Differential biological activities of mammalian Id proteins in muscle cells

Id proteins are helix-loop-helix (HLH) transcription factors that lack DNA- binding domains. These proteins form inactive heterodimers with basic HLH ( bHLH) factors, inhibiting their DNA-binding and transcriptional activities. Consistent with a proposed role for Id proteins as inhibitors of terminal differentiation, Idl and Id3 have been shown to negatively regulate myogene sis in cultured muscle cells. Here we have investigated the possibility tha t Id2 and/or Id4 can act in a similar manner. Surprisingly, while over-expr ession of Id2 resulted in inhibition of differentiation of Sol 8 myoblast c ells, overexpression of Id4 did not. Sol 8 cells stably transfected with Id 4 showed no apparent changes in expression of muscle-specific genes upon di fferentiation. DNA-binding activities present at the muscle creatine kinase (MCK) enhancer E-box and transcription of the MCK enhancer were not altere d in Id4-overexpressing cells, compared with vector-transfected cells. Id2 is also a more potent inhibitor of protein/DNA complex formation at the MCK -R enhancer E-box than Identified in vitro. Therefore, our data support the notion that members of the Id family might be involved in the regulation o f distinct developmental pathways. (C) 1999 Academic Press.