DNA Methylation and Histone Deacetylation Inhibitors as Potential Therapeutic Agents for the Reconstitution of TGF-β Signaling in Breast Cancer

Abstract

Neoplastic cells show reduced expression of {TGF-beta} receptor type {II} {(RII)} in comparison to the normal breast tissue and benign lesions. Also, loss of {RII} expression correlates with high-tumor grade and invasive carcinomas. This correlation is also reflected in established primary breast carcinoma cell lines. Absence/reduced expression of {RII} was not associated with {RII} gene deletion, amplification, or mutations. The {RII} promoter is devoid of a distinct {TATA} box and is dependent on Sp1 transcription factor for its activity. While {SpI} activates {RII} gene transcription another member of the Sp gene family, Sp3 acts as a repressor, thus suggesting {Sp1/Sp3} activities control {RII} gene expression. Loss of {RII} expression in breast cancer cells involves two epigenetic mechanisms, {DNA} methylation and histone deacetylation. Inhibition of {DNA} methylation by decitabine (5 aza 2'deoxycytidine) stimulated {RII} expression in breast cancer cells. However, loss of {RII} expression was not owing to methylation of the {RII} gene. The {RII} gene in breast cancer cells exhibited reduced Sp1 binding and increased Sp3 binding activities. Inhibition of {DNA} methylation led to increased {SpI} binding as a consequence of increased {SpI} protein stability and simultaneous loss of Sp3 binding owing to decreased Sp3 transcription, thus altering {Sp1/Sp3} activities leading to {RII} expression in breast cancer cells. In contrast, inhibition of histone deacetylation by trichostatin A {(TSA)} induced {RII} expression without changing activator Sp1 and repressor Sp3 binding activities. However, in the {TSA} treated breast cancer cells, histone acetyltransferase {(HATs)} p300 mediated acetylation of Sp3 and converted Sp3 from a repressor to an activator of {RII} gene transcription. {RII} regeneration following inhibition of {DNA} methylation as well as histone deacetylation restored {TGF-beta} response, thus suggesting that {DNA} methylation inhibitor decitabine and histone deacetylation inhibitor {TSA} either alone or in combination may be used as potential therapeutic agents for the reconstitution of {TGF-beta} tumor suppressor pathway in breast carcinomas.