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Protein kinase C has both stimulatory and suppressive effects on macrophage superoxide production
W A Phillips1, M Croatto, N Veis
1University of Melbourne, Department of Medicine, Royal Melbourne Hospital, Parkville, Victoria, Australia.
Abstract:
Unlike resident peritoneal macrophages (RPM) or tumor necrosis factor alpha (TNF alpha)-primed bone marrow-derived macrophages (BMM), unprimed BMM do not generate superoxide in response to the protein kinase C (PKC) activator, phorbol myristate acetate (PMA). However, these cells do contain significant levels of PKC activity. In contrast to PMA, zymosan induces the generation of superoxide in unprimed BMM, as well as in TNF alpha-primed BMM and RPM. Staurosporine, a potent PKC inhibitor, failed to affect the zymosan-induced production of superoxide by unprimed and TNF alpha-primed BMM and RPM, in spite of substantial inhibition of PMA-induced superoxide production by the primed BMM and RPM. However, when PKC was depleted from unprimed BMM by prolonged (24 h) treatment with phorbol dibutyrate (PdBt) (10(-7) M) the ability of zymosan to induce the production of superoxide was greatly diminished. Such a result could be interpreted as suggesting a role for PKC in the zymosan-induced response, a conclusion which contrasts with the inhibitor data. However, PKC depletion, in this case, is achieved via the PdBt-induced activation of PKC. It is thus possible that it is the initial activation of PKC, rather than its depletion, that suppresses superoxide production. Consistent with this interpretation, the co-stimulation of unprimed BMM with both zymosan and PMA resulted in a reduced superoxide release compared to zymosan alone. The activation of PKC therefore appears to have a suppressive effect on the generation of superoxide by unprimed cells. We thus conclude that PKC is not required for zymosan-induced superoxide production by either primed or unprimed macrophages and suggest that PKC may be involved in regulatory mechanisms restricting superoxide production by macrophages. However, since PMA alone can initiate the release of superoxide from primed BMM and RPM, it would appear that PKC can mediate both stimulatory and suppressive signals for macrophage superoxide production.
Insights
Protein kinase C (PKC) does not drive superoxide production in macrophages stimulated by zymosan. Instead, PKC activation appears to suppress superoxide generation, suggesting a regulatory role in macrophage responses.
Area of Science:
- Immunology
- Cell Biology
- Biochemistry
Background:
- Macrophages play a crucial role in innate immunity, producing superoxide as a key microbicidal agent.
- Protein kinase C (PKC) is involved in various cellular signaling pathways, including those regulating macrophage activation.
- Different macrophage populations (resident peritoneal macrophages, bone marrow-derived macrophages) exhibit distinct responses to stimuli.
Purpose of the Study:
- To investigate the role of PKC in zymosan-induced superoxide production by macrophages.
- To differentiate between PKC's potential stimulatory and suppressive roles in macrophage activation.
- To elucidate the signaling mechanisms underlying macrophage superoxide generation.
Main Methods:
- Utilized phorbol myristate acetate (PMA) as a PKC activator and staurosporine as a PKC inhibitor.
- Employed zymosan to induce superoxide production in various macrophage populations (unprimed BMM, TNF alpha-primed BMM, RPM).
- Investigated the effects of PKC depletion and co-stimulation with PMA and zymosan on superoxide release.
Main Results:
- Unprimed bone marrow-derived macrophages (BMM) did not produce superoxide in response to PMA, unlike primed BMM and resident peritoneal macrophages (RPM).
- Zymosan induced superoxide production in all tested macrophage types, irrespective of priming or PKC inhibition.
- PKC depletion via phorbol dibutyrate (PdBt) diminished zymosan-induced superoxide production, while co-stimulation with PMA and zymosan reduced superoxide release, indicating a suppressive role for activated PKC.
Conclusions:
- PKC is not essential for zymosan-induced superoxide production in macrophages.
- PKC activation appears to suppress superoxide generation, acting as a negative regulator in unprimed macrophages.
- PKC can mediate both stimulatory (in primed cells with PMA) and suppressive signals in macrophage superoxide production, highlighting its complex regulatory role.