Ubiquitination, localization, and stability of an anti-apoptotic BCL2-like protein, BCL2L10/BCLb, are regulated by Ubiquilin1

Abstract

We have previously shown that all six members of the anti-apoptotic BCL2 gene family can cooperate with (myelocytomatosis oncogene) MYC in a mouse model of leukemia, but three of them are significantly less potent contributors to leukemogenicity than the other three. The protein encoded by one of these less potent genes, BCL2L10∕BCLb, was recently shown to vary dramatically in many primary human cancers by immunohistochemistry, and the protein levels were inversely correlated with survival in patients with several cancer types. We examined BCLb mRNA in a panel of human cancer cell lines and did not observe the extensive variation in mRNA that would be required to explain the vast differences in protein levels. We found that the levels of BCLb protein diminish quickly after inhibition of protein synthesis with cycloheximide, so we searched for interacting proteins that might affect posttranslational stability of BCLb. Using a variety of approaches, including immunoaffinity and mass spectrometry, we identified a protein, Ubiquilin1 (Ubqln), that specifically interacts with BCLb, and not with other anti-apoptotic BCL2-like proteins. Ubqln stabilizes BCLb protein, while also promoting monoubiquitination on multiple lysine residues and relocation to the cytosol. Furthermore, primary lung adencarcinomas have more Ubqln mRNA than normal adjacent lung tissue, and higher Ubqln mRNA levels are associated with shorter survival of lung cancer patients, suggesting that potentiation of the anti-apoptotic potential of BCLb through regulation of its stability by Ubqln may be an important factor in tumor progression.

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).…”

“…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.…”

Ubiquilin 2 enhances osteosarcoma progression through resistance to hypoxic stress

“…Frequent expression of two HIF-1α downstream molecules, VEGF and platelet-derived growth factor (PdGF), is correlated with inferior event free-survival in osteosarcoma (19,20). With regard to malignant tumor effects related to UBQLN, it stabilizes BCL2L10/BCLb, one of the six anti-apoptotic members of the BCL2 family, inhibits apoptosis, and contributes to the survival of human lung cancer cells (9). Thus, the function of UBQLN in cancer cells is the same as that in osteosarcoma, in that it inhibits hypoxia-induced apoptosis in tumor cells.…”

“…Mutations of the UBQLN2 gene suppress ubiquitin-mediated proteasomal degradation, leading to the accumulation of inclusions composed of misfolded proteins linked to the onset of ALS (8). Moreover, higher UBQLN mRNA levels are associated with shorter survival of lung cancer patients and UBQLN is an important factor in tumor progression (9).…”

Regulation of receptor tyrosine kinases by Ubiquilin1.

“…These proteins are encoded by genes on different chromosomes: UBQLN1 (9q22, 589 or 561 amino acids), UBQLN2 (Xp11.21, 624 amino acids), UBQLN3 (11p15, 654 amino acids), UBQLN4 (1q21, 601 amino acids) and UBQLN-L (11p15.4, 475 amino acids) [1].…”