Molecular mechanisms conferring resistance/sensitivity to glucocorticoid-induced apoptosis during cytotoxic stress

• Main Author: Lynch, James Thomas
• Published: University of Manchester 2009
• Subjects: 571.6; Transcription : Acute Lymphocytic Leukaemia : Glucocorticoid Receptor : Apoptosis : Hypoxia : Phosphorylation
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.503678

During stress conditions, glucocorticoids are secreted and exert most of their physiological responses by binding to and modulating the transcriptional activity of the glucocorticoid receptor (GR). Once activated, GR can regulate numerous cellular processes including inflammation, development, growth, metabolism and apoptosis. Although glucocorticoids have been used in the treatment of leukaemia for over 50 years, with the molecular mechanisms by which steroids exert their pro-apoptotic effect, the pathways responsible for the development of resistance to glucocorticoid treatment, as well as their role in the programmed cell death in other tissue types have not been precisely defined. Research has demonstrated that glucocorticoid-induced apoptosis requires a transcriptionally active form of GR and is executed by the induction of the intrinsic pathway of apoptosis. In addition, GR is regulated by diverse types of cytotoxic stress; including UV irradiation and hypoxia, which alter the receptor’s transcriptional activity through multiple mechanisms. These include post-translational modifications, subcellular localisation and interaction of the receptor with co-regulator proteins. The aims of this study are to identify novel members of the Bcl-2 family that are regulated at the transcriptional level by GR in both leukaemia and other tissue types where glucocorticoids promote cell survival. In addition, the molecular crosstalk between signalling pathways activated by cytotoxic stress conditions and the mechanisms by which they differentially modulate the apoptotic response will be investigated. Results obtained in this study have identified putative glucocorticoid response elements in the promoters of the BH3-only pro-apoptotic gene NOXA and the anti-apoptotic gene Mcl-1 and confirmed that both NOXA and Mcl-1 are direct GR transcriptional targets. The glucocorticoid-mediated expression of NOXA and Mcl-1 alters their protein-protein interaction pattern, leading to the subsequent destabilisation of Mcl-1 in cell lines that undergo glucocorticoid-induced apoptosis. Investigation into the effects that other cytotoxic stress pathways have on GR function have revealed that serine 226 phosphorylation of GR by JNK occurs in a rapid and transient manner. Phosphorylation has inhibitory effects on the transcription of GR targets in a gene-specific manner, including the differential regulation of NOXA gene expression. During hypoxia, glucocorticoids differentially regulate the GR and HIF-1 target genes, NOXA and Mcl-1, altering the apoptotic response. This study has provided additional insight into the molecular mechanisms that govern glucocorticoid-induced programmed cell death and revealed mechanisms by which glucocorticoids and cytotoxic stress pathways crosstalk, regulating apoptosis.