Intermittent Fluorescence Oscillations in Lipid Droplets in a Live Normal and Lung Cancer Cell: Time-Resolved Confocal Microscopy

Polymeric microenvironment enhancing polarity response sensitivity for discriminating lipid droplets in cancer cells

Cellular micro-environment analysis via fluorescence probe has become a powerful method to explore the early-stage cancer diagnosis and pathophysiological process of relevant diseases. The polarity change of intracellular lipid droplets (LDs) is closely linked with disorders or diseases, which result in various physiological and pathological processes. However, the efficient design strategy for lipid droplet polarity probes with high sensitivity is lacking. To overcome this difficulty, two kinds of LDs-targeting and polarity-sensitive fluorescent probes containing carbazole and siloxane groups were rationally designed and synthesized. With the carbazole-based rotor and bridge-like siloxanes, two probes (P1 and P2) behave high sensitivity to polarity changes and show different fluorescent intensity in normal and cancer cells. Notably, polysiloxanes groups promoted the response sensitivity of the probes dramatically for the polymeric microenvironment. In addition, due to the polarity changes of LDs in cancer cells, the distinct fluorescent intensities in different channels of laser scanning confocal microscope were observed between NHA cell and U87 cells. This work could offer an opportunity to monitor the dynamic behaviors of LDs and further provide a powerful tool to be potentially applied in the early-stage diagnosis of cancer.

Photoactivable AIEgen-based Lipid-Droplet-Specific Drug Delivery Model for Live Cell Imaging and Two-Photon Light-Triggered Anticancer Drug Delivery

Lipid droplets (LDs) are dynamic complex organelles involved in various physiological processes, and their number and activity are linked to multiple diseases, including cancer. In this study, we have developed LD-specific near-infrared (NIR) light-responsive nano-drug delivery systems (DDSs) based on chalcone derivatives for cancer treatment. The reported nano-DDSs localized inside the cancer microenvironment of LDs, and upon exposure to light, they delivered the anticancer drug valproic acid in a spatiotemporally controlled manner. The developed systems, namely, 2'-hydroxyacetophenone-dimethylaminobenzaldehyde-valproic (HA-DAB-VPA) and 2'-hydroxyacetophenone-diphenylaminobenzaldehyde-valproic (HA-DPB-VPA) ester conjugates, required only two simple synthetic steps. Our reported DDSs exhibited interesting properties such as excited-state intramolecular proton transfer (ESIPT) and aggregation-induced emission (AIE) phenomena, which provided advantages such as AIE-initiated photorelease and ESIPT-enhanced rate of photorelease upon exposure to one- or two-photon light. Further, colocalization studies of the nano-DDSs by employing two cancerous cell lines (MCF-7 cell line and CT-26 cell line) and one normal cell line (HEK cell line) revealed LD concentration-dependent enhanced fluorescence intensity. Furthermore, systematic investigations of both the nano-DDSs in the presence and absence of oleic acid inside the cells revealed that nano-DDS HA-DPB-VPA accumulated more selectively in the LDs. This unique selectivity by the nano-DDS HA-DPB-VPA toward the LDs is due to the hydrophobic nature of the diphenylaminobenzaldehyde (mimicking the LD core), which significantly leads to the aggregation and ESIPT (at 90% volume of fw, ΦF = 20.4% and in oleic acid ΦF = 24.6%), respectively. Significantly, we used this as a light-triggered anticancer drug delivery model to take advantage of the high selectivity and accumulation of the nano-DDS HA-DPB-VPA inside the LDs. Hence, these findings give a prototype for designing drug delivery models for monitoring LD-related intracellular activities and significantly triggering the release of LD-specific drugs in the biological field.

