Regulation of LIM-domain-binding 1 protein expression by ubiquitination of Lys134

RNF12 Promotes Glioblastoma Malignant Proliferation via Destructing RB1 and Regulating MAPK Pathway

Background

RNF12 has been linked to a variety of biological activities, including the control of the MDM2/P53 pathway, although its additional functions remain unclear. RNF12 was discovered to be a new ubiquitin ligase (E3) for RB1, amongst the most frequently repressed proteins in cancer of human.

Method

Cell Counting Kit-8 was used to detect the cell proliferation; coimmunoprecipitation was used to determine that RNF12 interacts with RB1. Xenograft studies were used to verify the results.

Result

In vivo and in vitro RNF12 interacts with RB1 regardless of E3 ligase activity. The ubiquitination of RB1 by RNF12 had an effect on its stability. RNF12 inhibits the RB1 protein and stimulates the MAPK pathway, promoting the growth of GBMs.

Conclusion

Our findings show that RNF12 may operate as a tumour promoter by modulating the cancerous proliferation of glioblastoma by controlling the activity of a new RNF12/RB1/MAPK pathway regulatory axis and that this regulatory axis might be a valuable diagnostic focus in glioblastoma.

LDB1 Enforces Stability on Direct and Indirect Oncoprotein Partners in Leukemia

The LMO2/LDB1 macromolecular complex is critical in hematopoietic stem and progenitor cell specification and in the development of acute leukemia. This complex is comprised of core subunits of LMO2 and LDB1 as well as single-stranded DNA-binding protein (SSBP) cofactors and DNA-binding basic helix-loop-helix (bHLH) and GATA transcription factors. We analyzed the steady-state abundance and kinetic stability of LMO2 and its partners via Halo protein tagging in conjunction with variant proteins deficient in binding their respective direct protein partners. We discovered a hierarchy of protein stabilities (with half-lives in descending order) as follows: LDB1 > SSBP > LMO2 > TAL1. Importantly, LDB1 is a remarkably stable protein that confers enhanced stability upon direct and indirect partners, thereby nucleating the formation of the multisubunit protein complex. The data imply that free subunits are more rapidly degraded than those incorporated within the LMO2/LDB1 complex. Our studies provided significant insights into LMO2/LDB1 macromolecular protein complex assembly and stability, which has implications for understanding its role in blood cell formation and for therapeutically targeting this complex in human leukemias.

Identifying Regulatory Posttranslational Modifications of PD-L1: A Focus on Monoubiquitinaton

A set of high-affinity, high-specificity posttranslational modification (PTM) enrichment tools was developed to generate an unbiased snapshot of four key PTM profiles (tyrosine phosphorylation, acetylation, ubiquitination, and SUMOylation 2/3) for the clinically important protein programmed cell death ligand 1 (PD-L1). The results showed that epidermal growth factor (EGF) treatment induced tyrosine phosphorylation, acetylation, and ubiquitination of PD-L1. Further characterization of EGF-induced PD-L1 ubiquitination revealed a significant increase in mono- and multiubiquitination of PD-L1 that occurred on glycosylated PD-L1. EGF induced mono- and multiubiquitination of PD-L1 preceded EGF-induced increases in PD-L1 protein levels. Chemical inhibitors of the EGFR pathway, gefitnib and SCH772984, suppressed PD-L1 mono- and multiubiquitination, and inhibition of the ubiquitin E1 activating enzyme, with the chemical inhibitor PYR41, was sufficient to block EGF-stimulated increases in PD-L1 protein levels. This study highlights the significance of identifying novel PTMs for PD-L1 and reveals potentially critical regulatory mechanisms that may be valuable therapeutic targets. In a broader context, this report validates an approach whereby one can gain insight into novel mechanisms of action by a simple and unbiased analysis of a PTM profile of potentially any endogenous protein of interest.

LMO2 Oncoprotein Stability in T-Cell Leukemia Requires Direct LDB1 Binding

LMO2 is a component of multisubunit DNA-binding transcription factor complexes that regulate gene expression in hematopoietic stem and progenitor cell development. Enforced expression of LMO2 causes leukemia by inducing hematopoietic stem cell-like features in T-cell progenitor cells, but the biochemical mechanisms of LMO2 function have not been fully elucidated. In this study, we systematically dissected the LMO2/LDB1-binding interface to investigate the role of this interaction in T-cell leukemia. Alanine scanning mutagenesis of the LIM interaction domain of LDB1 revealed a discrete motif, R(320)LITR, required for LMO2 binding. Most strikingly, coexpression of full-length, wild-type LDB1 increased LMO2 steady-state abundance, whereas coexpression of mutant proteins deficient in LMO2 binding compromised LMO2 stability. These mutant LDB1 proteins also exerted dominant negative effects on growth and transcription in diverse leukemic cell lines. Mass spectrometric analysis of LDB1 binding partners in leukemic lines supports the notion that LMO2/LDB1 function in leukemia occurs in the context of multisubunit complexes, which also protect the LMO2 oncoprotein from degradation. Collectively, these data suggest that the assembly of LMO2 into complexes, via direct LDB1 interaction, is a potential molecular target that could be exploited in LMO2-driven leukemias resistant to existing chemotherapy regimens.

Early stages of induction of anterior head ectodermal properties in Xenopus embryos are mediated by transcriptional cofactor ldb1

BACKGROUND

Specific molecules involved in early inductive signaling from anterior neural tissue to the placodal ectoderm to establish a lens-forming bias, as well as their regulatory factors, remain largely unknown. In this study, we sought to identify and characterize these molecules.

RESULTS

Using an expression cloning strategy to isolate genes with lens-inducing activity, we identified the transcriptional cofactor ldb1. This, together with evidence for its nuclear dependence, suggests its role as a regulatory factor, not a direct signaling molecule. We propose that ldb1 mediates induction of early lens genes in our functional assay by transcriptional activation of lens-inducing signals. Gain-of-function assays demonstrate that the inductive activity of the anterior neural plate on head ectodermal structures can be augmented by ldb1. Loss-of-function assays show that knockdown of ldb1 leads to decreased expression of early lens and retinal markers and subsequently to defects in eye development.

CONCLUSIONS

The functional cloning, expression pattern, overexpression, and knockdown data show that an ldb1-regulated mechanism acts as an early signal for Xenopus lens induction.