Mechanism of monocyte activation and expression of proinflammatory cytochemokines by placenta growth factor

Abstract

Monocytes from patients with sickle cell disease (SCD) are in an activated state. However, the mechanism of activation of monocytes in SCD is not known. Our studies showed that placenta growth factor (PIGF) activated monocytes and increased mRNA levels of cytokines (tumor necrosis factor-α [TNF-α] and interleukin-1β [IL-1β]) and chemokines (monocyte chemotactic protein-1 [MCP-1], IL-8, and macrophage inflammatory protein-1β [MIP-1β]) in both normal monocytes and in the THP-1 monocytic cell line. This increase in mRNA expression of cytochemokines was also reflected in monocytes derived from subjects with SCD. We studied the PIGF-mediated downstream cellular signaling events that caused increased transcription of inflammatory cytochemokines and chemotaxis of THP-1 monocytes. PIGF-mediated cytochemokine mRNA and protein expression was inhibited by PD98059 and wortmannin, inhibitors of mitogen-activated protein kinase kinase (MAPK/MEK) kinase and phosphatidylinositol-3 (PI3) kinase, respectively, but not by SB203580, a p38 kinase inhibitor. PIGF caused a time-dependent transient increase in phosphorylation of extracellular signal-regulated kinase-1/2 (ERK-1/2), which was completely inhibited by wortmannin, indicating that activation of PI3 kinase preceded MEK activation. PIGF also induced transient phosphorylation of AKT. MEK and PI3 kinase inhibitors and antibody to Flt-1 abrogated PIGF-induced chemotaxis of THP-1 monocytes. Overexpression of a dominant-negative AKT or a dominant-negative PI3 kinase p85 subunit in THP-1 monocytes attenuated the PIGF-mediated phosphorylation of ERK-1/2, cytochemokine secretion, and chemotaxis. Taken together, these data show that activation of monocytes by PIGF occurs via activation of Flt-1, which results in activation of PI3 kinase/AKT and ERK-1/2 pathways. Therefore, we propose that increased levels of PIGF in circulation play an important role in the inflammation observed in SCD via its effects on monocytes. © 2003 by The American Society of Hematology.