Cell lipid droplet heterogeneity and altered biophysical properties induced by cell stress and metabolic imbalance

Lipid droplets (LD) are important regulators of lipid metabolism and are implicated in several diseases. However, the mechanisms underlying the roles of LD in cell pathophysiology remain elusive. Hence, new approaches that enable better characterization of LD are essential. This study establishes that Laurdan, a widely used fluorescent probe, can be used to label, quantify, and characterize changes in cell LD properties. Using lipid mixtures containing artificial LD we show that Laurdan GP depends on LD composition. Accordingly, enrichment in cholesterol esters (CE) shifts Laurdan GP from ~0.60 to ~0.70. Moreover, live-cell confocal microscopy shows that cells present multiple LD populations with distinctive biophysical features. The hydrophobicity and fraction of each LD population are cell type dependent and change differently in response to nutrient imbalance, cell density, and upon inhibition of LD biogenesis. The results show that cellular stress caused by increased cell density and nutrient overload increased the number of LD and their hydrophobicity and contributed to the formation of LD with very high GP values, likely enriched in CE. In contrast, nutrient deprivation was accompanied by decreased LD hydrophobicity and alterations in cell plasma membrane properties. In addition, we show that cancer cells present highly hydrophobic LD, compatible with a CE enrichment of these organelles. The distinct biophysical properties of LD contribute to the diversity of these organelles, suggesting that the specific alterations in their properties might be one of the mechanisms triggering LD pathophysiological actions and/or be related to the different mechanisms underlying LD metabolism.

A lipid droplet-targetable and biothiol-sensitive fluorescent probe for the diagnosis of cancer cells/tissues

Lipid droplets (LDs) have recently been reported as an attractive target for cancer diagnosis and treatment, owing to their special structure or microenvironment changes in cancer development and resistance. However, the relationship between the biothiol level of LDs and cancer is still poorly understood, partially owing to the absence of effective molecular tools. Herein, we developed a LD-targetable and biothiol-sensitive fluorescent probe, BTDA-RSS, by introducing 2,4-dinitrobenzenesulfonyl (DNBS) as the biothiol reaction group into a benzothiazolyl derivative. BTDA-RSS displayed a marked and rapid fluorescence turn-on response toward biothiols, due to the biothiol-triggered cleavage of DNBS to yield the highly fluorescent benzothiazolyl iminocoumarin BTDA. In addition, the probe shows significant LD-targetable ability, and has been applied for imaging endogenous/exogenous biothiol changes in LDs. Importantly, BTDA-RSS has successfully been utilized to distinguish cancerous cells/tissues from normal cells/tissues with excellent contrast. Surprisingly, we demonstrated for the first time the visualization of LD biothiols in surgical specimens from cancer patients, thereby holding great potential for the application of BTDA-RSS in the clinical diagnosis of human cancers.

A Near-Infrared Probe for Specific Imaging of Lipid Droplets in Living Cells

Lipid droplets (LDs) are involved in various physiological processes and associated with cancer development, and are regarded as a potential tumor marker for cancer diagnosis. Monitoring LDs is of prior importance to understand their involvement in biological mechanisms and the early detection of cancers. Highly sensitive and specific noninvasive fluorescent probes are particularly desirable for imaging LDs and cancer diagnosis. Herein, according to the high-viscosity and low-polarity microenvironment in LDs, we developed four easily prepared LDs-specific probes based on noncharged merocyanines. Among them, LD-1 absorbs and emits in the near-infrared (NIR) region with a large Stokes shift. Importantly, LD-1 displayed high sensitivity to high viscosity and low polarity, which allowed it to show high LDs-targeting ability. In cell imaging, LD-1 successfully probed the changes in LDs in the presence of oleic acid or during ferroptosis and was used to distinguish cancer cells from normal cells.

Three in One: Stimuli-Responsive Fluorescence, Solid-State Emission, and Dual-Organelle Imaging Using a Pyrene-Benzophenone Derivative

Small organic luminogens, owing to their contrasting stimuli-responsive fluorescence in solution along with strong emission in aggregated and solidstates, have been employed in optoelectronic devices, sensors, and bioimaging. Pyrene derivatives usually exhibit strong fluorescence and concentration-dependent excimer/aggregate emission in solution. However, the impacts of microenvironments on the monomer and aggregate emission bands and their relative intensities in solution, solid, and supramolecular aggregates are intriguing. The present study delineates a trade-off between the monomer and aggregate emissions of a pyrene-benzophenone derivative (ABzPy) in solution, in the solid-state, and in nanoaggregates through a combined spectroscopic and microscopic approach. The impact of external stimuli (viscosity, pH) on the aggregate emission was demonstrated using steady-state and time-resolved spectroscopy, including fluorescence correlation spectroscopy and fluorescence anisotropy decay analysis. The aggregate formation was noticed at a higher concentration (>10 μM) in solution, at 77 K (5 μM), and in the solid-state due to the π-π stacking interactions (3.6 Å) between two ABzPy molecules. In contrast, no aggregate formation was observed in the viscous medium as well as in a micellar environment even at a higher concentration of ABzPy (50 μM). The crystal structure analysis further shed light on the intermolecular hydrogen-bonding-assisted solid-state emission, which was found to be highly sensitive toward external stimuli like pH and mechanical forces. The broad emission band comprising both monomer and aggregate in the aqueous dispersion of nanoaggregates was used for the specific cellular imaging of lysosomes and lipid droplets, respectively.

