The role of retinoic acid in glioma growth control

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Flynn, Patricia Margaret
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University College Cork
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Tumours of the central nervous system are known as gliomas, arising from the astrocytes, oligodendrocytes, ependymal cells or from glial progenitor cells. Although a relatively rare diagnosis, there is disproportionate morbidity associated with a glioma diagnosis, owing to its diffuse and infiltrative nature and partly, in the restricted accessibility of the tumour to treatment. Several obstacles prevent effective treatment, namely, the blood brain barrier, peripheral inactivation of systemic treatment, outward tumoural convection pressures, and cancer stem cell resistance. Despite therapeutic advances, the relapse rate and the mortality linked to glioma remains high, with most patients surviving less than 2 years following diagnosis. The terminal differentiation of malignant cells using a differentiation agent such as retinoic acid (RA) could be a promising scientific advance in the treatment of this disease. Endogenous retinoic acid is the primary active metabolite of vitamin A. It is a small, lipophilic differentiation agent that acts as a ligand for a family of nuclear receptors (RARs) to regulate the expression of target genes. The main family of nuclear receptors comprises three genes, RARα, RARβ and RARγ, each coding for multiple isoforms. The selective stimulation of these isoforms with RA has been shown to mainly inhibit cellular proliferation but is also known in some cases to promote such proliferation. The use of all-trans retinoic acid as an agent of differentiation has also been highly successful in the treatment of acute promyelocytic leukaemia. The canonical retinoic acid pathway involves proteins that are responsible for the conversion of precursors of RA, their transport and the transcription of genes downstream of the RARs. In this thesis, gene expression of components of the canonical RA signalling pathway was analysed from existing microarray data for a panel of 1100 gliomas of various histological grades. The analysis was performed on the R2 Genomics Analysis and Visualization Platform. Expression of individual genes was extracted from the datasets and analysed according to WHO grade. The results showed that the expression of key components of the pathway was altered in high grade gliomas compared to the lower grades. The expression levels of RBP1, RBP2, and RBP3 (involved in the transport of retinol) were significantly altered in high grade glioma, with an increase in the expression levels of RBP1 and RPB2 and the decrease in expression level of RBP3. The genes involved in the oxidation of retinol to retinal, ADH1A, ADH1B, ADH1C, ADH4, ADH7, RDH5 RDH10, RDH11 and RDH16 were all significantly lower in high grade glioma. The expression levels of the genes involved in the oxidation of retinal to retinoic acid, ALDH8A1, and ALDH1A1 (RALDH1) are also significantly reduced in high grade glioma. The genes involved in the intranuclear transcription of the RA pathway are affected by high grade glioma. Retinoic acid receptor genes RARA, RARB, RARG, retinoid X receptor genes RXRA, RXRB, RXRG, transcriptional co-activator genes EP300, NCOA1, NCOA2, transcriptional co-repressor genes HDAC2, NCOR1, and NRPI1 are all significantly lower in high grade glioma with an increase in the expression of transcriptional repressor HDAC1. There is also significantly reduced expression of CYP26B1, involved in the metabolism of RA. Together, lower expression of the enzymes responsible for the intracellular formation of RA from its precursors and its intranuclear transcriptional machinery could potentially lead to a reduction in RA signalling. This suggests that the targeted activation of the RA pathways in gliomas with reduced signalling capacity could be used to regulate cancerous growth. Using two established glioblastoma cell lines, the growth altering properties of retinoic acid, some of its synthetic derivatives and specific retinoic acid receptor isoforms was assessed. Results show that manipulation of the retinoic acid signalling pathway by selectively stimulating different isoforms of the RARs can lead to changes in growth patterns that differ depending on the cellular context. Opposing growth patterns were produced in the two glioblastoma cell lines, A172 and U87-MG in response to ATRA, with growth suppression of the A172 cell line and proliferation of the U87-MG cell line. More detailed insight into the growth responses mediated by isoform specific retinoids and overexpression of isoforms highlighted the potential for suppressing the growth of the cell lines by targeting individual receptor isoforms. While further research is needed, these results show that targeting specific receptors in cell lines can lead to growth reduction and may pave the way to the use of isoform selective retinoids in the treatment of glioma.
Glioblastoma , Retinoic acid receptors , Retinoic acid , Gene expression analysis , Differential isoforms
Fylnn, P. M. 2021. The role of retinoic acid in glioma growth control. MRes Thesis, University College Cork.
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