The presence of C/EBPα and its degradation are both required for TRIB2-mediated leukaemia

Abstract

C/EBPα (p42 and p30 isoforms) is commonly dysregulated in cancer via the action of oncogenes, and specifically in acute myeloid leukaemia (AML) by mutation. Elevated TRIB2 leads to the degradation of C/EBPα p42, leaving p30 intact in AML. Whether this relationship is a cooperative event in AML transformation is not known and the molecular mechanism involved remains elusive.Using mouse genetics our data reveal that in the complete absence of C/EBPα TRIB2 was unable to induce AML. Only in the presence of C/EBPα p42 and p30 were TRIB2 and p30 able to cooperate to decrease the latency of disease. We demonstrate the molecular mechanism involved in degradation of C/EBPα p42 requires site-specific direct interaction between TRIB2 and C/EBPα p42 for the K48-specific ubiquitin-dependent proteasomal degradation of C/EBPα p42. This interaction and ubiquitination is dependent on a critical C-terminal lysine residue on C/EBPα. We show effective targeting of this pathway pharmacologically using proteasome inhibitors in TRIB2 positive AML cells. Together, our data show that excess p30 cooperated with TRIB2 only in the presence of p42 to accelerate AML and the direct interaction and degradation of C/EBPα p42 is required for TRIB2-mediated AML.

Tribbles in the 21st Century: The Evolving Roles of Tribbles Pseudokinases in Biology and Disease

“…However, a major mechanistic function of TRIBs in cancer cells appears to be the (inappropriate) association of TRIB proteins with substrate degradation and stability networks, leading to a subsequent imbalance in timely regulation of crucial transcriptional networks. For example, TRIB2-mediated degradation of the transcription factor C/EBPα is known to have an oncogenic role in the development of acute myeloid leukemia (AML) 54, 55, lung [56], and liver cancers 57, 58. These studies all point to abnormal regulation of TRIB transcription, translation, or protein turnover as disease drivers, and below we use the hematological system and other cancer models to describe how this is thought to work mechanistically at the cellular level in mammals.…”

“…Mechanistically, degradation of C/EBPα p42 by TRIB2 is known to occur via a proteasome-dependent pathway involving lysine 48 polyubiquitination [55]. Using mouse genetics, it was shown that the presence of C/EBPα is paradoxically required for TRIB2-induced AML, and only in the presence of the C/EBPα p42 isoform is a cooperative effect observed with TRIB2 and C/EBPα p30 [55].…”

“…Mechanistically, degradation of C/EBPα p42 by TRIB2 is known to occur via a proteasome-dependent pathway involving lysine 48 polyubiquitination [55]. Using mouse genetics, it was shown that the presence of C/EBPα is paradoxically required for TRIB2-induced AML, and only in the presence of the C/EBPα p42 isoform is a cooperative effect observed with TRIB2 and C/EBPα p30 [55]. Structure–function analysis of TRIB2 revealed that deletion or mutation of the TRIB signature C-terminal E3 ligase COP1-binding site (Figure 4A) prevented TRIB2-mediated degradation of C/EBPα and this correlated directly with a failure to induce AML in vivo [51].…”

“…Putative cooperating genes in TRIB1 and TRIB2-mediated AML include HOX pathway genes, such as HOXA9 , MEIS1 , NUP98 - HOXD13 , and C/EBPAp30 48, 55, 70, 71. Gene expression analysis has revealed that TRIB2 expression levels, while generally low in AML, are higher in PML-RARA -positive leukemia than in PML-RARA -negative leukemia [59].…”

Substrate binding allosterically relieves autoinhibition of the TRIB1 pseudokinase

“…Perhaps the most well-established role of Tribbles proteins is regulating CCAAT enhancer binding protein (C/EBP) family transcription factors (14)(15)(16)(17). The C/EBP family is comprised of 6 members in humans (α, β, γ, δ,ε, and ζ).…”