|dc.contributor.author||Patel, Brijeshkumar Shantilal||-|
|dc.description.abstract||Asthma is a chronic lung disease with complex inflammatory disorder. In asthma, airways are remodelled and narrowed, resulting into increased mucus production and contraction of muscles. It has been known that in asthma, airway smooth muscle (ASM) cells act as the primary effector and pivotal cell type involved in asthma. The current treatment suggests β2-agonists as drugs of choice for bronchodilation in ASM cells and long acting β2-agonists (LABA) combined with inhaled corticosteroid (ICS) are used as maintenance therapy in asthma. The mechanism of β2-agonists works via increased levels of cyclic adenosine mono phosphate (cAMP) in ASM cells. It is well established that a rapid increase in cAMP levels in ASM are critical for effective bronchial relaxation however, these intracellular cAMP levels is labile and reduced by phosphodiesterase (PDEs) enzymes. PDEs are intracellular regulators catalysing the breakdown of cAMP and/ cGMP (cyclic guanosine-5'-monophosphate) to their inactive forms. This makes β2-agonists less effective in relaxation of bronchial muscles. The molecular mechanism of PDE is still not clearly known and there are no approved PDE inhibitors in the market for asthma. Our hypothesis is that once we treat ASM cells with PDE inhibitors in addition to LABA and corticosteroid, the effectiveness of LABA and corticosteroid will enhance in airway inflammation.
In addition to the bronchial relaxation effect of β2-agonists, also causes anti-inflammatory properties due, in part, to their ability to upregulate mitogen-activated protein kinase phosphatase 1 (MKP-1) in a cAMP-dependant manner. Given that MKP-1 is cAMP-dependant and inhibition of PDE acts to increase β2-agonist-induced cAMP, it is possible that the presence of PDE inhibitors may enhance β2-adrenoceptor-mediated responses. Additionally, it would enable us to understand the molecular mechanism of these PDE inhibitors as a possible additive therapy for asthma in the future.
Among all PDE isoenzymes inhibition of PDE3 and PDE4 are the main targets for ASM cells. Utilizing primary cultures of ASM cells, we showed in chapter 3 that inhibitors of PDE4, but not PDE3, increase β2-agonist-induced cAMP, MKP-1 mRNA and protein upregulation. When cAMP was increased there was a concomitant increase in MKP-1 levels and significant inhibition of tissue necrosis factor α (TNFa) induced neutrophil chemoattractant cytokine, interleukin -8 (IL-8). This result was consistent with all PDE4 inhibitors examined but not for the PDE3 inhibitors. These findings reinforce cAMP-dependent control of MKP-1 expression in ASM cells.
Since MKP-1 is an integral part of the anti-inflammatory effect of PDE4 inhibitors in ASM cells, we were keen to see the effect of a newer generation of PDE4 inhibitor, Roflumilast N-oxide (RNO) for anti-inflammatory repression. RNO is approved for use in chronic obstructive pulmonary disease (COPD) and has demonstrated anti-inflammatory impact in vivo and in vitro. Up to the present time, however, the effect of RNO on the synthetic function of ASM cells is unknown. We address this herein in chapter 6 and investigate the effect of RNO on β2-adrenoceptor-mediated, cAMP-dependent responses in ASM cells in vitro, and whether RNO enhances steroid-induced repression of inflammation. RNO (0.001-1000 nM) alone had no effect on cAMP production from ASM cells, and significant potentiation of the formoterol-induced cAMP could only be achieved at the highest concentration of RNO tested (1000 nM). At this concentration, RNO exerted a small, but not significantly different, potentiation of formoterol-induced expression of anti-inflammatory MKP-1. Consequently, TNFα induced IL-8 secretion was unaffected by RNO in combination with formoterol. However, as there was the potential for PDE4 inhibitors and LABA to interact with corticosteroids to achieve superior anti-inflammatory efficacy, we examined whether RNO, alone or in combination with formoterol, enhanced the anti-inflammatory effect of dexamethasone. While RNO alone did not significantly enhance cytokine repression achieved with steroids, RNO in combination with formoterol significantly enhanced the anti-inflammatory effect of dexamethasone in ASM cells.
Once we had obtained reliable results from the PDE4 inhibitors we were intrigued to investigate the molecular mechanism of non-specific PDE inhibitors, mainly xanthine derivatives such as theophylline and doxophylline, for their formoterol-induced cAMP effect in ASM cells. We showed in chapter 4 and 5 that theophylline and doxophylline both did not potentiate cAMP release from ASM cells treated with formoterol. Moreover, theophylline (0.1-10 µM) did not increase formoterol-induced MKP-1 mRNA expression nor protein upregulation; consistent with the lack of cAMP generation. Similar trends observed for doxophylline (0.1-100 µM) where cAMP level was not enhanced. In asthma, adenosine receptor activates adenylate cylclase which leads to elevation of cAMP however doxophylline have lower affinity for adenosine receptor so does not induce cAMP. Theophylline (at 10 µM) was anti-inflammatory and repressed secretion of IL-8, produced in response to TNFa. Because theophylline’s effects were independent of PDE4 inhibition or anti-inflammatory MKP-1, we then wished to elucidate the novel mechanisms responsible. We investigated the impact of theophylline on protein phosphatase 2A (PP2A), a master controller of multiple inflammatory signalling pathways, and showed that theophylline increases TNFa induced PP2A activity in ASM cells. In spite of positive clinical data for doxophylline with exhibition of lesser side effects in recent studies, our results do not appear to have any significant repression of IL-8 in ASM cells. Further experiments with less variability in the results and tight control on cell plating techniques may provide better understanding of results and test our hypothesis more fully. Overall our study suggests that understanding the molecular mechanism of various PDE inhibitors in ASM cells offers an additive option as anti-inflammatory therapy in asthma.||en_AU|
|dc.publisher||University of Sydney||en_AU|
|dc.publisher||Faculty of Pharmacy||en_AU|
|dc.rights||The author retains copyright of this thesis. It may only be used for the purposes of research and study. It must not be used for any other purposes and may not be transmitted or shared with others without prior permission.||en_AU|
|dc.subject||airway smooth muscle||en_AU|
|dc.title||Exploring the mechanistic pathway of various phosphodiesterase inhibitors in airway smooth muscle cells||en_AU|
|dc.type.pubtype||Doctor of Philosophy Ph.D.||en_AU|
|dc.description.disclaimer||Access is restricted to staff and students of the University of Sydney . UniKey credentials are required. Non university access may be obtained by visiting the University of Sydney Library.||en_AU|
|Appears in Collections:||Sydney Digital Theses (University of Sydney Access only)|