The cooperation of cis-elements during M-cadherin promoter activation

M-cadherin is a skeletal muscle-specific transmembrane protein mediating the cell-cell adhesion of myoblasts during myogenesis. It is expressed in the proliferating satellite cells and highly induced by myogenic regulatory factors (MRFs) during terminal myogenic differentiation. Several conserved cis-elements, including 5 E-boxes, 2 GC boxes, and 1 conserved downstream element (CDE) were identified in the M-cadherin proximal promoter. We found that E-box-3 and -4 close to the transcription initiation site (TIS) mediated most of its transactivation by MyoD, the strongest myogenic MRF. Including of any one of the other E-boxes restored the full activation by MyoD, suggesting an essential collaboration between E-boxes. Stronger activation of M-cadherin promoter than that of muscle creatine kinase (MCK) by MyoD was observed regardless of culture conditions and the presence of E47. Furthermore, MyoD/E47 heterodimer and MyoD ∼ E47 fusion protein achieved similar levels of activation in differentiation medium (DM), suggesting high affinity of MyoD/E47 to E-boxes 3/4 under DM. We also found that GC boxes and CDE positively affected MyoD mediated activation. The CDE element was predicted to be the target of the chromatin-modifying factor Meis1/Pbx1 heterodimer. Knockdown of Pbx1 significantly reduced the expression level of M-cadherin, but increased that of N-cadherin. Using ChIP assay, we further found significant reduction in MyoD recruitment to M-cadherin promoter when CDE was deleted. Taken together, these observations suggest that the chromatin-modifying function of Pbx1/Meis1 is critical to M-cadherin promoter activation before MyoD is recruited to E-boxes to trigger transcription.