site polymorphisms and alternative splicing of the human CD13 gene--different splicing frequencies among patients with acute myeloid leukaemia and healthy individuals

Within the haematopoietic system, CD13/aminopeptidase N (APN), a transmembrane glycoprotein, is expressed on the surface of early committed progenitors of granulocytes and monocytes and by all cells of these lineages as they mature. CD13 is expressed on the majority of leukaemic myeloblasts in acute myeloid leukaemia (AML), and on leukaemic lymphoblasts in a small percentage of acute lymphoid leukaemia cases. Thus, anti-CD13 monoclonal antibodies are used as diagnostic markers in leukaemia typing. By systematically amplifying overlapping reverse transcription polymerase chain reaction (RT-PCR) amplicons throughout the CD13 mRNA, we identified two splice variants in which exon 3 and exon 14 were lost. Fourteen healthy individuals and 34 patients with AML were screened for these splice variants. All healthy individuals, and the majority of AML patients, had both splice variants but they represented less than 10% of the total RT-PCR-amplified CD13 product. Increased expression of both truncated CD13 mRNA forms were observed in 6% of AML patients, whereas no detectable exon 3 or exon 14 splice variants could be generated in 26% and 9% of AML patients respectively. The different splicing frequencies may reflect altered processing of pre-mRNA or expansion of certain cell types for some AML patients, even though no correlation existed to blast percentage, FAB classification, surface antigens or cytogenetic characteristics. In addition, we identified an intron of 506 bp between exon 1 and exon 2 as well as two sites of single nucleotide polymorphism with a heterozygosity index of about 0.5, making them useful as genetic markers.

Aminopeptidase-A. II. Genomic cloning and characterization of the rat promoter

Aminopeptidase-A (APA) has a widespread tissue distribution consistent with a role in the metabolism of circulating or locally produced ANG II or CCK-8. APA is also highly expressed in pre-B lymphocytes, but its role in lymphoid cell development is unknown. To begin to understand the basis for cell-specific regulation of APA expression, we sought to clone and characterize the rat gene promoter. Screening of a rat genomic library with a partial rat APA cDNA resulted in isolation of a 12-kb clone found to contain the first exon and >3 kb of 5'-flanking sequence. Primer extension of rat kidney mRNA indicated that the major transcription start site was 312 bp upstream of the translation start codon and 22 bp downstream from a TATA box. Constructs containing portions of the 5'-flanking region placed upstream of a chloramphenicol acetyltransferase reporter gene indicated that expression was cell specific and that high activity could be obtained with constructs containing as little as 110 bp of 5'-flanking region sequence. We further identified an upstream regulatory element between -1063 and -348 that suppressed transcription in a cell-specific manner. This element (termed upstream suppressor of APA, or USA) also suppressed transcription of a heterologous promoter. These results indicate that the organization and regulation of the rat APA is not consistent with it being a housekeeping gene and further suggest that rat APA gene transcription might be regulated through the presence of a novel strong upstream suppressor element.