Dioxin-induced thrombocyte aggregation in zebrafish

Protective effect of prostacyclin against pre-cardiac edema caused by 2,3,7,8-tetrachlorodibenzo-p-dioxin and a thromboxane receptor agonist in developing zebrafish

The role of prostaglandin pathways has been suggested in some toxicological responses to dioxins. Cyclooxygenase type 2b (COX2b), thromboxane synthase, and the thromboxane receptor (TP) pathway have been implicated in mediating 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced pre-cardiac edema in developing zebrafish at 55 h post fertilization (hpf). Pre-cardiac edema refers to edema located in a small cavity between the heart and body wall of zebrafish eleutheroembryos. In the present study, we assessed the role of prostacyclin, which counteracts some biological effects of thromboxane, in TCDD-induced pre-cardiac edema. Pre-cardiac edema induced by TCDD exposure (0.5 and 1 ppb) beginning at 24 hpf was markedly inhibited by exposure to beraprost (5 and 10 μM), a prostacyclin receptor (IP) agonist, beginning at 33 hpf. The preventive effect of beraprost was reduced by exposure to CAY10441 (10 μM), an IP antagonist starting at 33 hpf. Knockdowns of the IP receptor (IP-KD) with two different morpholinos caused edema by themselves and enhanced pre-cardiac edema caused by the low concentration of TCDD (0.5 ppb). On the other hand, short exposure beginning at 48 hpf to U46619 (7.5-30 μM), a thromboxane receptor agonist caused pre-cardiac edema, which was inhibited by exposure beginning at 48 hpf to both ICI-192,605 (24 μM), a TP antagonist, and beraprost. Expression of prostacyclin synthase was increased from fertilization, plateaued by 48 hpf, and was maintained until at least 96 hpf. Overall, the results demonstrate a preventive effect of prostacyclin on TCDD-induced pre-cardiac edema in developing zebrafish.

TCDD and omeprazole prime platelets through the aryl hydrocarbon receptor (AhR) non-genomic pathway

The role of the aryl hydrocarbon receptor (AhR) in hemostasis has recently gained increased attention. Here, we demonstrate, by qRT-PCR and western blot, that human platelets express both AhR mRNA and AhR protein. AhR protein levels increase in a dose dependent manner when incubated with either 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) or omeprazole. Treatment of platelets with puromycin blocks increased AhR protein synthesis in the presence of AhR activators. Additionally, treatment of platelets with either activator results in phosphorylation of p38MAPK and cPLA2, two key signaling molecules in platelet activation pathways. Using the AhR competitive inhibitors alpha naphthoflavone and CH-223191, we show that phosphorylation of p38MAPK is AhR dependent. Further, inhibition of p38MAPK blocks downstream cPLA2 phosphorylation induced by TCDD or omeprazole. Treatment with AhR activators results in platelet priming, as demonstrated by increased platelet aggregation, which is inhibited by AhR antagonists. Our data support a model of the platelet AhR non-genomic pathway in which treatment with AhR activators results in increased expression of the AhR, phosphorylation of p38MAPK and cPLA2, leading to platelet priming in response to agonist.

Breaking the mold: transcription factors in the anucleate platelet and platelet-derived microparticles

Platelets are small anucleate blood cells derived from megakaryocytes. In addition to their pivotal roles in hemostasis, platelets are the smallest, yet most abundant, immune cells and regulate inflammation, immunity, and disease progression. Although platelets lack DNA, and thus no functional transcriptional activities, they are nonetheless rich sources of RNAs, possess an intact spliceosome, and are thus capable of synthesizing proteins. Previously, it was thought that platelet RNAs and translational machinery were remnants from the megakaryocyte. We now know that the initial description of platelets as "cellular fragments" is an antiquated notion, as mounting evidence suggests otherwise. Therefore, it is reasonable to hypothesize that platelet transcription factors are not vestigial remnants from megakaryocytes, but have important, if only partly understood functions. Proteins play multiple cellular roles to minimize energy expenditure for maximum cellular function; thus, the same can be expected for transcription factors. In fact, numerous transcription factors have non-genomic roles, both in platelets and in nucleated cells. Our lab and others have discovered the presence and non-genomic roles of transcription factors in platelets, such as the nuclear factor kappa β (NFκB) family of proteins and peroxisome proliferator-activated receptor gamma (PPARγ). In addition to numerous roles in regulating platelet activation, functional transcription factors can be transferred to vascular and immune cells through platelet microparticles. This method of transcellular delivery of key immune molecules may be a vital mechanism by which platelet transcription factors regulate inflammation and immunity. At the very least, platelets are an ideal model cell to dissect out the non-genomic roles of transcription factors in nucleated cells. There is abundant evidence to suggest that transcription factors in platelets play key roles in regulating inflammatory and hemostatic functions.