Role of transcription factors in inflammatory lung diseases

Abstract

NF-κB was first identified as a nuclear factor that binds the decameric DNA sequence 5′-GGGACTTTCC-3′ within the intronic immunoglobulin kappa light chain enhancer in mature B cells.9 Binding of NF-κB to this DNA sequence is responsible for the inducible activity of the enhancer element present in the immunoglobulin gene. NF-κB is a member of the Rel family of proteins, a novel family of ubiquitous transcription factors sharing a common structural motif for DNA binding and dimerisation.10 The Rel family of transcription factors can be defined as a group of proteins that share sequence homology over a 300 amino acid region termed the NF-κB/Rel domain. These proteins can exist either as homodimers or heterodimers, each with a specific affinity for different decamer binding sites fitting the κB motif.10 Several different NF-κB proteins have been characterised.10 The classical activated form of NF-κB is a heterodimer which usually consists of two proteins, a 65 kD polypeptide (p65) subunit (also referred to as Rel A) and a 50 kD polypeptide (p50) subunit. Other subunits, such as p105, a p50 precursor (NF-κB1), p100, a precursor of p52 (NF-κB2), c-Rel and Rel B are present in different forms of NF-κB. These subunits form various homodimers and heterodimers which are likely to have different affinities for DNA and different transactivation potentials of the complexes required to activate different sets of specific genes. In unstimulated cells NF-κB is found in the cytoplasm as an inactive non-DNA binding form, associated with an inhibitor protein called inhibitory κB (IκB) which masks the nuclear translocation signal and so prevents NF-κB from entering the nucleus (fig1).10 Upon cell stimulation—for example, by cytokines—specific kinases phosphorylate the IκB-α (IκB kinase complex) and the p105/p65 complex leading to their ubiquitination (transfer of the ubiquitin molecule).11 Ubiquitination of p105 results in an increase in the rate at which p105 is processed to p50, whereas ubiquitination of IκB-α serves as a signal to the proteasome complex (proteolytic enzymes) to degrade rapidly and completely the IκB unit in the cytoplasm.12 This critical release of NF-κB (p65/p50) from IκB results in the translocation of active p65/p50 into the nucleus where it binds to specific motifs in the promoter regions of target genes. The p50 subunit within the p50/p65 heterodimer greatly facilitates DNA binding, whereas the p65 subunit is required for adequate transactivation.10 A p50/p50 homodimer also binds to κB sites but inhibits, rather than triggers, transcription.10The importance of NF-κB subunits is highlighted by the fact that transgenic animals, which do not possess the genes coding for the p50 or Rel B components of NF-κB, have defects in both immune and inflammatory responses.13