Regulation of lipolysis by 14-3-3 proteins on human adipocyte lipid droplets

Chemical Probe-Enabled Lipid Droplet Proteomics

Emerging evidence indicates that lipid droplets (LDs) play important roles in lipid metabolism, energy homeostasis, and cell stress management. Notably, dysregulation of LDs is tightly linked to numerous diseases, including lipodystrophies, cancer, obesity, atherosclerosis, and others. The pivotal physiological roles of LDs have led to an exploration of research in recent years. The functions of LDs are inherently connected to the composition of their proteome. Current methods for profiling LD proteins mostly utilize LD fractionation, including those based on proximity-based labeling techniques. Global profiling of the LD proteome in live cells without the isolation of LDs is still challenging. Herein, we disclose two small-molecule chemical probes, termed LDF and LDPL. Both LDF/LDPL are small in size and could freely and specifically migrate within the lipid context of LDs. Consequently, they were successfully used for live-cell fluorescence imaging of LDs and from animal tissues. We further showed that LDPL was capable of large-scale profiling of the LD proteome without the need of LD isolation. By using LDPL, 1584 high-confidence proteins, most of which could be annotated to prominent LD functions, were next identified. Importantly, further validation studies by using representative "hit" proteins revealed that CHMP6 and PRDX4 could act as the lipophagy receptor and lipolysis suppressor, respectively. Our results thus confirmed for the first time that LDPL is a powerful chemical tool for in situ profiling of LD proteomes. With the ability to provide a deeper understanding of LD proteomics from the native cellular environments, our newly developed strategy may be used in future to decipher the dynamics and molecular mechanism of LDs in various diseases.

Tools to Dissect Lipid Droplet Regulation, Players, and Mechanisms

Spurred by the authors' own recent discovery of reactive metabolite-regulated nexuses involving lipid droplets (LDs), this perspective discusses the latest knowledge and multifaceted approaches toward deconstructing the function of these dynamic organelles, LD-associated localized signaling networks, and protein players. Despite accumulating knowledge surrounding protein families and pathways of conserved importance for LD homeostasis surveillance and maintenance across taxa, much remains to be understood at the molecular level. In particular, metabolic stress-triggered contextual changes in LD-proteins' localized functions, crosstalk with other organelles, and feedback signaling loops and how these are specifically rewired in disease states remain to be illuminated with spatiotemporal precision. We hope this perspective promotes an increased interest in these essential organelles and innovations of new tools and strategies to better understand context-specific LD regulation critical for organismal health.

The β2 adrenergic receptor cross-linked interactome identifies 14-3-3 proteins as regulating the availability of signaling-competent receptors

The emerging picture of G protein-coupled receptor function suggests that the global signaling response is an integrated sum of a multitude of individual receptor responses, each regulated by their local protein environment. The β2 adrenergic receptor (B2AR) has long served as an example receptor in the development of this model. However, the mechanism and the identity of the protein-protein interactions that govern the availability of receptors competent for signaling remain incompletely characterized. To address this question, we characterized the interactome of agonist-stimulated B2AR in human embryonic kidney 293 cells using FLAG coimmunoprecipitation coupled to stable isotope labeling by amino acids in cell culture and mass spectrometry. Our B2AR cross-linked interactome identified 190 high-confidence proteins, including almost all known interacting proteins and 6 out of 7 isoforms of the 14-3-3 family of scaffolding proteins. Inhibiting 14-3-3 proteins with the peptide difopein enhanced isoproterenol-stimulated adrenergic signaling via cAMP approximately 3-fold and increased both miniGs and arrestin recruitment to B2AR more than 2-fold each, without noticeably changing EC50 with respect to cAMP signaling or effector recruitment upon stimulation. Our results show that 14-3-3 proteins negatively regulate downstream signaling by inhibiting access of B2AR to effector proteins. We propose that 14-3-3 proteins maintain a dynamic pool of B2AR that has reduced signaling efficacy in response to acute agonist stimulation, limiting the number of signaling-competent receptors at the plasma membrane. SIGNIFICANCE STATEMENT: This study presents a new interactome of the agonist-stimulated β2 adrenergic receptor, a paradigmatic G protein-coupled receptor that is both a model system for members of this class and an important signaling protein in respiratory, cardiovascular, and metabolic regulation. We identify 14-3-3 proteins as responsible for restricting β2 adrenergic receptor access to signaling effectors and maintaining a receptor population that is insensitive to acute stimulation by agonists.

