Receptor desensitization, or decreased responsiveness of a receptor to agonist stimulation, represents a regulatory process with the potential to have a significant impact on cell behavior. P2Y(2), a G-protein-coupled receptor activated by extracellular nucleotides, undergoes desensitization at many tissues, including the vascular endothelium. Endothelial cells from a variety of vascular beds are normally exposed to extracellular nucleotides released from damaged cells and activated platelets. The purpose of the present study was to compare P2Y(2) receptor desensitization observed in endothelial cells derived from bovine retina, a model of microvascular endothelium, and human umbilical vein endothelial cells (HUVECs), a model of a large blood vessel endothelium. P2Y(2) receptor desensitization was monitored by following changes in UTP-stimulated intracellular free Ca(2 +) in single cells using fura-2 microfluorometry. Both endothelial cell models exhibited desensitization of the P2Y(2) receptor after stimulation with UTP. However, the cells differed in the rate, dependence on agonist concentration, and percentage of maximal desensitization. These results suggest differential mechanisms of P2Y(2) receptor desensitization and favors heterogeneity in extracellular nucleotide activity in endothelial cells according to its vascular bed origin.
"This suggests that co-activation of the two signaling pathways could lead to intensified but differential physiological effects in situ, with the nucleotides as fast but transient effectors, and EGF as a delayed but more persistent effector. We have not investigated the mechanism underlying the decay of the response to nucleotides, but agonist-induced desensitization accompanied by receptor internalization has been demonstrated (Hoffmann et al., 2008; Sanabria et al., 2008). "
[Show abstract][Hide abstract] ABSTRACT: The adult subventricular zone (SVZ) contains astrocyte-like stem cells capable of generating new neurons for the olfactory bulb. Adult neurogenesis is driven by a variety of signal systems that can induce synergistic or opposing cellular responses. It is therefore important to gain insight into the underlying downstream signaling pathways. We have previously shown that the nucleotides ADPbetaS and UTP induce rapid Ca2+ transients in cultured SVZ-derived adult neural progenitors and augment growth-factor-mediated progenitor cell proliferation. Here, we investigated signaling pathways elicited by ADPbetaS, UTP and epidermal growth factor (EGF). All three agonists elicit ERK1/2 and CREB phosphorylation but the temporal characteristics differ between the nucleotides and EGF. Differentiation of the progenitors alters the receptor profile. Oligodendrocytes and young neurons, but not astrocytes, lose responsiveness to the agonists. Inhibition experiments are indicative of an ADPbetaS-elicited EGF receptor transactivation. Whereas UTP acts via the P2Y2 receptor, ADPbetaS exerts its function via the P2Y1 receptor and the P2Y13 receptor. Our data demonstrate that nucleotides and EGF induce converging, but also differential, intracellular signaling pathways and suggest that they carry the potential to act synergistically in the control of cell proliferation and cell survival in adult neurogenesis.
[Show abstract][Hide abstract] ABSTRACT: Microvascular endothelial cells play an essential role in inflammatory diseases. Functional heterogeneity between microvascular segments in normal organ homeostasis has been appreciated for a long time, and more recent studies have revealed heterogeneity in endothelial reactivity to inflammatory stimuli as well. This review summarizes the state-of-the-art knowledge regarding endothelial cell responses to the proinflammatory cytokines tumor necrosis factor alpha, interleukin-1beta, and the bacterial product lipopolysaccharide. It focuses on similarities and differences in reactivity between endothelial cell subsets in vitro and endothelial cells in their pathophysiological environment in vivo, and culminates into a mainly theoretical model of possible intracellular control mechanisms that can assist to ultimately explain the molecular causes of endothelial heterogeneity. The last part of this review contains some pharmacological considerations, and, with the aim to further unravel the molecular basis of in vivo endothelial heterogeneity, descriptions of new techniques that will be essential to achieve this.
Seminars in Thrombosis and Hemostasis 04/2010; 36(3):246-64. DOI:10.1055/s-0030-1253448 · 3.88 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Each segment of the vascular system has its own function, based on its location in the body. The endothelial cells lining
these vascular segments are an intrinsic part of this segmental variation, and it is not surprising that throughout the vascular
bed they have a highly heterogenic appearance. This chapter starts with a description of the heterogenic phenotype of endothelial
cells in the body in quiescent conditions. Focus is next on endothelial engagement in inflammatory processes and how the microvascular
endothelial cells in the different organs and within an organ respond to tumor necrosis factor a, interleukin-1, and lipopolysaccharide
as inflammatory stimuli. Studies on endothelial responsiveness both in vitro in culture systems and in vivo in the intact body are discussed. From a pharmacological point of view, knowledge of the molecular basis of heterogeneity
in endothelial cell behavior will be critical for successful drug development to counteract endothelial cell engagement in
disease. This chapter therefore concludes with a short overview of technological advancements that may assist in unveiling
the mechanisms responsible. When applied to address microvascular endothelial heterogeneity, major steps forward in endothelial
are anticipated that will assist in defining the right molecular targets for the microvascular segments involved in the pathology
Tobias Hasenberg, Severin Mühleder, Andrea Dotzler, Sophie Bauer, Krystyna Labuda, Wolfgang Holnthoner, Roland Lauster, Uwe Marx
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