Peripheral blood platelet-derived microparticles (PMPs) circulate in blood and may
interact directly with target cells affecting their various biological functions.
To investigate the effect of human PMPs on hematopoiesis, we first phenotyped them
for expression of various surface molecules and subsequently studied various biological
responses of normal stem/progenitor (CD34(+)) and more differentiated precursor cells
as well as several leukemic cell lines to PMPs.
We found that, in addition to platelet-endothelium attachment receptors (CD41, CD61
and CD62), PMPs express G-protein-coupled seven transmembrane-span receptors such
as CXCR4 and PAR-1; cytokine receptors including TNF-RI, TNF-RII, and CD95; and ligands
such as CD40L and PF-4. Moreover, we found that several of these receptors could be
transferred by PMPs to the membranes of normal as well as malignant cells and observed
that PMPs: 1) chemoattract these cells, 2) increase their adhesion, proliferation,
and survival, and 3) activate in these cells various intracellular signaling cascades
including MAPK p42/44, PI-3K-AKT, and STAT proteins. The biological effects of PMPs
were only partly reduced by heat inactivation or trypsin digest, indicating that,
in addition to the protein components of PMPs, lipid components are also responsible
for their biological activity.
We conclude that PMPs modulate biological functions of hematopoietic cells and postulate
that they play an important but as yet not fully understood role in intercellular
cross-talk in hematopoiesis. Further studies, however, are needed to identify the
PMP components that exert specific biological effects.