Mechanisms involved in the bidirectional effects of protein kinase C activators on neutrophil responses to leukotriene B4.
Three protein kinase C (PKC) activators (PMA, mezerein, and a diacylglycerol) had bidirectional effects on human polymorphonuclear neutrophil (PMN) degranulation responses to leukotriene (LT) B4. Lower concentrations of the three agents enhanced, whereas higher concentrations inhibited, release of lysozyme and beta-glucuronidase stimulated by the arachidonic acid metabolite. Contrastingly, the activators inhibited but never enhanced LTB4-induced Ca2+ transients. We examined the causes for these varying effects. Each PKC activator reduced PMN specific binding of [3H]LTB4. Scatchard analyses revealed that PMA (greater than or equal to 0.16 nM) decreased the number of high affinity LTB4 receptors. The receptor losses correlated closely with inhibition of Ca2+ transients. PMN pretreated with 0.5 nM PMA for 5 min retained approximately 50% of their high affinity LTB4 receptors. These cells responded to 10 nM LTB4 with reduced but still substantial rises in cytosolic Ca2+, enhanced PKC mobilization, and increased granule enzyme release. The latter two effects appeared calcium-dependent because sequential exposure to PMA and LTB4 did not synergistically stimulate PKC mobilization or degranulation in PMN that were: 1) Ca2(+)-depleted; 2) challenged with 5 nM PMA; or 3) treated with LTB4 for 5 min before PMA. Each of the latter treatments completely interfered with the extent or timing of LTB4-induced Ca2+ transients. Accordingly, we suggest that the response-specific, bidirectional effects of PKC activators on LTB4 result from two opposing mechanisms. First, PKC activators down-regulate LTB4 high affinity receptors and thereby reduce those PMN responses that are not elicited by activated PKC (i.e., Ca2+ transients). Second, LTB4, by elevating cytosolic Ca2+, increases the amount of PKC mobilized by PKC activators and thereby promotes PKC-dependent responses (e.g., degranulation). The two mechanisms may be pertinent to the bidirectional effects of PKC activators on various other agonists. Furthermore, PKC, by down-regulating receptors, may serve as a physiologic stop signal for terminating function and producing a poststimulatory state of desensitization.
O'Flaherty
J T
JT
Department of Medicine, Wake Forest University Medical Center, Winston-Salem, NC 27103.
Redman
J F
JF
Jacobson
D P
DP
D002118:Calcium / Q000502:physiology; D015536:Down-Regulation; D004789:Enzyme Activation / Q000187:drug effects; D006801:Humans; D066298:In Vitro Techniques; D015759:Ionomycin / Q000494:pharmacology; D007975:Leukotriene B4 / Q000494:pharmacology*; D009504:Neutrophils / Q000502:physiology*; D010980:Platelet Membrane Glycoproteins; D011493:Protein Kinase C / Q000378:metabolism*; D011956:Receptors, Cell Surface / Q000378:metabolism; D043562:Receptors, G-Protein-Coupled; D011971:Receptors, Immunologic / Q000378:metabolism; D018102:Receptors, Leukotriene B4; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D013997:Time Factors
D010980:Platelet Membrane Glycoproteins; D011956:Receptors, Cell Surface; D043562:Receptors, G-Protein-Coupled; D011971:Receptors, Immunologic; D018102:Receptors, Leukotriene B4; C039351:platelet activating factor receptor; D007975:Leukotriene B4; D015759:Ionomycin; D011493:Protein Kinase C; D013755:Tetradecanoylphorbol Acetate; D002118:Calcium