TGFβ1 enhances MAD1 expression and stimulates promoter-bound Pol II phosphorylation: basic functions of C/EBP, SP and SMAD3 transcription factors

We observed that in addition to G-CSF, the cytokine transforming growth factor β (TGFβ1) rapidly induced the expression of MAD1 mRNA and protein in promyelocytic tumor cells. Moreover we found that C/EBP and SP transcription factors cooperated in regulating the expression of MAD1. This cooperativity was dependent on the respective binding sites in the proximal promoter, with the CCAAT boxes being bound by C/EBPα/β heterodimers. Both C/EBP and SP transcription factors bound constitutively to DNA without obvious changes in response to TGFβ1. In addition SMAD3 stimulated the MAD1 reporter, cooperated with C/EBPα and was bound to the core promoter region. Thus SMAD3 appears to be a potential link between TGFβ1 signaling and C/EBP regulated promoter activity. Moreover TGFβ1 stimulated the phosphorylation of polymerase II at serine 2 and its progression into the gene body, consistent with enhanced processivity.Our findings suggest that C/EBP and SP factors provide a platform of transcription factors near the core promoter of the MAD1 gene that participate in mediating signal transduction events emanating from different cytokine receptors. SMAD3, a target of TGFβ1 signaling, appears to be functionally relevant. We suggest that a key event induced by TGFβ1 at the MAD1 promoter is the recruitment or activation of cofactors, possibly in complex with C/EBP, SP, and SMAD3 transcriptional regulators, that control polymerase activity.The MYC/MAX/MAD network of transcriptional regulators is essential to control many aspects of cell physiology [1]. MYC was originally identified as oncogene in several different chicken retroviruses. Subsequently the three human MYC genes, MYC, MYCN and MYCL were found deregulated in the large majority of human tumors [2]. The potent capacity of MYC to transform cells has also been supported by a large number of studies in both primary cells and established cell lines and in animal models. Central to the ability to transform cells is MYC's function a