Tunable NIR AIE-active optical materials for lipid droplet imaging in typical model organisms and photodynamic therapy

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.

Constructing lipid droplet-targeting photosensitizers based on coumarins with NIR emission

The development of photosensitizers (PSs) with subcellular targeting capability has raised interest for photodynamic therapy (PDT) research. In this work, two coumarin-based photosensitizers (C-S-2 and C-S-3) were designed and synthesized via expanding their π-conjugation, introducing strong electron-donor and acceptor groups, and adopting sulfur substitution strategy. These sulfured-coumarins exhibited near-infrared emission (greater than 650 nm), lipid droplet-targeting ability and obvious photocytotoxicity under laser irradiation. In particular, C-S-3 exhibited better photostability, superior lipid droplet-targeting capability, and stronger photodynamic effect on cancer cells than C-S-2.

Recent progress in metal complexes functionalized nanomaterials for photodynamic therapy

Metal complexes have shown promise as photosensitizers for cancer diagnosis and therapeutics. However, the vast majority of metal photosensitizers are not ideal and associated with several limitations including pharmacokinetic limitations, off-target toxicity, fast systemic clearance, poor membrane permeability, and hypoxic tumour microenvironments. Metal complex functionalized nanomaterials have the potential to construct multifunctional systems, which not only overcome the above defects of metal complexes but are also conducive to modulating the tumour microenvironment (TME) and employing combination therapies to boost photodynamic therapy (PDT) efficacy. In this review, we first introduce the current challenges of photodynamic therapy and summarize the recent research strategies (such as metal coordination bonds, self-assembly, π-π stacking, physisorption, and so on) used for preparing metal complexes functionalized nanomaterials in the application of PDT.

Lipid droplet targeting-guided hypoxic photodynamic therapy with curcumin analogs

Two photosensitizers (CCOH and CCN) were designed and synthesized by introducing coumarin into the curcumin (CUR) structure. Compared with CUR, more reactive oxygen species (ROS) were generated by CCOH and CCN in type I and II synergy upon light irradiation. Cell experiments indicated that CCN with an excellent LD-targeting effect could be used to monitor the changes in the morphology and number of LDs in tumor cells during PDT.

A Robust Phenotypic Screening Assay Utilizing Human Podocytes to Identify Agents that Modulate Lipid Droplets

Lipid droplets (LDs), initially thought to be mere lipid storage structures, are highly dynamic organelles with complex functions that control cell fate and behavior. In recent years, their relevance as therapeutic targets for a wide array of human diseases has been well established. Consequently, efforts to develop tools to study them have intensified, including assays that can accurately track LD levels in clinically relevant cell-based models. We previously reported that LD accumulation destines podocytes for lipotoxicity and cell death in renal diseases of metabolic and nonmetabolic origin. We also showed that LD accumulation in those cells serves as both a marker for disease progression and as a therapeutic target. Here, we describe a robust phenotypic screening method, using differentiated human podocytes, for identifying small-molecule compounds that rescue podocytes from LD accumulation and lipotoxicity under cellular stress. Major assay advances include 1) the use of a solvatochromic dye to improve LD staining, reduce background noise, and improve detection accuracy, 2) use of confocal imaging to reduce vertical overlap of LDs and enable accurate counting, 3) combining membrane and cytoskeleton stains to improve cell segmentation in confocal mode, and 4) use of an optimized spot detection algorithm that requires minimal configuration per individual run. The assay is robust and yields a Z-factor that is consistently >0.5.

Illuminating Sub-Cellular Organelles by Small Molecule AIEgens

Sub-cellular organelles play a critical role in a myriad biological phenomena. Consequently, organelle structures and functions are invariably highjacked in diverse diseases including metabolic disorders, aging, and cancer. Hence, illuminating organelle dynamics is crucial in understanding the diseased states as well as developing organelle-targeted next generation therapeutics. In this review, we outline the novel small molecules which show remarkable aggregation-induced emission (AIE) properties due to restriction in intramolecular motion (RIM). We outline the examples of small molecules developed to image organelles like mitochondria, endoplasmic reticulum (ER), Golgi, lysosomes, nucleus, cell membrane and lipid droplets. These AIEgens have tremendous potential for next-generation phototherapy.

Aggregation-Induced Emission Luminogens for Enhanced Photodynamic Therapy: From Organelle Targeting to Tumor Targeting

Photodynamic therapy (PDT) has attracted much attention in the field of anticancer treatment. However, PDT has to face challenges, such as aggregation caused by quenching of reactive oxygen species (ROS), and short 1O2 lifetime, which lead to unsatisfactory therapeutic effect. Aggregation-induced emission luminogen (AIEgens)-based photosensitizers (PSs) showed enhanced ROS generation upon aggregation, which showed great potential for hypoxic tumor treatment with enhanced PDT effect. In this review, we summarized the design strategies and applications of AIEgen-based PSs with improved PDT efficacy since 2019. Firstly, we introduce the research background and some basic knowledge in the related field. Secondly, the recent approaches of AIEgen-based PSs for enhanced PDT are summarized in two categories: (1) organelle-targeting PSs that could cause direct damage to organelles to enhance PDT effects, and (2) PSs with tumor-targeting abilities to selectively suppress tumor growth and reduce side effects. Finally, current challenges and future opportunities are discussed. We hope this review can offer new insights and inspirations for the development of AIEgen-based PSs for better PDT effect.

A Novel Fluoro-Pyrazine-Bridged Donor-Accepter-Donor Fluorescent Probe for Lipid Droplet-Specific Imaging in Diverse Cells and Superoxide Anion Generation

PURPOSE

Lipid droplets (LDs) are dynamic organelles which associated with many metabolic processes. Reliable long-term imaging of LD is of great importance in LD-based therapy and research. Conventional fluorescent probes suffer from poor photostability and difficulty of preparation, which compromise their LD imaging ability. In this study, we aim to provide a novel and universal fluorescent probe for LD-specific imaging in both eukaryotic and prokaryotic cells. The versatile and potential applications of the probe were also evaluated.

