Platelet Factor 4 (CXC Chemokine Ligand 4) Differentially Regulates Respiratory Burst, Survival, and Cytokine Expression of Human Monocytes by Using Distinct Signaling Pathways

Abstract

Platelet factor 4 (PF4; CXCL4) is an abundant platelet α-granule CXC chemokine with unique functions. Although lacking a chemotactic activity, PF4 initiates a signal transduction cascade in human monocytes leading to the induction of a broad spectrum of acute and delayed functions including phagocytosis, respiratory burst, survival, and the secretion of cytokines. Surprisingly, although these monocyte functions are well defined, only very limited information exists on the specific signaling pathways that are involved in the regulation of these biological responses. By using specific inhibitors and direct phosphorylation/activation studies, we show in the present study that PF4-mediated respiratory burst is dependent on a very rapid activation of PI3K, Syk, and p38 MAPK. Moreover, monocyte survival and differentiation instead is controlled by a delayed activation of Erk, with an activity peak after 6 h of stimulation. The inhibition of Erk completely reverted PF4-mediated protection against apoptosis. Finally, even though JNK is rapidly activated in PF4-treated monocytes, it is dispensable for the regulation of survival and respiratory burst. However, PF4-induced up-regulation of chemokine and cytokine mRNA and protein requires a sustained activation of JNK and Erk. Taken together, PF4-stimulated immediate monocyte functions (oxygen radical formation) are regulated by p38 MAPK, Syk, and PI3K, whereas delayed functions (survival and cytokine expression) are controlled by Erk and JNK.

CXCL4‐induced monocyte survival, cytokine expression, and oxygen radical formation is regulated by sphingosine kinase 1

“…By contrast to SKI, DMS pretreatment of unstimulated cells also results in decreased ROS formation, and increased cell death (data not shown). These data indicate that CXCL4-mediated protection from apoptosis is controlled by SphK.In a recent report we have demonstrated that several cytokines and chemokines were induced in CXCL4-treated monocytes [3]. To examine whether cytokine/chemokine expression is also regulated by SphK, monocytes were preincubated in the presence or absence of a constant dosage of SKI (9 mM).…”

“…In a recent report we have demonstrated that several cytokines and chemokines were induced in CXCL4-treated monocytes [3]. To examine whether cytokine/chemokine expression is also regulated by SphK, monocytes were preincubated in the presence or absence of a constant dosage of SKI (9 mM).…”

“…Immunodetection was performed as described in detail elsewhere [3,42]. Band intensities on blot membranes were quantified using Odyssey software 2.1 and presented either as integrated intensities or fold induction from unstimulated control.…”

“…However, a successful defense requires not only the presence of monocytes at inflammatory sites but also fast and effective mechanisms for their activation. In previous reports we described monocyte activation by CXC chemokine ligand 4 (CXCL4; platelet factor 4) [1][2][3]. CXCL4 belongs to the family of CXC-chemokines and is rapidly released in high concentrations upon platelet activation [4,5].…”

“…Studying signaling pathways involved in CXCL4-mediated monocyte activation, we could demonstrate that besides three members of the MAPK-family (p38, Erk, and JNK), Syk and PI3K became activated. While oxygen radical formation requires p38, Syk, and PI3K activity, apoptosis is regulated by Erk, and cytokine/chemokine production by Erk and JNK [3].Over the past decade, it has become abundantly clear that sphingolipids and their metabolites are key signaling molecules. Sphingolipids are ubiquitous components of cell membranes and their metabolites ceramide, sphingosine, and sphingosine-1-phosphate (S1P) have important physiological functions, including regulation of cell growth and survival (for review, see references [10][11][12][13]).…”