Immortalization of primary human keratinocytes by the helix-loop-helix protein, Id-1

Basic helix-loop-helix (bHLH) DNA-binding proteins have been demonstrated t o regulate tissue-specific transcription within multiple cell lineages. The Id family of helix-loop-helix proteins does not possess a basic DNA-bindin g domain and functions as a negative regulator of bHLH proteins. Overexpres sion of Id proteins within a variety of cell types has been shown to inhibi t their ability to differentiate under appropriate conditions. We demonstra te that ectopic expression of Id-1 leads to activation of telomerase activi ty and immortalization of primary human keratinocytes. These immortalized c ells have a decreased capacity to differentiate as well as activate phospho rylation of the retinoblastoma protein. Additionally, these cells acquire a n impaired p53-mediated DNA-damage response as a late event in immortalizat ion, We conclude that bHLH proteins play a pivotal role in regulating norma l keratinocyte growth and differentiation, which can be disrupted by the im mortalizing functions of Id-1 through activation of telomerase activity and inactivation of the retinoblastoma protein.