Characterizing the protein-protein interaction between MDM2 and 14-3-3σ; proof of concept for small molecule stabilization

Mouse Double Minute 2 (MDM2) is a key negative regulator of the tumor suppressor protein p53. MDM2 overexpression occurs in many types of cancer and results in the suppression of WT p53. The 14-3-3 family of adaptor proteins are known to bind MDM2 and the 14-3-3σ isoform controls MDM2 cellular localization and stability to inhibit its activity. Therefore, small molecule stabilization of the 14-3-3σ/MDM2 protein-protein interaction (PPI) is a potential therapeutic strategy for the treatment of cancer. Here, we provide a detailed biophysical and structural characterization of the phosphorylation-dependent interaction between 14-3-3σ and peptides that mimic the 14-3-3 binding motifs within MDM2. The data show that di-phosphorylation of MDM2 at S166 and S186 is essential for high affinity 14-3-3 binding and that the binary complex formed involves one MDM2 di-phosphorylated peptide bound to a dimer of 14-3-3σ. However, the two phosphorylation sites do not simultaneously interact so as to bridge the 14-3-3 dimer in a 'multivalent' fashion. Instead, the two phosphorylated MDM2 motifs 'rock' between the two binding grooves of the dimer, which is unusual in the context of 14-3-3 proteins. In addition, we show that the 14-3-3σ-MDM2 interaction is amenable to small molecule stabilization. The natural product fusicoccin A forms a ternary complex with a 14-3-3σ dimer and an MDM2 di-phosphorylated peptide resulting in the stabilization of the 14-3-3σ/MDM2 PPI. This work serves as a proof-of-concept of the drugability of the 14-3-3/MDM2 PPI and paves the way toward the development of more selective and efficacious small molecule stabilizers.

Aster-B-dependent estradiol synthesis protects female mice from diet-induced obesity

Aster proteins mediate the nonvesicular transport of cholesterol from the plasma membrane (PM) to the endoplasmic reticulum (ER). However, the importance of nonvesicular sterol movement for physiology and pathophysiology in various tissues is incompletely understood. Here we show that loss of Aster-B leads to diet-induced obesity in female but not in male mice, and that this sex difference is abolished by ovariectomy. We further demonstrate that Aster-B deficiency impairs nonvesicular cholesterol transport from the PM to the ER in ovaries in vivo, leading to hypogonadism and reduced estradiol synthesis. Female Aster-B-deficient mice exhibit reduced locomotor activity and energy expenditure, consistent with established effects of estrogens on systemic metabolism. Administration of exogenous estradiol ameliorates the diet-induced obesity phenotype of Aster-B-deficient female mice. These findings highlight the key role of Aster-B-dependent nonvesicular cholesterol transport in regulating estradiol production and protecting females from obesity.

Structural insights into the complex of oncogenic KRas4BG12V and Rgl2, a RalA/B activator

About a quarter of total human cancers carry mutations in Ras isoforms. Accumulating evidence suggests that small GTPases, RalA, and RalB, and their activators, Ral guanine nucleotide exchange factors (RalGEFs), play an essential role in oncogenic Ras-induced signalling. We studied the interaction between human KRas4B and the Ras association (RA) domain of Rgl2 (Rgl2RA), one of the RA-containing RalGEFs. We show that the G12V oncogenic KRas4B mutation changes the interaction kinetics with Rgl2RA The crystal structure of the KRas4BG12V: Rgl2RA complex shows a 2:2 heterotetramer where the switch I and switch II regions of each KRasG12V interact with both Rgl2RA molecules. This structural arrangement is highly similar to the HRasE31K:RALGDSRA crystal structure and is distinct from the well-characterised Ras:Raf complex. Interestingly, the G12V mutation was found at the dimer interface of KRas4BG12V with its partner. Our study reveals a potentially distinct mode of Ras:effector complex formation by RalGEFs and offers a possible mechanistic explanation for how the oncogenic KRas4BG12V hyperactivates the RalA/B pathway.

