The transcriptional regulator peroxisome proliferator-activated receptor-gamma coactivator-1 alpha (PGC-1 alpha) controls mitochondrial biogenesis and energy homeostasis. Although physical exercise induces PGC-1 alpha expression in muscle, the underlying mechanism of this effect has remained incompletely understood. We recently identified a novel MUscle-enriched isoform of PGC-1 alpha transcript (designated PGC-1 alpha-b) that is derived from a previously unidentified first exon. We have now cloned and characterized the human PGC-1 alpha-b promoter. The muscle-specific transcription factors MyoD and MRF4 transactivated this promoter through interaction with a proximal E-box motif. Furthermore, either forced expression of Ca2+- and calmodulin-dependent protein kinase IV (CaMKIV), calcineurin A, or the p38 mitogen-activated protein kinase (p38 MAPK) kinase MKK6 or the intracellular accumulation of cAMP activated the PGC-1 alpha-b promoter in Cultured myoblasts through recruitment of cAMP response element (CRE)-binding protein (CREB) to a putative CRE located downstream of the E-box. Our results thus reveal a potential molecular basis for isoform-specific regulation of PGC-1 alpha expression in contracting muscle. (C) 2009 Elsevier Inc. All rights reserved.