BS69 has been shown to interact with several viral and cellular proteins, including E1A, EBNA2, LMP1, MGA and Myb (Hateboer et al, 1995; Ladendorff et al, 2001; Masselink et al, 2001; Ansieau & Leutz, 2002; Wan et al, 2006). In one case, BS69 (or an alternatively spliced isoform of BS69) acts to bridge EBV-encoded LMP1 and TRAF6 in the LMP1-induced JNK pathway (Wan et al, 2006). However, in many other cases, BS69 functions to repress the transcriptional activity of its associated factors (Masselink & Bernards, 2000; Wei et al, 2003; Ekblad et al, 2005). How BS69 represses transcription is unclear. Presumably, it does so by recruiting molecules such as histone deacetylases, as BS69 does not contain domains with intrinsic repressive activity (Velasco et al, 2006). Other than the features above, the normal cellular function of BS69 remains unclear. Furthermore, we do not know the actual target genes controlled by BS69; therefore, the present work represents the first example, to our knowledge, in which several BS69 target genes, for example, p21Cip1, MDM2 and Fas, are identified. We have shown that BS69 regulates these genes through p53; however, as not all known p53 target genes are affected by BS69, it suggests that BS69 only selectively associates with a fraction of p53-responsive promoters. By fine-tuning the expression levels of p21Cip1, BS69 could function as a ‘sensor’ in the normal cellular growth programme. In response to the loss of BS69 function—either owing to downregulation of its expression levels by some yet to be identified extracellular signals or to sequestration by another protein—cells will enter the senescence state as an intrinsic cellular defense mechanism. The presence of other transforming genes might result in full-blown cell transformation (Land et al, 1983; Serrano et al, 1997).
Two connected yet distinct senescence pathways have been identified: one consisting of p53/p21Cip1 and the second p16INK4a/Rb (Beausejour et al, 2003; Chan et al, 2005; Herbig et al, 2006). Although RasV12-induced senescence involves both pathways (Serrano et al, 1997), cellular senescence induced by the knockdown of either BS69 or p400 involves only p53/p21Cip1 (Fig 3; supplementary Fig S1 online; Chan et al, 2005). However, it remains unclear which upstream stimuli preferentially use the p53/p21Cip1 pathway to trigger senescence. This might explain why BS69 mRNA levels were only partly decreased in both RasV12-induced and replication-induced senescent cells (supplementary Fig S3A online). Consistently, overexpression of BS69 is not sufficient to bypass RasV12-induced and replication-induced senescence (supplementary Fig S3B,C online). As to the relationship between BS69 and p400, in many aspects, BS69 behaves like p400 as both proteins interact with adenovirus E1A protein (Hateboer et al, 1995; Fuchs et al, 2001) and knockdown of either BS69 or p400 in IMR90 cells induces senescence (Chan et al, 2005). Interestingly, we found that BS69 forms a complex with p400 in cells. We propose that a transcription repressive complex, consisting of p400, BS69 and possibly some other yet to be identified proteins, interacts with p53 to repress its transcriptional activity (Fig 4E). Loss of either protein relieves the repression on p53 and promotes cells to enter the senescence state through the p53/p21Cip1 pathway. In the future, it is important to identify other protein components in the p400–BS69 complex, to understand how these proteins interact with each other in the complex, and to find out exactly how these proteins interact with p53 and repress its transcriptional activity. Only by doing so can we obtain a clearer picture of the role of BS69 in cellular senescence and get more insights into the normal physiological function of BS69.