UBE1L represses PML/RAR{alpha} by targeting the PML domain for ISG15ylation

Area of Science:

• Oncology
• Molecular Biology
• Biochemistry

Background:

• Acute promyelocytic leukemia (APL) is driven by the PML/RARalpha oncogenic fusion protein.
• All-trans-retinoic acid (ATRA) induces clinical remission by promoting PML/RARalpha degradation.
• Existing knowledge suggests ATRA triggers PML/RARalpha degradation, but the precise mechanisms and targeted domains are not fully elucidated.

Purpose of the Study:

• To investigate the independent mechanisms of PML/RARalpha degradation mediated by retinoic acid (RA) and ubiquitin-activating enzyme-E1-like (UBE1L).
• To determine if the ubiquitin-like protein ISG15 directly conjugates with PML/RARalpha.
• To explore the roles of specific protein domains and deconjugases in these degradation pathways.

Main Methods:

• Transient transfection experiments using different PML/RARalpha domains.
• Treatment with all-trans-retinoic acid (ATRA).
• Assessment of protein expression, degradation, and ISG15ylation in response to ATRA, UBE1L, proteasomal inhibitors, and deconjugase inhibition.

Main Results:

• RA treatment preferentially down-regulated the RARalpha domain of PML/RARalpha.
• UBE1L targeted the PML domain for repression and induced ISG15ylation of this domain.
• The ISG15 deconjugase UBP43/USP18 opposed UBE1L-mediated degradation but not RA-dependent degradation.
• Proteasomal inhibition blocked both RA- and UBE1L-mediated degradation.

Conclusions:

• Retinoic acid (RA) induces PML/RARalpha degradation via distinct mechanisms targeting different protein domains.
• UBE1L-mediated degradation involves ISG15ylation of the PML domain.
• These findings reveal two convergent pathways for PML/RARalpha degradation, enhancing understanding of ATRA's antineoplastic effects in APL.