An adipocentric perspective of pancreatic lipotoxicity in diabetes pathogenesis

Obesity and diabetes represent two increasing and invalidating public health issues that often coexist. It is acknowledged that fat mass excess predisposes to insulin resistance and type 2 diabetes mellitus (T2D), with the increasing incidence of the two diseases significantly associated. Moreover, emerging evidence suggests that obesity might also accelerate the appearance of type 1 diabetes (T1D), which is now a relatively frequent comorbidity in patients with obesity. It is a common clinical finding that not all patients with obesity will develop diabetes at the same level of adiposity, with gender, genetic, and ethnic factors playing an important role in defining the timing of diabetes appearance. The adipose tissue (AT) expandability hypothesis explains this paradigm, indicating that the individual capacity to appropriately store energy surplus in the form of fat within the AT determines and prevents the toxic deposition of lipids in other organs, such as the pancreas. Thus, we posit that when the maximal storing capacity of AT is exceeded, individuals will develop T2D. In this review, we provide insight into mechanisms by which the AT controls pancreas lipid content and homeostasis in case of obesity to offer an adipocentric perspective of pancreatic lipotoxicity in the pathogenesis of diabetes. Moreover, we suggest that improving AT function is a valid therapeutic approach to fighting obesity-associated complications including diabetes.

Personal exposure to airborne organic pollutants and lung function changes among healthy older adults

Epidemiological evidence on the impact of airborne organic pollutants on lung function among the elderly is limited, and their underlying biological mechanisms remain largely unexplored. Herein, a longitudinal panel study was conducted in Jinan, Shandong Province, China, involving 76 healthy older adults monitored over a span of five months repetitively. We systematically evaluated personal exposure to a diverse range of airborne organic pollutants using a wearable passive sampler and their effects on lung function. Participants' pulmonary function indicators were assessed, complemented by comprehensive multi-omics analyses of blood and urine samples. Leveraging the power of interaction analysis, causal inference test (CIT), and integrative pathway analysis (IPA), we explored intricate relationships between specific organic pollutants, biomolecules, and lung function deterioration, elucidating the biological mechanisms underpinning the adverse impacts of these pollutants. We observed that bis (2-chloro-1-methylethyl) ether (BCIE) was significantly associated with negative changes in the forced vital capacity (FVC), with glycerolipids mitigating this adverse effect. Additionally, 31 canonical pathways [e.g., high mobility group box 1 (HMGB1) signaling, phosphatidylinositol 3-kinase (PI3K)/AKT pathway, epithelial mesenchymal transition, and heme and nicotinamide adenine dinucleotide (NAD) biosynthesis] were identified as potential mechanisms. These findings may hold significant implications for developing effective strategies to prevent and mitigate respiratory health risks arising from exposure to such airborne pollutants. However, due to certain limitations of the study, our results should be interpreted with caution.

14-3-3ζ regulates adipogenesis by modulating chromatin accessibility during the early stages of adipocyte differentiation

We previously established the scaffold protein 14-3-3ζ as a critical regulator of adipogenesis and adiposity, but the temporal specificity of its action during adipocyte differentiation remains unclear. To decipher if 14-3-3ζ exerts its regulatory functions on mature adipocytes or on adipose precursor cells (APCs), we generated Adipoq14-3-3ζKO and Pdgfra14-3-3ζKO mouse models. Our findings revealed a pivotal role for 14-3-3ζ in APC differentiation in a sex-dependent manner, whereby male and female Pdgfra14-3-3ζKO mice display impaired or potentiated weight gain, respectively, as well as fat mass. To better understand how 14-3-3ζ regulates the adipogenic transcriptional program in APCs, CRISPR-Cas9 was used to generate TAP-tagged 14-3-3ζ-expressing 3T3-L1 preadipocytes. Using these cells, we examined if the 14-3-3ζ nuclear interactome is enriched with adipogenic regulators during differentiation. Regulators of chromatin remodeling, such as DNMT1 and HDAC1, were enriched in the nuclear interactome of 14-3-3ζ, and their activities were impacted upon 14-3-3ζ depletion. The interactions between 14-3-3ζ and chromatin-modifying enzymes suggested that 14-3-3ζ may control chromatin remodeling during adipogenesis, and this was confirmed by ATAC-seq, which revealed that 14-3-3ζ depletion impacted the accessibility of up to 1,244 chromatin regions corresponding in part to adipogenic genes, promoters, and enhancers during the initial stages of adipogenesis. Moreover, 14-3-3ζ-dependent chromatin accessibility was found to directly correlate with the expression of key adipogenic genes. Altogether, our study establishes 14-3-3ζ as a crucial epigenetic regulator of adipogenesis and highlights the usefulness of deciphering the nuclear 14-3-3ζ interactome to identify novel pro-adipogenic factors and pathways.