D-Limonene inhibits the occurrence and progression of LUAD through suppressing lipid droplet accumulation induced by PM2.5 exposure in vivo and in vitro

BACKGROUND

PM_2.5 exposure is associated with lung adenocarcinoma (LUAD), but the mechanism is unclear. The lack of understanding impedes our effort on prevention. This study examined a possible mechanism of lung cancer caused by PM_2.5 exposure, and aimed to find a potential intervention for people living in PM_2.5 polluted regions.

METHODS

Electron microscopy and oil-red staining were conducted to examine the lipid droplet accumulation. Masson's trichrome staining, colony forming, scratch assay and transwell experiment were conducted to evaluate the effect of PM_2.5 exposure and D-limonene intervention on the occurrence and progression of LUAD. Potential intervention targets were found by RNA-Seq and verified by luciferase reporter assay. MiR-195 KO mice constructed with CRISPR/Cas9 technology were used to investigate the pivotal role of D-limonene-miR-195-SREBP1/FASN axis. Cohort analysis of lung cancer patients, human LUAD tissues staining and human intervention trial were also conducted to validate the results of cell and animal experiments.

RESULTS

Our results showed that PM_2.5 exposure induced accumulation of lipid droplets in LUAD cells which accompanied by increased malignant cellular behaviors. PM_2.5 exposure led to cleaved N-SREBP1 translocation into nucleus, which activated the de novo lipogenesis pathway. Same changes were also observed in normal lung epithelial cells and normal lung tissue, and mice developed pulmonary fibrosis after long-term exposure to PM_2.5. Furthermore, in a cohort of 11,712 lung cancer patients, significant lipid metabolism disorders were observed in higher PM_2.5 polluted areas. In view of that, D-limonene was found to inhibit the changes in lipid metabolism through upregulating the expression of miR-195, which inhibited the expression of lipogenic genes (SREBF1/FASN/ACACA) specifically. And a small human intervention trial showed that serum miR-195 was upregulated after oral intake of D-limonene.

CONCLUSION

Our findings reveal a new mechanism of pulmonary fibrosis and LUAD that is related to PM_2.5 exposure-induced lipid droplet accumulation. We also demonstrate that D-limonene-miR-195-SREBP1/FASN axis is a potential preventive intervention for mediating the progression and development of LUAD induced by PM_2.5 exposure. Trial registration Chinese Clinical Trial Registry, ChiCTR2000030200. Registered 25 February 2020, http://www.chictr.org.cn/showproj.aspx?proj=48013.

Lipid droplet polarity decreases during the pathology of muscle injury as revealed by a polarity sensitive sensor

Revealing the relationship between lipid droplets (LDs)polarity and disease is indispensable in clinicopathological diagnosis. So far, muscle injury is often ignored as it is not life-threatening as cardiovascular and cerebrovascular diseases, making the exploration of the internal relationship between muscle injury and LDs polarity a gray area. Herein, a fluorescent probe (CCB) with powerful polar-sensitive as well as precise LDs targeting was designed for visualizing the LDs polarity in the pathology of muscle injury. By means of the probe CCB, the identification of cancer cells and the monitoring of LDs polarity changes in dysfunctional cells were successfully realized. Furthermore, the penetration ability of CCB in tissues of mice was tested to verify the applicability of the probe in organisms. Importantly, by CCB, the relationship between muscle damage and LDs polarity was explored, revealing that muscle damage caused a significant decrease in LDs polarity accompanied by a significant increase in fluorescence. Most importantly, it is the first time to reveal the relationship between muscle damage and LDs polarity. Therefore, the probe CCB will be a powerful monitoring platform for diagnosing related diseases caused by abnormal LDs polarity.

