ROLE OF NEWLY SYNTHESIZED CYCLOOXYGENASES IN ASPIRIN RESPONSIVENESS IN THE HUMAN MEGAKARYOBLASTIC CELL LINE MEG01

Abstract

Background: Aspirin is one of the most commonly used drugs in cardiovascular medicine. It interferes with platelet aggregation by inhibiting cyclooxygenases in blood platelets and megakaryocytes. Low-dose aspirin has at least two distinct cellular targets that result in a long-lasting antiplatelet effect over a 24-hour dosing interval. The platelet (COX)-1 is the first target, whose blockage occurs pre-systemically in portal blood and is cumulative with repeated daily dosing. The second target is the megakaryocyte (MK) COX-1 and COX-2 isozymes, whose acetylation is dependent on systemic bioavailability and contributes to the long-lasting duration of thromboxane A2 inhibition, as newly released platelets express acetylated COX isozymes derived from bone marrow progenitors for a significant portion of the 24-hour dosing cycle. Aspirin has been shown to have variability in its response among patients. However, the difference in the recovery of cyclooxygenase activity could be the consequence of a difference in the stability of cyclooxygenases or an enhanced rate of enzyme renewal. Because of the restricted accessibility of MKs in vivo in humans, and to study the effect of aspirin on MKs, the use of a megakaryocytic cell line is therefore of great importance.

Aims: In this project, we aim to detect the expression of several genes in the arachidonic acid metabolism pathway, investigate if aspirin affects their expression and if aspirin modifies the decay of these genes.

Methods: Meg01 cells were treated with 10 µM of ASA for 30 minutes, then plated in a 12 well plate for 24 or 72 hours. Cells were then centrifuged at 300g for 5 minutes followed by the addition of 200 μl trizol or qiazol to the pellet to extract RNA, then RT-PCR was performed. To investigate whether aspirin modifies the half-life of cyclooxygenases mRNA, the levels of gene expression were determined at different time points (0, 1, 3, 18, and 24 hours) by RT-PCR after the addition of actinomycin D, an inhibitor of the transcription machinery.

Results: The expression of several genes in MEG01 and CHRF-288 showed that both MEG01 and CHRF-288 express most of the genes essential for prostanoid synthesis and signaling. Investigating the effects of ASA on gene expression of MEG01, we found that treatment of cells for 24 hours resulted in a statistically significant decrease in COX-2 gene (around 40% decrease ± 10%, p<0.02) compared to vehicle-treated MEG01. No significant change was observed for the other genes. Treatment of the cells for 72 hours resulted in a significant increase in the gene expression of TXAS (around 78% increase ± 27%, p<0.05) compared to vehicle-treated cells. Comparing ASA 24 hours and 72 hours cases, there was a significant increase in expression for most genes in the 72 hours treatment period. Investigating the effect of ASA on RNA stability of these genes, the results show that there was no modification on the mRNA of PTGS1, while there was a stabilization of mRNA at lower levels for COX-2 and PGES1. mRNA for TXAS decreased and gave half-life around 3 hours in the treatment group, while TXAR mRNA in the treatment group was stabilized at 100% preventing further increase in the expression. For EP1, EP2, and EP4 the mRNA was stabilized at lower values in the treatment group compared to the control group. However, in EP3 the half-life of the mRNA was around 6 hours in the presence of ASA compared to around 18 hours without ASA.