Related Experiment Videos
Diminished protein kinase C-activated arachidonate metabolism accompanies rat macrophage differentiation in the lung
M Peters-Golden1, R W McNish, J K Brieland
1Department of Internal Medicine, University of Michigan, Ann Arbor.
Abstract:
Alveolar macrophages (AM) differ from other macrophage (m phi) populations in their profile of eicosanoids synthesized from arachidonic acid (AA)3. Little information is available regarding possible differences in the regulation of AA metabolism among various m phi populations. In our study, we compared the ability of cultured resident rat AM and peritoneal m phi (PM) to release and metabolize AA in response to exogenous activators of protein kinase C (PKC). When stimulated with PMA, prelabeled PM released free [3H]AA in a dose-dependent manner over the concentration range 1 to 100 nM. As assessed by HPLC, PMA-stimulated PM metabolized AA to a variety of predominantly cyclooxygenase products. The dose-dependent synthesis of PGE2 by unlabeled PM stimulated with PMA was confirmed using RIA. The ability of PMA to trigger AA release and metabolism in PM was a function of its capacity to activate PKC, as indicated by the following: 1) an additional activator of PKC, oleoyl acetylglycerol, also triggered PM AA metabolism, whereas phorbol didecanoate, which lacks the ability to activate PKC, did not; 2) two structurally unrelated inhibitors of PKC activation (staurosporine and sphinganine) both abrogated PMA induced AA release in PM; and 3) pretreatment for 18 h with high dose PMA (used to deplete cellular PKC), but not phorbol didecanoate, rendered PM refractory to subsequent PMA stimulation of AA release. In contrast to PM, AM cultured in identical fashion failed to release or metabolize AA in response to either PMA or oleoyl acetylglycerol. PM and AM were also compared for their ability to release extracellular superoxide anion in response to PMA; once again, PM exhibited significantly greater release than did AM. Inasmuch as this unresponsiveness to activation of PKC distinguishes AM from other m phi populations, we conclude that it is a unique consequence of m phi differentiation in the lung. Moreover, because both AA metabolism and the respiratory burst are affected, this refractoriness appears to reflect a defect at some proximal level in PKC-mediated signaling.
Insights
Alveolar macrophages (AM) do not metabolize arachidonic acid (AA) when stimulated via protein kinase C (PKC), unlike peritoneal macrophages (PM). This unresponsiveness in AM is a unique feature of lung macrophage differentiation.
Area of Science:
- Immunology
- Cell Biology
- Biochemistry
Background:
- Alveolar macrophages (AM) exhibit distinct eicosanoid synthesis profiles compared to other macrophage populations.
- Limited data exists on the regulatory mechanisms governing arachidonic acid (AA) metabolism across diverse macrophage populations.
- Understanding these differences is crucial for elucidating macrophage function in pulmonary and systemic immunity.
Purpose of the Study:
- To compare the capacity of cultured rat alveolar macrophages (AM) and peritoneal macrophages (PM) to release and metabolize AA.
- To investigate the role of protein kinase C (PKC) activation in regulating AA metabolism in AM and PM.
- To identify potential differences in PKC-mediated signaling pathways between AM and PM.
Main Methods:
- Cultured rat AM and PM were stimulated with exogenous activators of protein kinase C (PKC), such as phorbol 12-myristate 13-acetate (PMA) and oleoyl acetylglycerol.
- Arachidonic acid (AA) release was quantified using radiolabeling ([3H]AA) and High-Performance Liquid Chromatography (HPLC).
- Eicosanoid production (e.g., PGE2) was measured by Radioimmunoassay (RIA), and superoxide anion release was assessed.
Main Results:
- Peritoneal macrophages (PM) released and metabolized exogenous arachidonic acid (AA) in a dose-dependent manner upon stimulation with PMA, primarily producing cyclooxygenase products like PGE2.
- PM's AA release and metabolism were confirmed to be PKC-dependent, as evidenced by responses to other PKC activators and inhibition by PKC inhibitors.
- In contrast, alveolar macrophages (AM) failed to release or metabolize AA in response to PMA or oleoyl acetylglycerol, indicating unresponsiveness to PKC activation.
Conclusions:
- Alveolar macrophages (AM) exhibit a unique unresponsiveness to protein kinase C (PKC)-mediated activation of arachidonic acid (AA) metabolism, distinguishing them from peritoneal macrophages (PM).
- This refractoriness in AM appears to be a consequence of macrophage differentiation within the lung microenvironment.
- The defect likely resides at a proximal level in the PKC-mediated signaling pathway, affecting both AA metabolism and the respiratory burst.