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Production of the effector molecule thymidine by human lung alveolar macrophages
Abstract:
Cultured human alveolar macrophages (HAM phi) were found to produce soluble factors which inhibit tritiated thymidine [( 3H]-TdR) incorporation into tumour cells in vitro. We present evidence that thymidine (TdR) can be detected in HAM phi culture supernatants by thin-layer chromatography. Moreover, TdR secretion by HAM phi is an active process. Using [6-14C]-orotic acid as an early precursor to TdR and [3H]-TMP as the phosphorylation product, we were able to show that cultured HAM phi transformed them into labelled TdR. The very efficient thin-layer chromatography of the labelled metabolites was backed up by high-pressure liquid chromatography of nucleotides. HAM phi produce TdR by de novo synthesis and dephosphorylation. This phenomenon is due to the lack of thymidine kinase in normal mature macrophages. Since TdR, in high concentrations, can inhibit DNA synthesis through the 'TdR blockade' phenomenon, it is suggested that TdR secretion by HAM phi is a mechanism of non-specific modulation of tumour cell growth but highly restricted to the immediate macrophage microenvironment in vivo. The effectiveness of the thymidine gradient may thus be quite narrow, but is worthy of interest.
Insights
Human alveolar macrophages secrete thymidine, inhibiting tumor cell growth. This occurs via de novo synthesis and dephosphorylation due to a lack of thymidine kinase, potentially modulating the tumor microenvironment.
Area of Science:
- Immunology
- Cell Biology
- Biochemistry
Background:
- Cultured human alveolar macrophages (HAM phi) produce soluble factors that inhibit tumor cell proliferation.
- Thymidine (TdR) is implicated as a potential inhibitory factor secreted by macrophages.
Purpose of the Study:
- To investigate the production and mechanism of thymidine secretion by cultured human alveolar macrophages.
- To determine if thymidine secretion by HAM phi influences tumor cell growth in vitro.
Main Methods:
- Detection of thymidine in macrophage culture supernatants using thin-layer chromatography.
- Tracing the de novo synthesis of thymidine using radiolabeled precursors ([6-14C]-orotic acid and [3H]-TMP).
- Analysis of nucleotide metabolism and dephosphorylation pathways via high-pressure liquid chromatography.
Main Results:
- Cultured human alveolar macrophages actively secrete thymidine into their supernatant.
- Macrophages convert orotic acid and TMP into labeled thymidine, indicating de novo synthesis and dephosphorylation.
- The absence of thymidine kinase in mature macrophages explains the accumulation and secretion of thymidine.
Conclusions:
- Human alveolar macrophages synthesize and secrete thymidine, a process facilitated by their lack of thymidine kinase.
- Secreted thymidine can inhibit tumor cell DNA synthesis through the 'thymidine blockade' phenomenon.
- This macrophage-mediated thymidine secretion may represent a localized mechanism for modulating tumor cell growth within the immediate microenvironment.