Small molecule based fluorescent chemosensors for imaging the microenvironment within specific cellular regions

The microenvironment (local environment), including viscosity, temperature, polarity, hypoxia, and acidic-basic status (pH), plays indispensable roles in cellular processes. Significantly, organelles require an appropriate microenvironment to perform their specific physiological functions, and disruption of the microenvironmental homeostasis could lead to malfunctions of organelles, resulting in disorder and disease development. Consequently, monitoring the microenvironment within specific organelles is vital to understand organelle-related physiopathology. Over the past few years, many fluorescent probes have been developed to help reveal variations in the microenvironment within specific cellular regions. Given that a comprehensive understanding of the microenvironment in a particular cellular region is of great significance for further exploration of life events, a thorough summary of this topic is urgently required. However, there has not been a comprehensive and critical review published recently on small-molecule fluorescent chemosensors for the cellular microenvironment. With this review, we summarize the recent progress since 2015 towards small-molecule based fluorescent probes for imaging the microenvironment within specific cellular regions, including the mitochondria, lysosomes, lipid drops, endoplasmic reticulum, golgi, nucleus, cytoplasmic matrix and cell membrane. Further classifications at the suborganelle level, according to detection of microenvironmental factors by probes, including polarity, viscosity, temperature, pH and hypoxia, are presented. Notably, in each category, design principles, chemical synthesis, recognition mechanism, fluorescent signals, and bio-imaging applications are summarized and compared. In addition, the limitations of the current microenvironment-sensitive probes are analyzed and the prospects for future developments are outlined. In a nutshell, this review comprehensively summarizes and highlights recent progress towards small molecule based fluorescent probes for sensing and imaging the microenvironment within specific cellular regions since 2015. We anticipate that this summary will facilitate a deeper understanding of the topic and encourage research directed towards the development of probes for the detection of cellular microenvironments.

Detecting lipid droplets polarity: Silicone-based unique fluorescent probe for cancer diagnosis in living cells

Fluorescent probes for monitoring polarity of lipid droplets (LDs) are essential tools in pathological research, especially cancer related. Herein, we have designed a biocompatible and novel fluorescent probe (TDCQ) with intramolecular charge transfer mechanism, which consists of a naphthalimide moiety accepting electron and a triphenylamine fragment providing electron. In view of polarity-sensitivity and AIE characteristic, TDCQ specially aggregates on the LDs in cells by remarkable green dots fluorescent. Due to the variation of LDs numbers in normal cells and cancer cells, the probe emits stronger green fluorescence in cancer cells but weaker in normal cells. Moreover, TDCQ has outstanding photostability and low toxicity, permitting green fluorescence to persist for a valid time in cells. This article demonstrates that the capacity of TDCQ for facilitating the in-depth study of LDs and applying to the identification of cancer cells.

A fluorogenic probe for dynamic tracking of lipid droplets’ polarity during the evolution of cancer

Exploring the changes in the polarity of intracellular lipid droplets (LDs) during the evolution of cancer is important for cancer detection and treatment.

Mechanisms and Future of Non-Small Cell Lung Cancer Metastasis

Lung cancer, renowned for its fast progression and metastatic potency, is rising to become a leading cause of death globally. It has been long observed that lung cancer is particularly ept in spawning distant metastasis at its early stages, and it can readily colonize virtually any human organ. In recent years, cancer research has shed light on why lung cancer is endowed with its exceptional ability to metastasize. In this review, we will take a comprehensive look at the current research on lung cancer metastasis, including molecular pathways, anatomical features and genetic traits that make lung cancer intrinsically metastatic, as we go from lung cancer’s general metastatic potential to the particular metastasis mechanisms in multiple organs. We highly concerned about the advanced discovery and development of lung cancer metastasis, indicating the importance of lung cancer specific gene mutations, heterogeneity or biomarker discovery, and discussing potential opportunities and challenges. We will also introduce some current treatments that targets certain metastatic strategies of non-small cell lung cancer (NSCLC). Advances made in these regards could be critical to our current knowledge base of lung cancer metastasis.

Lipid metabolism gene-wide profile and survival signature of lung adenocarcinoma

Background

Lung cancer has high morbidity and mortality across the globe, and lung adenocarcinoma (LUAD) is the most common histologic subtype. Disordered lipid metabolism is related to the development of cancer. Analysis of lipid-related transcriptome helps shed light on the diagnosis and prognostic biomarkers of LUAD.

