Insights Into the Biogenesis and Emerging Functions of Lipid Droplets From Unbiased Molecular Profiling Approaches

Moving the fat: Emerging roles of rab GTPases in the regulation of lipid droplet contact sites

Lipid droplets (LDs) play crucial roles in lipid metabolism, energy homeostasis, and cellular stress. Throughout their lifecycle, LDs establish membrane contact sites (MCSs) with the endoplasmic reticulum, mitochondria, peroxisomes, endosomes, lysosomes, and phagosomes. LD MCSs are dynamically generated in response to metabolic or immune cues to ensure that LD lipids (and proteins) are timely delivered to optimize valuable substrates and avoid lipotoxicity. It is increasingly evident that many Rab GTPases are involved in LD dynamics. Here, we summarize our current understanding of how and when Rab proteins dynamically drive the generation of LD MCSs and regulate a variety of LD functions.

Inhibition of LPCAT3 exacerbates endoplasmic reticulum stress and HBV replication

BACKGROUND

Altered phospholipid metabolism plays a key role in changing the immune microenvironment and severely affecting T-cell function. LPCAT3 is one of the vital enzymes regulating phospholipid metabolism. This study aims to verify the effect of LPCAT3 on HBV replication in vitro and the chronic progression of hepatitis B infection based on the results of lipidomic.

METHODS

Untargeted lipidomic analysis was employed to scrutinize discrepancies in lipid metabolites between 40 HBV-infected patients and those who spontaneously cleared the virus. Subsequently, enzyme-linked immunosorbent assay (ELISA), enzyme-linked immunospot assay (ELISPOT), western blotting (WB) and quantitative polymerase chain reaction (qPCR) were utilized to investigate LPCAT3 expression and assess HBV replication and endoplasmic reticulum stress (ERS).

RESULTS

A comparative analysis between HBV-infected patients and those experiencing spontaneous clearance revealed significant disparities in 24 lipid metabolites. Among these, phosphatidylcholine (PC) and lysophosphatidylcholine (LPC), constituting half (12/24) of the identified metabolites, were identified as substrates and products of LPCAT3. In vitro studies demonstrated that inhibiting LPCAT3 led to elevated expression levels of hepatitis B surface antigen (HBsAg), HBV-DNA, and interferon-γ (IFN-γ) (P < 0.05), indicative of heightened HBV replication. Furthermore, LPCAT3 inhibition significantly upregulated the expression of genes associated with ERS (P < 0.05).

CONCLUSIONS

Inhibiting LPCAT3 significantly correlates with HBV replication and induces inflammation by enhancing ERS. We hypothesize that LPCAT3 serves as a potential biomarker for hepatitis B virus replication and chronic progression. Furthermore, these findings elucidate the malignant progression of HBV infection from the standpoint of lipid metabolism, offering a novel insight for subsequent mechanistic exploration or therapeutic studies.

LAY SUMMARY

LPCAT3 inhibition enhances endoplasmic reticulum stress and HBV replication by altering the membrane phospholipid composition and promotes chronic hepatitis B progression.

Surface tension-driven sorting of human perilipins on lipid droplets

Perilipins (PLINs), the most abundant proteins on lipid droplets (LDs), display similar domain organization including amphipathic helices (AH). However, the five human PLINs bind different LDs, suggesting different modes of interaction. We established a minimal system whereby artificial LDs covered with defined polar lipids were transiently deformed to promote surface tension. Binding of purified PLIN3 and PLIN4 AH was strongly facilitated by tension but was poorly sensitive to phospholipid composition and to the presence of diacylglycerol. Accordingly, LD coverage by PLIN3 increased as phospholipid coverage decreased. In contrast, PLIN1 bound readily to LDs fully covered by phospholipids; PLIN2 showed an intermediate behavior between PLIN1 and PLIN3. In human adipocytes, PLIN3/4 were found in a soluble pool and relocated to LDs upon stimulation of fast triglyceride synthesis, whereas PLIN1 and PLIN2 localized to pre-existing LDs, consistent with the large difference in LD avidity observed in vitro. We conclude that the PLIN repertoire is adapted to handling LDs with different surface properties.

