Induction of transforming growth factor-β receptor type II expression in estrogen receptor-positive breast cancer cells through SP1 activation by 5-aza-2'-deoxycytidine

Previous studies suggest that estrogen receptor-positive (ER⁺) breast cancer cells acquire resistance to transforming growth factor-β (TGF-β) because of reduced expression levels of TGF-β receptor type H (RII). We now report that treatment of ER⁺ breast cancer cells with the DNA methyltransferase inhibitor 5-aza-2'-deoxycytidine (5-aza-2'-dC) leads to accumulation of RII transcript and protein in three different cell lines. RII induction restored TGF-β response in MCF-7L breast cancer cells as indicated by the enhanced activity of a TGF-β responsive promoter-reporter construct (p3TP-Lux). A transiently transfected RII promoter-reporter element (RII- chloramphenicol acetyltransferase) showed an increase in activity in the 5- aza-2'-dC-treated MCF-7L cells compared with untreated cells, suggesting the activation of a transactivator of RII transcription. Using electrophoretic mobility shift assays, the enhanced binding of proteins from 5-aza-2'-dC- treated MCF-7L nuclear extracts to radiolabeled Sp1 oligonucleotides was demonstrated. An RII promoter-chloramphenicol acetyltransferase construct containing a mutation in the Sp1 site was not expressed in the 5-aza-2'-dC- treated MCF-7L cells, further demonstrating that induction of Sp1 activity by 5-aza-2'-dC in the MCF-7L cells was critical to RII expression. Northern analysis indicated that 5-aza-2'-dC treatment did not affect the Sp1 transcript levels. Western blot analysis revealed an increase of Sp1 protein in the 5-aza-2'-dC-treated MCF-7L cells, but there was no change in the c- Jun levels. Studies after cyclohexamide treatment suggested an increase in the Sp1 protein stability from the 5-aza-2'-dC-treated MCF-7L extracts compared with untreated control extracts. These results indicate that the transcriptional repression of RII in the ER⁺ breast cancer cells is caused by suboptimal activity of Sp1, whereas treatment with 5-aza2'-dC stabilizes the protein thus increasing steady-state Sp1 levels and thereby leads to enhanced RII transcription and subsequent restoration of TGF-β sensitivity.