Methods

In this study, expression analysis of 1045 lipid metabolism-related genes was performed between LUAD tumors and normal tissues derived from the Cancer Genome Atlas Lung Adenocarcinoma (TCGA-LUAD) cohort. The interaction network of differentially expressed genes (DEGs) was constructed to identify the hub genes. The association between hub genes and overall survival (OS) was evaluated and formed a model to predict the prognosis of LUAD using a nomogram. The model was validated by another cohort, GSE13213.

Results

A total of 217 lipid metabolism-related DEGs were detected in LUAD. Genes were significantly enriched in glycerophospholipid metabolism, fatty acid metabolic process, and eicosanoid signaling. Through network analysis and cytoHubba, 6 hub genes were identified, including INS, LPL, HPGDS, DGAT1, UGT1A6, and CYP2C9. High expression of CYP2C9, UGT1A6, and INS, and low expressions of DGAT1, HPGDS, and LPL, were associated with worse overall survival for 1925 LUAD patients. The model showed that the high-risk score group had a worse OS, and the validated cohort showed the same result.

Conclusions

In this study, a signature of 6 lipid metabolism genes was constructed, which was significantly associated with the diagnosis and prognosis of LUAD patients. Thus, the gene signature can be used as a biomarker for LUAD.

Light-Up Lipid Droplets Dynamic Behaviors Using a Red-Emitting Fluorogenic Probe

Intracellular lipid metabolism occurs in lipid droplets (LDs), which is critical to the survival of cells. Imaging LDs is an intuitive way to understand their physiology in live cells. However, this is limited by the availability of specific probes that can properly visualize LDs in vivo. Here, an LDs-specific red-emitting probe is proposed to address this need, which is not merely with an ultrahigh signal-to-noise (S/N) ratio and a large Stokes shift (up to 214 nm) but also with superior resistance to photobleaching. The probe has been successfully applied to real-time tracking of intracellular LDs behaviors, including fusion, migration, and lipophagy processes. We deem that the proposed probe here offers a new possibility for deeper understanding of LDs-associated behaviors, elucidation of their roles and mechanisms in cellular metabolism, and determination of the transition between adaptive lipid storage and lipotoxicity as well.

A novel hybrid of 3-benzyl coumarin seco-B-ring derivative and phenylsulfonylfuroxan induces apoptosis and autophagy in non-small-cell lung cancer

BACKGROUND

Compound 6, as a novel hybrid of 3-benzyl coumarin seco-B-ring derivative and nitric oxide (NO) donor phenylsulfonylfuroxan, has the potential to develop into an anticancer drug because it displays significant antiproliferation activitity for various solid cancer cell lines including non-small-cell lung cancer (NSCLC) cells.

PURPOSE

We attempt to uncover the capacities of compound 6 to induce apoptosis and autophagy in NSCLC cells, as well as the underlying mechanism involved in this process.

METHODS

The effect of compound 6 on cell viability was evaluated in A549 cells by MTT assay. Apoptosis was mainly detected by flow cytometry. The induction of autophagy was observed by transmission electron microscopy (TEM), confocal microscopy as well as western-blotting technique. The expression of all related-proteins including PI3K/Akt/mTOR signaling pathway were also examined by western-blotting technique.

RESULTS

Above all, distinct growth inhibition and caspase-dependent apoptosis were detected in A549 cells administered with compound 6. Then, we confirmed the induction of autophagy triggered by compound 6 in A549 cells. Noticeably, blocking autophagy using a series of inhibitors and ATG5 siRNA had little effect on the cytotoxicity of compound 6, elucidating nonprotective autophagy triggered in NSCLC cells. Further research illustrated that PI3K/Akt/mTOR signaling pathway was involved in compound 6-induced apoptosis, and 3-MA as well as LY294002 had synergistic inhibiting effect on proliferation of A549 cells through the pathway mentioned above.

CONCLUSION

These findings raise a rationale that this 3-benzyl coumarin seco-B-ring derivative and phenylsulfonylfuroxan hybrid could be a promising candidate for developing as a therapeutic agent toward NSCLC, and the combination therapy through PI3K/Akt/mTOR signaling pathway may result in optimized treatment outcomes.