The link between alterations in circadian rhythms and lipid metabolism in bipolar disorder: the hypothesis of lipid droplets

Bipolar disorder (BD) is a neuropsychiatric illness characterized by recurrent episodes of mania and depression, leading to significant cognitive and functional impairments, psychiatric and metabolic comorbidities, and substantial healthcare costs. The complex nature and lack of specific biomarkers for BD make it a daily challenge for clinicians. Therefore, advancing our understanding of BD pathophysiology is essential to identify novel diagnostic biomarkers and potential therapeutic targets. Although its neurobiology remains unclear, circadian disruption and lipid alterations have emerged as key hallmarks of BD. Lipids are essential components of the brain and play a critical role in regulating synaptic activity and neuronal development. Consequently, alterations in brain lipids may contribute to the neuroanatomical changes and reduced neuroplasticity observed in BD. Lipid droplets, which regulate the storage of neutral lipids, buffer the levels of toxic lipids within cells. These dynamic organelles adapt to cellular needs, and their dysregulated accumulation has been implicated in several pathological conditions. Notably, lipid droplets and different classes of lipids exhibit rhythmic oscillations throughout the 24-hour cycle, suggesting a link between lipid metabolism, circadian rhythms, and lipid droplets. In this review, we explore the impairment of circadian rhythms and lipid metabolism in BD and present evidence that circadian clocks regulate lipid droplet accumulation. Importantly, we propose the "hypothesis of lipid droplets for BD," which posits that impaired lipid metabolism in BD is closely linked to alterations in lipid droplet homeostasis driven by circadian clock disruption.

The target of rapamycin signaling pathway regulates vegetative development, aflatoxin biosynthesis, and pathogenicity in Aspergillus flavus

The target of rapamycin (TOR) signaling pathway is highly conserved and plays a crucial role in diverse biological processes in eukaryotes. Despite its significance, the underlying mechanism of the TOR pathway in Aspergillus flavus remains elusive. In this study, we comprehensively analyzed the TOR signaling pathway in A. flavus by identifying and characterizing nine genes that encode distinct components of this pathway. The FK506-binding protein Fkbp3 and its lysine succinylation are important for aflatoxin production and rapamycin resistance. The TorA kinase plays a pivotal role in the regulation of growth, spore production, aflatoxin biosynthesis, and responses to rapamycin and cell membrane stress. As a significant downstream effector molecule of the TorA kinase, the Sch9 kinase regulates aflatoxin B1 (AFB1) synthesis, osmotic and calcium stress response in A. flavus, and this regulation is mediated through its S_TKc, S_TK_X domains, and the ATP-binding site at K340. We also showed that the Sch9 kinase may have a regulatory impact on the high osmolarity glycerol (HOG) signaling pathway. TapA and TipA, the other downstream components of the TorA kinase, play a significant role in regulating cell wall stress response in A. flavus. Moreover, the members of the TapA-phosphatase complexes, SitA and Ppg1, are important for various biological processes in A. flavus, including vegetative growth, sclerotia formation, AFB1 biosynthesis, and pathogenicity. We also demonstrated that SitA and Ppg1 are involved in regulating lipid droplets (LDs) biogenesis and cell wall integrity (CWI) signaling pathways. In addition, another phosphatase complex, Nem1/Spo7, plays critical roles in hyphal development, conidiation, aflatoxin production, and LDs biogenesis. Collectively, our study has provided important insight into the regulatory network of the TOR signaling pathway and has elucidated the underlying molecular mechanisms of aflatoxin biosynthesis in A. flavus.

Lipid droplets provide metabolic flexibility for cancer progression

A hallmark of cancer cells is their remarkable ability to efficiently adapt to favorable and hostile environments. Due to a unique metabolic flexibility, tumor cells can grow even in the absence of extracellular nutrients or in stressful scenarios. To achieve this, cancer cells need large amounts of lipids to build membranes, synthesize lipid-derived molecules, and generate metabolic energy in the absence of other nutrients. Tumor cells potentiate strategies to obtain lipids from other cells, metabolic pathways to synthesize new lipids, and mechanisms for efficient storage, mobilization, and utilization of these lipids. Lipid droplets (LDs) are the organelles that collect and supply lipids in eukaryotes and it is increasingly recognized that the accumulation of LDs is a new hallmark of cancer cells. Furthermore, an active role of LD proteins in processes underlying tumorigenesis has been proposed. Here, by focusing on three major classes of LD-resident proteins (perilipins, lipases, and acyl-CoA synthetases), we provide an overview of the contribution of LDs to cancer progression and discuss the role of LD proteins during the proliferation, invasion, metastasis, apoptosis, and stemness of cancer cells.

Localization, traffic and function of Rab34 in adipocyte lipid and endocrine functions

BACKGROUND

Excessive lipid accumulation in the adipose tissue in obesity alters the endocrine and energy storage functions of adipocytes. Adipocyte lipid droplets represent key organelles coordinating lipid storage and mobilization in these cells. Recently, we identified the small GTPase, Rab34, in the lipid droplet proteome of adipocytes. Herein, we have characterized the distribution, intracellular transport, and potential contribution of this GTPase to adipocyte physiology and its regulation in obesity.

