Defective ubiquitin-mediated degradation of antiapoptotic Bfl-1 predisposes to lymphoma

Abstract

The antiapoptotic Bcl-2 family member Bfl-1 is up-regulated in many human tumors in which nuclear factor-kappaB (NF-kappaB) is implicated and contributes significantly to tumor cell survival and chemoresistance. We previously found that NF-kappaB induces transcription of bfl-1 and that the Bfl-1 protein is also regulated by ubiquitin-mediated proteasomal degradation. However, the role that dysregulation of Bfl-1 turnover plays in cancer is not known. Here we show that ubiquitination-resistant mutants of Bfl-1 display increased stability and greatly accelerated tumor formation in a mouse model of leukemia/lymphoma. We also show that tyrosine kinase Lck is up-regulated and activated in these tumors and leads to activation of the IkappaB kinase, Akt, and extracellular signal-regulated protein kinase signaling pathways, which are key mediators in cancer. Coexpression of Bfl-1 and constitutively active Lck promoted tumor formation, whereas Lck knockdown in tumor-derived cells suppressed leukemia/lymphomagenesis. These data demonstrate that ubiquitination is a critical tumor suppression mechanism regulating Bfl-1 function and suggest that mutations in bfl-1 or in the signaling pathways that control its ubiquitination may predispose one to cancer. Furthermore, because bfl-1 is up-regulated in many human hematopoietic tumors, this finding suggests that strategies to promote Bfl-1 ubiquitination may improve therapy.

Antiapoptotic potency of Bcl-2 proteins primarily relies on their stability, not binding selectivity

“…In contrast, Bcl-2, Bcl-w, and Bcl-xL were stable, with half-lives of about 20 hours. This agrees with published data on the untagged proteins, 14,22,36,37 indicating that GFP fusion did not influence the turnover of the Bcl-2 proteins. Accordingly, the turnover of GFP-Bcl-2 and GFP-Mcl-1 was virtually identical to that of endogenous Bcl-2 and Mcl-1 in the same cells (supplemental Figure 7).…”

“…6,13,19 These results do not fully agree (Figure 2 line 1) and do not necessarily reflect binding in cells, particularly because the mitochondrial membrane context is lacking. 20,21 Also, removal of the membrane anchor from the prosurvival Bcl-2 proteins can alter binding properties, [22][23][24] and BH3-peptides may not represent the full-length proteins. 25,26 Most likely for these reasons, BH3-binding profiles of Bak and Bax determined in vitro differ greatly from those determined by coimmunoprecipitation from cells 27 ( Figure 2).…”

“…The dichotomy in antiapoptotic capacity of the prosurvival Bcl-2 proteins is not restricted to therapy resistance. An Em-Myc-driven leukemogenesis model revealed that Bcl-2, Bcl-xL, and Bcl-w also have a greater oncogenic potency than Bcl-B, Bfl-1, and Mcl-1, 2 whereas stabilized versions of Bcl-B 36 and Bfl-1 22 were as potent in accelerating tumorigenesis as Bcl-xL. Interestingly, a stabilized form of Mcl-1 expressed from the endogenous gene locus was recently found to largely rescue the dramatic developmental deficiencies of Bcl-2 2/2 mice.…”

Role of the pro-survival molecule Bfl-1 in melanoma

“…One study by Yang et al (2005) added green fluorescent protein (GFP) to Bfl-1 to discover the intracellular localisation and instead found that fusion with GFP at either the C-or N-terminus converted Bfl-1 into a potent pro-apoptotic protein and induced cell death 426 . However this discovery has not been confirmed by other research groups such as Fan et al 427 and Simmons et al 371 , who have used GFP to tag Bfl-1, as commercial antibodies which recognise the protein have only become available in recent years.…”

BCL2A1: the underdog in the BCL2 family

“…Ubiquitin-resistant and hence more stable mutants of BCL2A1 have been described to promote lymphoma, highlighting the importance of post-translational regulation of BCL2A1 in a physiological context. 86 Although not tumorigenic by itself in mice, 63 enforced expression of BCL2A1 was found to accelerate myc-induced leukemogenesis upon engraftment in mice. However, when compared with other anti-apoptotic BCL2 proteins, the effect of BCL2A1 overexpression on the survival of leukemic mice was less pronounced.…”