An E box in the exon 1 promoter regulates insulin-like growth factor-I expression in differentiating muscle cells.
Insulin-like growth factor (IGF)-I expression is subject to complex temporal and spatial regulation. Endocrine synthesis occurs in the liver, where transcription is initiated from promoters located in either exon 1 (P1) or in exon 2 (P2), whereas local transcription is mainly initiated from P1. IGF-I is expressed in a range of tissues and, in particular, is an important regulator of skeletal muscle mass, although the mechanisms of tissue-specific regulation remain to be fully characterized. Gene regulation in skeletal muscle is associated with the E box DNA element (5'-CANNTG-3') recognized by myogenic regulatory factors (MRFs), such as MyoD1. Transcription element profiling identified a hypothetical myogenic E box (sequence 5'-CAGCTG-3') within P1, immediately upstream of the major muscle transcriptional start site, and we sought to test its activity in differentiating C2C12 myoblasts. We found P1-driven IGF-I mRNA expression to be associated with myogenic differentiation and, moreover, that a single base-pair mutation in the E box specifically reduced expression in myofibers. A synthetic enhancer construct containing a triplet repeat of the E box was active in muscle cells and strongly induced in myofibers. The capacity of a double-stranded IGF-I E box probe (but not one bearing a single-base pair alteration) to bind C2C12 nuclear lysates increased with myogenesis, and a transactivation assay demonstrated that the E box was recognized by E protein-MRF heterodimers. Mechanisms of tissue-specific gene activation are of increasing biological interest, and we have identified a cis-element able to direct muscle-specific IGF-I gene expression.
Clore Laboratory, Univ. of Buckingham, Hunter St., Buckingham, MK18 1EG, UK.
A S
AS
McLellan
T
T
Kealey
K
K
Langlands
D000818:Animals; D002454:Cell Differentiation / Q000235:genetics; D002460:Cell Line; D024721:E-Box Elements / Q000235:genetics*; D005091:Exons / Q000235:genetics; D018507:Gene Expression Regulation, Developmental / Q000235:genetics*; D007334:Insulin-Like Growth Factor I / Q000235:genetics*; D051379:Mice; D032446:Myoblasts / Q000166:cytology* / Q000378:metabolism*; D011401:Promoter Regions, Genetic / Q000235:genetics; D015533:Transcriptional Activation
D007334:Insulin-Like Growth Factor I