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Myeloperoxidase: its structure and expression during myeloid differentiation
Abstract:
Myeloperoxidase (MPO) is a major protein present in myeloid cells and is used by these cells to help kill microbes. The human promyelocytic HL-60 line can be induced to differentiate to granulocytes or macrophagelike cells. Poly (A) containing RNA was isolated from HL-60 granulocytes, HL-60 macrophages, HL-60 blasts, and normal human granulocytes. The mRNA was translated in a reticulocyte lysate system in the presence of 35S-methionine. The MPO was precipitated from the lysate with rabbit IgG antiserum to human MPO. The resulting precipitate from HL-60 blasts gave a major band of radioactivity of approximately 77,000 daltons and another band at approximately 46,000 daltons on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The MPO identity of the labeled bands was confirmed by cold competition. The relative mRNA activity expressed as a percentage of radioactivity incorporated into MPO (77,000-dalton band) as compared with total trichloracetic acid (TCA) precipitable radioactivity was 0.2%. Negligible mRNA activity for MPO was present in HL-60 granulocytes, HL-60 macrophages, and normal human granulocytes. Pulse-chase experiments showed that MPO was an approximate 75,000-dalton major band and 77,000-dalton minor band of radioactivity after HL-60 blasts were labeled for 1/2 hour with 35S-methionine and the cell lysate immunoprecipitated and subjected to SDS-PAGE. The chase experiments (one to 24 hours) showed that the 77,000- and 75,000-dalton bands of radioactivity were replaced with two major bands (55,000 and 15,000 daltons) and one minor band (approximately 39,000 daltons) of radioactivity. Six-hour 35S-methionine labeling experiments showed that the relative rate of MPO synthesis compared with total TCA precipitable radioactivity was 0.5% in HL-60 blasts and almost negligible in HL-60 macrophages and granulocytes, normal human granulocytes, and B-lymphocytes. The KG-1 myeloblasts and KG-1a early myeloblasts synthesized a small amount of the 75,000-dalton MPO protein. Although HL-60 cells no longer synthesized MPO after differentiation, HL-60 granulocytes and HL-60 macrophages continued to contain MPO as measured by enzyme activity.(ABSTRACT TRUNCATED AT 400 WORDS)
Insights
Myeloperoxidase (MPO) is synthesized by HL-60 blasts but not by differentiated HL-60 cells. This study investigates MPO synthesis and processing in myeloid cell differentiation.
Area of Science:
- Biochemistry
- Molecular Biology
- Cell Biology
Background:
- Myeloperoxidase (MPO) is a key enzyme in myeloid cells, crucial for microbial killing.
- The human promyelocytic HL-60 cell line serves as a model for studying myeloid differentiation.
- Understanding MPO synthesis during differentiation is vital for hematological research.
Purpose of the Study:
- To investigate the synthesis and processing of Myeloperoxidase (MPO) in differentiating HL-60 cells.
- To compare MPO mRNA activity and protein synthesis in HL-60 blasts versus differentiated HL-60 cells and normal granulocytes.
- To elucidate the post-translational modifications and degradation pathways of MPO in myeloid cells.
Main Methods:
- Isolation of poly (A)-containing RNA from various HL-60 cell types and normal granulocytes.
- In vitro translation of mRNA using a reticulocyte lysate system with 35S-methionine labeling.
- Immunoprecipitation of MPO using rabbit IgG antiserum to human MPO.
- Analysis of radiolabeled MPO by SDS-PAGE and pulse-chase experiments.
- Enzyme activity assays to measure MPO levels in different cell types.
Main Results:
- HL-60 blasts showed significant MPO mRNA activity, translating to approximately 77,000 and 46,000 dalton MPO precursors.
- Differentiated HL-60 cells (granulocytes, macrophages) and normal granulocytes exhibited negligible MPO mRNA activity.
- Pulse-chase experiments revealed MPO processing from initial ~75/77 kDa bands to ~55/15 kDa bands over time.
- HL-60 blasts synthesized MPO at 0.5% of total protein synthesis, while differentiated cells showed negligible synthesis.
- KG-1 myeloblasts and KG-1a early myeloblasts synthesized small amounts of MPO protein.
Conclusions:
- Myeloperoxidase (MPO) synthesis is restricted to the blast stage in HL-60 cells and is downregulated upon differentiation.
- Post-translational processing and potential degradation pathways significantly alter MPO protein forms during myeloid cell maturation.
- Differentiated HL-60 cells and normal granulocytes retain MPO enzyme activity despite lacking active synthesis, indicating pre-existing protein levels.