Identification of gene products that control lipid droplet size in yeast using a high-throughput quantitative image analysis

Lipid droplets (LDs) are important organelles involved in energy storage and expenditure. LD dynamics has been investigated using genome-wide image screening methods in yeast and other model organisms. For most studies, genes were identified using two-dimensional images with LD enlargement as readout. Due to imaging limitation, reduction of LD size is seldom explored. Here, we aim to set up a screen that specifically utilizes reduced LD size as the readout. To achieve this, a novel yeast screen is set up to quantitatively and systematically identify genes that regulate LD size through a three-dimensional imaging-based approach. Cidea which promotes LD fusion and growth in mammalian cells was overexpressed in a yeast knockout library to induce large LD formation. Next, an automated, high-throughput image analysis method that monitors LD size was utilized. With this screen, we identified twelve genes that reduced LD size when deleted. The effects of eight of these genes on LD size were further validated in fld1 null strain background. In addition, six genes were previously identified as LD-regulating genes. To conclude, this methodology represents a promising strategy to screen for players in LD size control in both yeast and mammalian cells to aid in the investigation of LD-associated metabolic diseases.

Live Cell Microscopy: A Physical Chemistry Approach

Probing dynamics of intracellular components using physical chemistry techniques is a remarkable bottom-up approach for understanding the structures and functions of a biological cell. In this "Feature Article", we give an overview on local polarity, solvation, viscosity, acid-base property, red-ox processes (thiol-disulfide exchange), and gene silencing at selected intracellular components inside a live cell. Significant differences have been observed between cancer cells and their noncancer counterparts. We demonstrate that thiol-disulfide exchange, calcium oscillation, and gene silencing are manifested in time dependence of fluorescence intensity. We show that fluorescent gold nanoclusters may be used in drug delivery (e.g., doxorubicin) and selective killing of cancer cells. Further, we discuss dynamics and structural changes of DNA quadruplexes and i-motifs, induced by different external conditions (e.g., pH) and additives (e.g., K⁺ and other target specific small molecules). We demonstrate that peptidomimetic analogues have high specificity over double-stranded DNA for binding with i-motifs and G-quadruplexes. These results may have significant biological implications.

Rational design of a lipid-droplet-polarity based fluorescent probe for potential cancer diagnosis

We have rationally designed a robust fluorescent probe CTPA for potential cancer diagnosis by monitoring LD numbers and polarity variation. With the outstanding solvatochromism and high specificity for LDs of CTPA, the diagnosis of cancer can be achieved not only at the cellular levels but also in organs and living mice for the first time.

Bone Marrow Adipocyte: An Intimate Partner With Tumor Cells in Bone Metastasis

The high incidences of bone metastasis in patients with breast cancer, prostate cancer and lung cancer still remains a puzzling issue. The "seeds and soil" hypothesis suggested that bone marrow (soil) may provide a favorable "niche" for tumor cells (seed). When seeking for effective ways to prevent and treat tumor bone metastasis, most researchers focus on tumor cells (seed) but not the bone marrow microenvironment (soil). In reality, only a fraction of circulating tumor cells (CTCs) could survive and colonize in bone. Thus, the bone marrow microenvironment could ultimately determine the fate of tumor cells that have migrated to bone. Bone marrow adipocytes (BMAs) are abundant in the bone marrow microenvironment. Mounting evidence suggests that BMAs may play a dominant role in bone metastasis. BMAs could directly provide energy for tumor cells, enhance the tumor cell proliferation, and resistance to chemotherapy and radiotherapy. BMAs are also known for releasing some inflammatory factors and adipocytokines to promote or inhibit bone metastasis. In this review, we made a comprehensive summary for the interaction between BMAs and bone metastasis. More importantly, we discussed the potentially promising methods for the prevention and treatment of bone metastasis. Genetic disruption and pharmaceutical inhibition may be effective in inhibiting the formation and pro-tumor functions of BMAs.

Physical chemistry in a single live cell: confocal microscopy

A confocal microscope can be used to differentiate between cancer and non-cancer cells, and to enrich our knowledge of 3D tumor spheroids and drug delivery.

Spectral mapping of 3D multi-cellular tumor spheroids: time-resolved confocal microscopy

The tumor micro-environment of 3D multicellular spheroids and their interaction with a drug molecule are studied using time resolved confocal microscopy.