Myelomonocytic cell lineage expression of the neutrophil elastase gene
Human neutrophil elastase (NE) functions as a powerful serine protease capable of attacking a broad range of proteins. To examine the cellular site(s) of NE gene expression, a 0.65-kilobase cDNA (pPB15) complementary to the coding region of the NE gene was cloned from the cell line U937 using an oligonucleotide based on the known NE protein sequence. The sequence of pPB15 demonstrated that it coded for the 173 C-terminal residues of the 218 amino acids that comprise the mature NE protein, plus an additional 3' 60 base pairs prior to the in-frame stop codon, suggesting the NE mRNA contains sequences for a 20-residue C-terminal "pro" peptide that is not found in the mature protein. Northern analysis using 32P-labeled pPB15 as a probe revealed that neutrophils do not contain detectable NE mRNA transcripts despite the fact that this cell carries large amounts of this protein. Furthermore, resting and activated blood monocytes also contained no detectable NE mRNA transcripts, although these cells also carry detectable NE. In contrast, bone marrow precursor cells contained NE transcripts, suggesting the NE gene is expressed in blood precursor cells. In this regard, evaluation of HL-60 cells, a human cell line with myelomonocytic lineage features, demonstrated NE transcripts in resting cells and increased NE mRNA levels when the cells were induced toward the myelocytic lineage with dimethyl sulfoxide. However, when the HL-60 cells were induced toward the monocytic lineage with phorbol 12-myristate 13-acetate, NE transcripts were lost even though transcripts for interleukin-1 beta were plentiful. Together, these observations are consistent with the concept that the NE gene is not expressed in the blood cells that carry the protein, but in bone marrow precursors that express NE transcripts about the time of commitment to the myelocytic series.