METHODS

We used one-step Suzuki coupling reaction to synthesize a fluoro-pyrazine-bridged donor-acceptor-donor fluorescent probe (T-FP-T). The fluorescent properties and stability of T-FP-T were detected. Then, LD-specific imaging and dynamic movement tracking capabilities of T-FP-T were studied in fungus, bacteria, plant and animal tissues. The biosafety and photodynamic toxicity of the probe under different light irradiation were characterized.

RESULTS

T-FP-T showed large Stokes shift, superior brightness, excellent photostability, low toxicity. T-FP-T exhibited significant overlaps with adipophilin antibody or the commercial LD probe (LipidSpot™) in the cytoplasm, but not with Mitotracker red, Lysotracker red and Peroxisome Labeling dye. Moreover, T-FP-T also showed efficient superoxide anion generation capability under white LED light irradiation. The viability of Hela cells co-treated with T-FP-T and 1-h white LED light irradiation decreased to 62%.

CONCLUSIONS

All these outstanding capabilities make T-FP-T a new efficient LD-specific imaging probe. The generated superoxide anion from T-FP-T under white LED light irradiation could cause obvious cell death, which will inspire broad study in LD-targeted photodynamic therapy.

D-π-A structure fluorophore: NIR emission, response to viscosity, detection cyanide and bioimaging of lipid droplets

Intracellular viscosity, an important microenvironment factor, is closely involved in various cell processes as well as diseases. On the other hand, cyanide is one of the most hazardous chemicals for human health and environments. However, a NIR fluorescent probe for both response to viscosity and detection of cyanide remains vacant. Herein, we reported a D-π-A structure fluorophore (named CTR) which exhibited NIR emission and fluorescent enhancement response to viscosity via the molecular rotor strategy. Furthermore, CTR displayed fluorescent and colorimetric response to cyanide. Notably, test strips stained with CTR were fabricated, which could serve as an efficient and suitable cyanide test kit. Moreover, CTR could selectively accumulate in lipid droplets and visualize the metabolism of lipid droplets in live cells. These findings would provide new avenue to design fluorescent probe for effective response to viscosity, detection of cyanide, and bioimaging of lipid droplets in live cells.

Lipid Droplets in Cancer: From Composition and Role to Imaging and Therapeutics

Cancer is the second most common cause of death worldwide, having its origin in the abnormal growth of cells. Available chemotherapeutics still present major drawbacks, usually associated with high toxicity and poor distribution, with only a small fraction of drugs reaching the tumour sites. Thus, it is urgent to develop novel therapeutic strategies. Cancer cells can reprogram their lipid metabolism to sustain uncontrolled proliferation, and, therefore, accumulate a higher amount of lipid droplets (LDs). LDs are cytoplasmic organelles that store neutral lipids and are hypothesized to sequester anti-cancer drugs, leading to reduced efficacy. Thus, the increased biogenesis of LDs in neoplastic conditions makes them suitable targets for anticancer therapy and for the development of new dyes for cancer cells imaging. In recent years, cancer nanotherapeutics offered some exciting possibilities, including improvement tumour detection and eradication. In this review we summarize LDs biogenesis, structure and composition, and highlight their role in cancer theranostics.

Near-infrared emitting gold-silver nanoclusters with large Stokes shifts for two-photon in vivo imaging

Near-infrared emitting bi-metallic gold/silver nanoclusters with large Stokes shifts were manufactured through one-pot synthesis. The gold/silver nanoclusters exhibit strong NIR fluorescence due to the silver effect, which can be applied as a two-photon fluorescent contrast agent for in vivo bioimaging.

Recent Advances in AIEgen-Based Photodynamic Therapy and Immunotherapy

Cancer, one of the leading causes of death, has seriously threatened public health. However, there is still a lack of effective treatments. Nowadays, photodynamic therapy (PDT), relying on photosensitizers to trigger the generation of reactive oxygen species (ROS) for killing cancer cells, has been emerging as a noninvasive anti-cancer strategy. To enhance the overall anti-cancer efficacy of PDT, various approaches including molecular design and combination with other therapeutic techniques have been proposed and implemented. Especially, photodynamic immunotherapy that can effectively evoke the body's immune response has attracted much attention. Recently, a class of photosensitizers with aggregation-induced emission (AIE) character have shown unique promises, taking advantage of their profound fluorescence and ROS-generating ability in the aggregation state. Despite the promising results demonstrated by several groups, the associated studies are few and the mechanism of such AIEgen-based photodynamic immunotherapy has not been fully understood. This review discusses the recent advances in the AIEgen-based enhanced PDT with a special focus on the AIE photosensitizers for photodynamic immunotherapy, aiming to inspire more opportunities for in-depth investigation of the working principles in this emerging anti-cancer approach.

A Novel Photosensitizer for Lipid Droplet-Location Photodynamic Therapy

Lipid droplets (LDs), an extremely important cellular organelle, are responsible for the storage of neutral lipids in multiple biological processes, which could be a potential target site for photodynamic therapy (PDT) of cancer. Herein, a lipid droplet–targeted photosensitizer (BODSeI) is developed, allowing for fluorescence imaging–guided PDT. Owing to the location of lipid droplets, BODSeI demonstrates enhanced PDT efficiency with an extremely low IC50 value (around 125 nM). Besides, BODSeI shows good biocompatibility and high photostability. Therefore, BODSeI is promising for droplet-location PDT, which may trigger wide interest for exploring the pathway of lipid droplet–location PDT.