Endocrine Abstracts

Introduction:: The HPA axis is essential for mammalian life. Proopiomelanocortin (POMC), expressed in corticotroph cells of the pituitary, is the master activator of the axis, and a classical negative feedback loop exists whereby glucocorticoids from the adrenals repress its expression. Glucocorticoids are commonly prescribed medicines (10 million prescriptions in UK per year), but when used long-term, suppression of the HPA axis is a major side effect, with risk of life-threatening adrenal failure even after withdrawal of treatment.

Hypothesis:: Glucocorticoids induce de novo DNA methylation of POMC or POMC-activating transcription factors, accounting for long-term inhibition even after withdrawal of treatment.

Methods:: Methylation patterns were studied by bisulphite sequencing in murine ACTH-expressing (AtT20) and non-expressing (3T3-L1) cell lines, before and after long-term culture in dexamethasone or vehicle. Gene expression was assessed by qPCR.

Results:: Expression of POMC and Tpit was dramatically reduced in AtT20 cells, whereas that of NeuroD1 and Pitx1, two POMC-activating transcription factors, was not. Dexamethasone treatment was associated with de-novo methylation of the Tpit promoter region, which was methylated in 3T3-L1 cells under normal culture conditions. Tpit and POMC expression was rescued by the histone deacetylase inhibition.

Conclusion:: To our knowledge this is the first example of de novo DNA methylation induced by glucocorticoids. These data may have important implications for use of epigenetic modifying agents after chronic glucocorticoid therapy to allow recovery of the HPA axis.