METHODS

3T3-L1 and human primary preadipocytes were differentiated in vitro and Rab34 distribution and trafficking were analyzed using markers of cellular compartments. 3T3-L1 adipocytes were transfected with expression vectors and/or Rab34 siRNA and assessed for secretory activity, lipid accumulation and expression of proteins regulating lipid metabolism. Proteomic and protein interaction analyses were employed for the identification of the Rab34 interactome. These studies were combined with functional analysis to unveil the role played by the GTPase in adipocytes, with a focus on the actions conveyed by Rab34 interacting proteins. Finally, Rab34 regulation in response to obesity was also evaluated.

RESULTS

Our results show that Rab34 localizes at the Golgi apparatus in preadipocytes. During lipid droplet biogenesis, Rab34 translocates from the Golgi to endoplasmic reticulum-related compartments and then reaches the surface of adipocyte lipid droplets. Rab34 exerts distinct functions related to its intracellular location. Thus, at the Golgi, Rab34 regulates cisternae integrity as well as adiponectin trafficking and oligomerization. At the lipid droplets, this GTPase controls lipid accumulation and lipolysis through its interaction with the E1-ubiquitin ligase, UBA1, which induces the ubiquitination and proteasomal degradation of the fatty acid transporter and member of Rab34 interactome, FABP5. Finally, Rab34 levels in the adipose tissue and adipocytes are regulated in response to obesity and related pathogenic insults (i.e., fibrosis).

CONCLUSIONS

Rab34 plays relevant roles during adipocyte differentiation, including from the regulation of the oligomerization (i.e., biological activity) and secretion of a major adipokine with insulin-sensitizing actions, adiponectin, to lipid storage and mobilization from lipid droplets. Rab34 dysregulation in obesity may contribute to the altered adipokine secretion and lipid metabolism that characterize adipocyte dysfunction in conditions of excess adiposity.

Host Lipid Manipulation by Intracellular Bacteria: Moonlighting for Immune Evasion

Lipids are complex organic molecules that fulfill energy demands and sometimes act as signaling molecules. They are mostly found in membranes, thus playing an important role in membrane trafficking and protecting the cell from external dangers. Based on the composition of the lipids, their fluidity and charge, their interaction with embedded proteins vary greatly. Bacteria can hijack host lipids to satisfy their energy needs or to conceal themselves from host cells. Intracellular bacteria continuously exploit host, from their entry into host cells utilizing host lipid machinery to exiting through the cells. This acquisition of lipids from host cells helps in their disguise mechanism. The current review explores various mechanisms employed by the intracellular bacteria to manipulate and acquire host lipids. It discusses their role in manipulating host membranes and the subsequence impact on the host cells. Modulating these lipids in macrophages not only serve the purpose of the pathogen but also modulates the macrophage energy metabolism and functional state. Additionally, we have explored the intricate pathogenic relationship and the potential prospects of using this knowledge in lipid-based therapeutics to disrupt pathogen dominance.

Defensive-lipid droplets: Cellular organelles designed for antimicrobial immunity

Microbes have developed many strategies to subvert host organisms, which, in turn, evolved several innate immune responses. As major lipid storage organelles of eukaryotes, lipid droplets (LDs) are an attractive source of nutrients for invaders. Intracellular viruses, bacteria, and protozoan parasites induce and physically interact with LDs, and the current view is that they "hijack" LDs to draw on substrates for host colonization. This dogma has been challenged by the recent demonstration that LDs are endowed with a protein-mediated antibiotic activity, which is upregulated in response to danger signals and sepsis. Dependence on host nutrients could be a generic "Achilles' heel" of intracellular pathogens and LDs a suitable chokepoint harnessed by innate immunity to organize a front-line defense. Here, we will provide a brief overview of the state of the conflict and discuss potential mechanisms driving the formation of the 'defensive-LDs' functioning as hubs of innate immunity.

Multifaceted Nature of Lipid Droplets in Viral Interactions and Pathogenesis

Once regarded as inert organelles with limited and ill-defined roles, lipid droplets (LDs) have emerged as dynamic entities with multifaceted functions within the cell. Recent research has illuminated their pivotal role as primary energy reservoirs in the form of lipids, capable of being metabolized to meet cellular energy demands. Their high dynamism is underscored by their ability to interact with numerous cellular organelles, notably the endoplasmic reticulum (the site of LD genesis) and mitochondria, which utilize small LDs for energy production. Beyond their contribution to cellular bioenergetics, LDs have been associated with viral infections. Evidence suggests that viruses can co-opt LDs to facilitate their infection cycle. Furthermore, recent discoveries highlight the role of LDs in modulating the host's immune response. Observations of altered LD levels during viral infections suggest their involvement in disease pathophysiology, potentially through production of proinflammatory mediators using LD lipids as precursors. This review explores these intriguing aspects of LDs, shedding light on their multifaceted nature and implications in viral interactions and disease development.