Activation of p53 transcriptional activity by SMRT: a histone deacetylase 3-independent function of a transcriptional corepressor

The silencing mediator of retinoic acid and thyroid hormone receptors (SMRT) is an established histone deacetylase 3 (HDAC3)-dependent transcriptional corepressor. Microarray analyses of MCF-7 cells transfected with control or SMRT small interfering RNA revealed SMRT regulation of genes involved in DNA damage responses, and the levels of the DNA damage marker γH2AX as well as poly(ADP-ribose) polymerase cleavage were elevated in SMRT-depleted cells treated with doxorubicin. A number of these genes are established p53 targets. SMRT knockdown decreased the activity of two p53-dependent reporter genes as well as the expression of p53 target genes, such as CDKN1A (which encodes p21). SMRT bound directly to p53 and was recruited to p53 binding sites within the p21 promoter. Depletion of GPS2 and TBL1, components of the SMRT corepressor complex, but not histone deacetylase 3 (HDAC3) decreased p21-luciferase activity. p53 bound to the SMRT deacetylase activation domain (DAD), which mediates HDAC3 binding and activation, and HDAC3 could attenuate p53 binding to the DAD region of SMRT. Moreover, an HDAC3 binding-deficient SMRT DAD mutant coactivated p53 transcriptional activity. Collectively, these data highlight a biological role for SMRT in mediating DNA damage responses and suggest a model where p53 binding to the DAD limits HDAC3 interaction with this coregulator, thereby facilitating SMRT coactivation of p53-dependent gene expression.

Aberrant expression and modification of silencing mediator of retinoic acid and thyroid hormone receptors involved in the pathogenesis of tumoral cortisol resistance

Ectopic ACTH syndrome (EAS) accounts for 10-15% of cases of Cushing's syndrome and is mostly caused by small cell lung cancers or thymic carcinoids. EAS is characterized by tumoral cortisol resistance, whose underlying mechanism remains unknown. In this study, we reported that silencing mediator of retinoic acid and thyroid hormone receptors (SMRT), a major nuclear corepressor, was aberrantly expressed in ACTH-secreting thymic carcinoids. Overexpression and knockdown of SMRT in the ACTH-secreting AtT-20 cell line demonstrated that SMRT participated in the negative feedback of dexamethasone-mediated suppression of proopiomelanocortin. Posttranslational modification by the small ubiquitin-like modifiers (SUMO), i.e. SUMOylation plays an important role in fine-tuning transcriptional activities. SUMOylation of SMRT was observed in dexamethasone-resistant cell lines. Moreover, overexpression of the deSUMOylation enzyme enhanced the suppression of proopiomelanocortin by dexamethasone in AtT-20 cells. An evolutionarily conserved consensus SUMOylation site was identified close to the histone deacetylase 3 recruiting domain of SMRT, which might interfere with the recruiting process. These results suggested that aberrant expression and modification of SMRT might be involved in the pathogenesis of tumoral cortisol resistance. A therapeutic approach targeting SMRT SUMOylation might be developed for EAS patients.