Switching TTP on to turn off inflammation in COPD

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Chronic Obstructive Pulmonary Disease (COPD) is the third leading cause of death worldwide. It is a preventable and treatable disease, yet despite this, its incidence is on the rise. COPD treatments are symptomatic based and do not target the root cause of COPD. As such, there is a need for novel anti-inflammatories. The action of the potent anti-inflammatory protein tristetraprolin (TTP) is hypothesised to be through its tightly regulated phosphorylation. Phosphorylation of human TTP at Serine (S) residue 60 and S186 causes TTP to become inactive, while its dephosphorylation at those sites by protein phosphatase 2A (PP2A) causes it to become activated and bind target pro-inflammatory mRNA. To date, this has been proven in mice but has never been directly shown in humans. The benefits of activating PP2A in the inflammatory setting have been shown previously with the use of PP2A activators fingolimod hydrochloride (FTY720) and AAL(s). However, they were not as successful at reducing inflammation as knock-in TTP mice where the sites were unable to be phosphorylated. As such, more efficacious drugs need to be developed. Using the A549 inflammatory cell model, this study aimed to screen 13 newly synthesised FTY720 and AAL(s) analogues for their cytotoxicity and anti-inflammatory impact. Based on these results, 8 new analogues were chosen for further mice study. In addition to this, the parallel reaction monitoring (PRM) proteomics method was developed and optimised for the detection and quantification of TTP phosphorylation at specifically S60 and S186. Using this developed method, it was shown for the very first time that phosphorylation of TTP at S60 and S186 significantly increases in the inflammatory context. This method was then applied to two FTY720 and AAL(s) analogues to explore whether their anti-inflammatory effect was through TTP dephosphorylation at S60 and S186. Both AAL(s) and the analogues were anti-inflammatory due to a mechanism other than TTP which remains unknown. Finally, the potential of utilising the PRM method in the context of exploring the function of other phosphorylation sites on TTP, specifically S218 and S228 by GSK-3β, is discussed. In conclusion, this study has shown that in the context of inflammation in human cells, TTP is significantly phosphorylated at S60 and S186 hinting at the potential of anti-inflammatories which target those two specific sites.
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