Transcriptional response to retinoic acid in corticotroph tumor cells
CRH plays a central role in the regulation of the HPA axis, mediating endocrine and behavioural responses to various stressors. At the pituitary level, CRH stimulates POMC gene expression and ACTH synthesis. ACTH overproduction by pituitary adenomas leads to excessive glucocorticoid secretion, which causes Cushing's syndrome associated with high morbidity and ultimately mortality unless treated. Nevertheless, effective drug therapies for Cushing's syndrome do not currently exist. CRH signal transduction pathway plays a causal role in the development of several diseases like anxiety, depressive disorders and stress-associated pathologies. However, all existing antidepressive drugs possess high degree of non-responders and undesirable side effects. Thus, novel methods to regulate and inhibit CRH signalling might be useful for the treatment of Cushing's syndrome and affective disorders.
In AtT-20 cells, we have demonstrated that RA inhibits basal and CRH induced -POMC transcription and -ACTH secretion and decreases cell proliferation. Moreover, when athymic nude mice were inoculated with corticotroph tumor cells, control animals develop large subcutaneous corticotroph tumors, whereas no tumors were observed in mice injected with RA -treated cells. Administration of RA to mice that already had experimental ACTH-secreting tumors resulted in the inhibition of tumor growth. Plasma levels of ACTH and cortisol were reduced in RA treated mice, compared with controls. In an ongoing study, preliminary data from dogs suffering from Cushing's disease show that the treatment with RA decreases ACTH and cortisol levels and improves clinical symptoms in respect to control animals treated with ketoconazole. Nevertheless a key limitation of retinoid therapy is that the concentrations required for anticancer actions cause several adverse effects, including defects in liver function, conjunctivitis, mucositis and severe photosensitivity.
Fig. 1 Effect of retinoic acid (RA) treatment in nude mice injected with AtT-20 corticotrophinoma cells and in dogs suffering from Cushing‘s disease. In both cases, treatment with retinoic acid decreased tumor growth and serum corticosterone/ cortisol levels
Despite all the recent information about the action of RA, the molecular pathways governing RA action are still not elucidated. Since RA inhibits CRH signalling in corticotroph cells, understanding the mechanisms of action of RA might provide new targets for the treatment of Cushing's disease and affective disorders. The aim of our study is to provide novel insights into the molecular pathways governing RA action and in this way to identify genes that inhibit the CRH-signal transduction pathway. For this reason we analyzed the transcriptional response to RA in AtT-20 cells by applying cDNA microarray technology. In this experiment we compared AtT-20 cells responsive and resistant to RA treatment, in basal and RA treated conditions. The expression of several genes was found to be differentially modulated by RA in responsive AtT-20 cells with respect to basal without being changed in retinoid resistant AtT-20 cells. Microarray data of some candidate genes were confirmed using RT-PCR. Among these genes we selected AF08 because its activation inhibits the effects of CRH. The involvement of AF08 on pituitary pathogenesis and affective disorders is currently under investigation.
Research groups involved:
Marcelo Páez-Pereda, Ph.D., Affectis Pharmaceutical AG, Munich, Germany
Eduardo Arzt, Ph.D., University of Buenos Aires, Argentina
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