Aster-dependent nonvesicular transport facilitates dietary cholesterol uptake

Intestinal absorption is an important contributor to systemic cholesterol homeostasis. Niemann-Pick C1 Like 1 (NPC1L1) assists in the initial step of dietary cholesterol uptake, but how cholesterol moves downstream of NPC1L1 is unknown. We show that Aster-B and Aster-C are critical for nonvesicular cholesterol movement in enterocytes. Loss of NPC1L1 diminishes accessible plasma membrane (PM) cholesterol and abolishes Aster recruitment to the intestinal brush border. Enterocytes lacking Asters accumulate PM cholesterol and show endoplasmic reticulum cholesterol depletion. Aster-deficient mice have impaired cholesterol absorption and are protected against diet-induced hypercholesterolemia. Finally, the Aster pathway can be targeted with a small-molecule inhibitor to manipulate cholesterol uptake. These findings identify the Aster pathway as a physiologically important and pharmacologically tractable node in dietary lipid absorption.

Aster-dependent non-vesicular transport facilitates dietary cholesterol uptake

Intestinal cholesterol absorption is an important contributor to systemic cholesterol homeostasis. Niemann-Pick C1 Like 1 (NPC1L1), the target of the drug ezetimibe (EZ), assists in the initial step of dietary cholesterol uptake. However, how cholesterol moves downstream of NPC1L1 is unknown. Here we show that Aster-B and Aster-C are critical for non-vesicular cholesterol movement in enterocytes, bridging NPC1L1 at the plasma membrane (PM) and ACAT2 in the endoplasmic reticulum (ER). Loss of NPC1L1 diminishes accessible PM cholesterol in enterocytes and abolishes Aster recruitment to the intestinal brush border. Enterocytes lacking Asters accumulate cholesterol at the PM and display evidence of ER cholesterol depletion, including decreased cholesterol ester stores and activation of the SREBP-2 transcriptional pathway. Aster-deficient mice have impaired cholesterol absorption and are protected against diet-induced hypercholesterolemia. Finally, we show that the Aster pathway can be targeted with a small molecule inhibitor to manipulate dietary cholesterol uptake. These findings identify the Aster pathway as a physiologically important and pharmacologically tractable node in dietary lipid absorption.

One-Sentence Summary

Identification of a targetable pathway for regulation of dietary cholesterol absorption.

Clinical Consequences of Variable Results in the Measurement of Free Thyroid Hormones: Unusual Presentation of a Family with a Novel Variant in the THRB Gene Causing Resistance to Thyroid Hormone Syndrome

INTRODUCTION

Resistance to thyroid hormone β (RTHβ) is an inherited syndrome caused by dominant negative variants in the THRB gene (NM_000461.5). The clinical picture of RTHβ is variable, and patients harboring the same variant may display different degrees of disease severity.

CASE PRESENTATION

A 30-year-old man presented with thyrotoxicosis and central hyperthyroidism and was found to have a novel variant in the exon 10 of THRB gene (c.C1282G, p.L428V), located within the third hot spot region of the C-terminal of the receptor. Surprisingly, the same variant was found in two other relatives with an apparent normal thyroid function at initial screening. After exclusion of a TSH-secreting adenoma and serum interference in the proband, and the finding that exogenous levothyroxine failed to suppress the TSH in the brother affected by nodular goiter, relatives' thyroid function tests (TFTs) were reassessed with additional analytical method revealing biochemical features consistent with RTHβ in all carriers of the p.L428V variant. Functional studies showed a slightly impaired in vitro transcriptional activity of p.L428V. Interestingly' the expression of the human p.L428V thyroid hormone receptor beta in the zebrafish embryo background generated a phenotype consistent with RTHβ.

CONCLUSION

Variable results of TFTs on some immunoassays can be a cause of RTHβ diagnostic delay, but the genotype-phenotype correlation in this family and functional studies support p.L428V as a novel THRB variant expanding the spectrum of gene variants causing RTHβ. In vivo, rather than in vitro, functional assays may be required to demonstrate the dominant negative action of THRB variants.