Transcriptional regulation of the prothrombin gene in muscle

Thrombin has been shown to mediate neurite retraction in neurons and synapse elimination at the neuromuscular junction. The presence of prothrombin mRNA has been demonstrated in brain and in muscle, but extra-hepatic regulation of the prothrombin gene has not been investigated. To identify cis-acting DNA elements involved in the expression of the prothrombin gene in muscle, we have isolated and analyzed a 1.3-kilobase pair promoter region of the mouse prothrombin gene. Using a series of transiently transfected plasmid constructs in which gene segments of the prothrombin promoter were linked to the luciferase gene, we have identified a sequence, -302 to -210, essential for prothrombin promoter activity in C2-myotubes. Fine analysis revealed that deletion of nucleotides between -248 and -235 eliminated prothrombin promoter activity in C2-myotubes. Furthermore, electrophoretic mobility shift assays demonstrated that a nuclear factor present in C2-myotubes, but not in C2-myoblasts or HepG2 hepatocytes, specifically binds to the sequence -241 to -225. Substitutional mutation of nucleotides -237 to -231 abolished myotube-specific promoter activity and inhibited the nuclear factor binding. Quantitative reverse transcription polymerase chain reaction demonstrated the expression of prothrombin mRNA in myotubes, but not in myoblasts, of primary, C2, and G8 muscle cells. This result correlates with the lack of prothrombin promoter activity in C2-myoblasts. The data thus suggest that a myotube-specific nuclear factor binds to a cis-acting sequence encompassing the core nucleotides -237 to -231 and plays a critical role in muscle-specific, differentiation-dependent expression of the mouse prothrombin gene.