Abstract
The ankyrin repeat-containing protein with a suppressor of cytokine signaling box-2 (ASB2) gene was identified as a retinoic acid-response gene and a target of the promyelocytic leukemia-retinoic acid receptor-alpha oncogenic protein characteristic of acute promyelocytic leukemia. Expression of ASB2 in myeloid leukemia cells inhibits growth and promotes commitment, recapitulating an early step known to be critical for differentiation. Here we show that ASB2, by interacting with the Elongin BC complex, can assemble with Cullin5.Rbx1 to form an E3 ubiquitin ligase complex that stimulates polyubiquitination by the E2 ubiquitin-conjugating enzyme Ubc5. This is a first indication that a member of the ASB protein family, ASB2, is a subunit of an ECS (Elongin C-Cullin-SOCS box)-type E3 ubiquitin ligase complex. Altogether, our results strongly suggest that ASB2 targets specific proteins to destruction by the proteasome in leukemia cells that have been induced to differentiate.
Highlights
Identification of genes controlling proliferation and differentiation of myeloid cells is critical for understanding how myelopoiesis is dysregulated by chromosomal abnormalities in leukemia cells
The a suppressor of cytokine signaling box-2 (ASB2) gene was originally identified as an retinoic acid (RA)-inducible gene whose expression recapitulates early differentiation events critical to induced differentiation of myeloid leukemia cells [19]
We demonstrated that ASB2 interacts with the Elongin BC complex through the binding of Elongin C to the ASB2 BC box
Summary
Identification of genes controlling proliferation and differentiation of myeloid cells is critical for understanding how myelopoiesis is dysregulated by chromosomal abnormalities in leukemia cells. A few PML-RAR␣ target genes have been identified including (i) PRAM-1, a novel molecular adaptor of myeloid cells [18], (ii) ASB2, a member of the suppressors of cytokine signaling (SOCS) protein family [19], (iii) p21WAF1/CIP1, a mediator of growth inhibition that plays a role during commitment to differentiation of RA-treated APL cell [20], (iv) ubiquitin-activating enzyme E1-like (UBE1L), which mediates PML-RAR␣ ubiquitination and subsequent degradation [21], and (v) the CCAAT/enhancer binding proteins (C/ EBP)  and ⑀, which are myeloid transcription factors [22, 23]. Expression of the ASB2 protein in myeloid leukemia cells inhibits growth and promotes chromatin condensation, which are characteristics of commitment known as critical to differentiation of myeloid
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