Insulin resistance is a clinical symptom of type 2 diabetes that causes profound dysregulation of glucose and lipid metabolism. In the diabetic liver, insulin no longer suppresses glucose production via the intracellular signaling or transcriptional pathways that typically suppress gluconeogenic gene expression. However, resistance to insulin does not suppress fatty acid synthesis via the transcription factor SREBP1c, resulting in lipid accumulation and exacerbating diabetic symptoms. In a new article in Molecular Endocrinology, the Puigserver Lab shows that by binding to target promoters, the transcription factor Yin Yang 1 (YY1) represses expression of genes encoding enzymes for glucose and fatty acid synthesis. At the same time, YY1 activates expression of genes encoding fatty acid oxidation enzymes. As a consequence, decreased genetic dosage of YY1 in mouse liver leads to diabetic-like symptoms such as hyperglycemia, dyslipidemia, hepatic lipid accumulation, and insulin resistance. This phenotype mimics diabetic symptoms and makes targeting YY1 an attractive approach for treating insulin-resistant diabetes.
Lymphangioleiomyomatosis (LAM) is a destructive lung disease specific to women and is associated with the metastasis of Tuberin (TSC2)-null cells with hyperactive mTORC1 (mammalian target of rapamycin complex 1) activity. Clinical trials with the mTORC1 inhibitor rapamycin have revealed partial efficacy but are not curative. Pregnancy appears to exacerbate LAM, suggesting that estrogen (E2) may play a role in the unique features of LAM. As reported in PNAS, the Blenis Lab uses a LAM patient-derived cell line (bearing bi-allelic TSC2 inactivation) to demonstrate that E2 stimulates a robust and biphasic activation of extracellular signal-regulated kinase 2 (ERK2) and transcription of the epithelial-to-mesenchymal transition (EMT)-associated late response gene Fra1. In a carefully orchestrated collaboration, activated mTORC1/S6K1 signaling enhances the translation efficiency of Fra1 mRNA transcribed by the E2-ERK2 pathway, through S6K1-dependent eukaryotic translation initiation factor 4B (eIF4B) phosphorylation. This finding indicates that targeting the E2-ERK pathway in combination with the mTORC1 pathway may be an effective combination therapy for LAM.
Modular Cullin-RING E3 ubiquitin ligases (CRLs) use substrate binding adaptor proteins to specify target ubiquitylation. Many of the ~200 human CRL adaptor proteins remain poorly studied due to a shortage of efficient methods to identify biologically relevant substrates. As reported in Molecular Cell, the Harper Lab has developed Parallel Adaptor Capture (PAC) proteomics as a new approach for the identification of CRL substrates. They used the method to systematically identify candidate targets for the leucine-rich repeat family of F-box proteins (FBXLs) that function with SKP1-CUL1-F-box protein (SCF) E3s. This led to the identification of dozens on candidate substrates across the FBXL family. In validation experiments, they identified the unstudied F-box protein FBXL17 as a regulator of the NFR2 oxidative stress pathway, and demonstrated that FBXL17 controls the transcription of the NRF2 target HMOX1 via turnover of the transcriptional repressor BACH1 in the absence or presence of extrinsic oxidative stress. This work identifies a role for SCFFBXL17 in controlling the threshold for NRF2-dependent gene activation and provides a framework for elucidating the functions of CRL adaptor proteins.
The endoplasmic reticulum (ER) is a continuous membrane system consisting of the nuclear envelope and a peripheral network of membrane tubules and sheets. ER sheets often form stacks, an arrangement that is likely required to accommodate a maximum number of membrane-bound polysome for secretory protein synthesis. How sheets are connected with one another was unknown until recently. As reported in Cell, the Rapoport Lab and their collaborators used a novel automated serial thin sectioning electron microscopy technique to analyze the 3D structure of stacked ER sheets of mouse neurons and of professional secretory cells of the salivary gland. The team discovered that ER sheets are connected by a novel membrane motif consisting of continuous twisted membrane surfaces with helical edges that have left or right-handedness. A theoretical model indicates that this configuration corresponds to a minimum of elastic energy of the sheet surface and its edges. The three dimensional structure resembles a parking garage and likely allows the optimal packing of ER sheets in the restricted volume of a cell.
Accumulation of mutant p53 has been recognized as an important factor that promotes cancer progression and metastasis. Thus, strategies that promote the degradation of mutant p53 might be beneficial for the treatment of cancers. In a recent issue of Genes & Development, Vakifahmetoglu-Norberg et al. demonstrate that blocking autophagy may lead to the degradation of mutant p53 through activating chaperone mediated autophagy, a lysosomal dependent degradation mechanism. This research provides a new mechanism by which mutant p53 might be degraded and the possibility of activating chaperone mediated autophagy as a new treatment for cancers with mutant p53.
In an online PNAS article, the Spiegelman Lab reports on what happens when brown, white, and beige fat cells are exposed to a range of cold temperatures in vitro. They found that cooler temperatures can directly induce white and beige fat to activate a transcriptional program leading to thermogenesis, the generation of heat from chemical energy. Unlike thermogenesis induced by brown fat, this mechanism does not require norepinephrine, the primary chemical messenger of the sympathetic nervous system. See additional news coverage in Science and The Scientist.
Histones and histone-binding proteins play important role in activating and silencing eukaryotic genes. How binding of factors to the nucleosome may change the properties of chromatin to mediate gene activation or silencing is not understood. A recent crystal structure by Wang et al. (PNAS, 2013) of the nucleosome complexed with the BAH domain of the heterochromain protein Sir3 sheds new light on this question.
Heterochromatic gene silencing in the budding yeast S. cerevisiae requires a nucleosome binding protein, Sir3, and specific histone amino acids, in particular a conserved basic patch region spanning amino acids 16 to 20 in the N-terminal tail of histone H4. Wang and colleagues now provide evidence that association of Sir3 with the nucleosome induces interactions between the N-terminal tail of histone H4 and nucleosomal DNA. The Moazed Lab had previously shown that a conserved region in the N-terminus of Sir3, called the BAH domain, is a nucleosome- and histone tail-binding domain. In this paper, they show that while some histone H4 side chains are critical for binding of Sir3 to the nucleosome, two arginines (R17 and R19), which are critical for silencing in vivo, have no effect on binding of Sir3 to nucleosomes in vitro. To understand the paradoxical requirement for these arginines in gene silencing, the authors determined the 3.1 Å resolution crystal structure of the Sir3-BAH domain in complex with the nucleosome. Consistent with their biochemical data, the structure shows that R17 and R19 point away from the BAH domain and make electrostatic contacts with phosphates of the nucleosomal DNA backbone. In contrast, other histone side chains, which are also critical for silencing, make extensive bonding interactions with the BAH domain. These observations suggest a new role for histone tails in gene silencing and heterochromatin formation, beyond assembly platforms for recruitment of regulatory factors.