Correlating two-photon excited fluorescence imaging of breast cancer cellular redox state with seahorse flux analysis of normalized cellular oxygen consumption

Near-simultaneous quantification of glucose uptake, mitochondrial membrane potential, and vascular parameters in murine flank tumors using quantitative diffuse reflectance and fluorescence spectroscopy

The shifting metabolic landscape of aggressive tumors, with fluctuating oxygenation conditions and temporal changes in glycolysis and mitochondrial metabolism, is a critical phenomenon to study in order to understand negative treatment outcomes. Recently, we have demonstrated near-simultaneous optical imaging of mitochondrial membrane potential (MMP) and glucose uptake in non-tumor window chambers, using the fluorescent probes tetramethylrhodamine ethyl ester (TMRE) and 2-N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)-2-deoxyglucose (2-NBDG). Here, we demonstrate a complementary technique to perform near-simultaneous in vivo optical spectroscopy of tissue vascular parameters, glucose uptake, and MMP in a solid tumor model that is most often used for therapeutic studies. Our study demonstrates the potential of optical spectroscopy as an effective tool to quantify the vascular and metabolic characteristics of a tumor, which is an important step towards understanding the mechanisms underlying cancer progression, metastasis, and resistance to therapies.

Enhanced quantification of metabolic activity for individual adipocytes by label-free FLIM

Fluorescence lifetime imaging microscopy (FLIM) of intrinsic fluorophores such as nicotinamide adenine dinucleotide (NADH) allows for label-free quantification of metabolic activity of individual cells over time and in response to various stimuli, which is not feasible using traditional methods due to their destructive nature and lack of spatial information. This study uses FLIM to measure pharmacologically induced metabolic changes that occur during the browning of white fat. Adipocyte browning increases energy expenditure, making it a desirable prospect for treating obesity and related disorders. Expanding from the traditional two-lifetime model of NADH to a four-lifetime model using exponential fitting and phasor analysis of the fluorescence decay results in superior metabolic assessment compared to traditional FLIM analysis. The four lifetime components can also be mapped to specific cellular compartments to create a novel optical ratio that quantitatively reflects changes in mitochondrial and cytosolic NADH concentrations and binding states. This widely applicable approach constitutes a powerful tool for studies where monitoring cellular metabolism is of key interest.

Visualization of Breast Cancer Metabolism Using Multimodal Nonlinear Optical Microscopy of Cellular Lipids and Redox State

Label-free nonlinear optical microscopy (NLOM) based on two-photon excited fluorescence (TPEF) from cofactors nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FAD+) is widely used for high-resolution cellular redox imaging. In this work, we combined three label-free NLOM imaging methods to quantitatively characterize breast cancer cells and their relative invasive potential: (i) TPEF optical redox ratio (ORR = FAD+/NADH + FAD+), (ii) coherent Raman scattering of cellular lipids, and (iii) second harmonic generation of extracellular matrix (ECM) collagen. 3D spheroid models of primary mammary epithelial (PME) cells and breast cancer cell lines (T47D and MDA-MB-231) were characterized based on their unique ORR and lipid volume fraction signatures. Treatment with 17β-estradiol (E2) increased glycolysis in both PME and T47D ER+ breast cancer acini. However, PME cells displayed increased lipid content with no effect on ECM, while T47D cells had decreased lipid storage (P < 0.001) and significant reorganization of collagen. By measuring deuterated lipids synthesized from exogenously administered deuterium-labeled glucose, treatment of T47D cells with E2 increased both lipid synthesis and consumption rates. These results confirm that glucose is a significant source for the cellular synthesis of lipid in glycolytic breast cancer cells, and that the combination of cellular redox and lipid fraction imaging endpoints is a powerful approach with new and complementary information content.Significance: These findings provide unique insight into metabolic processes, revealing correlations between cancer metastasis and cellular redox state, lipid metabolism, and extracellular matrix. Cancer Res; 78(10); 2503-12. ©2018 AACR.

Protein-bound NAD(P)H Lifetime is Sensitive to Multiple Fates of Glucose Carbon

While NAD(P)H fluorescence lifetime imaging (FLIM) can detect changes in flux through the TCA cycle and electron transport chain (ETC), it remains unclear whether NAD(P)H FLIM is sensitive to other potential fates of glucose. Glucose carbon can be diverted from mitochondria by the pentose phosphate pathway (via glucose 6-phosphate dehydrogenase, G6PDH), lactate production (via lactate dehydrogenase, LDH), and rejection of carbon from the TCA cycle (via pyruvate dehydrogenase kinase, PDK), all of which can be upregulated in cancer cells. Here, we demonstrate that multiphoton NAD(P)H FLIM can be used to quantify the relative concentrations of recombinant LDH and malate dehydrogenase (MDH) in solution. In multiple epithelial cell lines, NAD(P)H FLIM was also sensitive to inhibition of LDH and PDK, as well as the directionality of LDH in cells forced to use pyruvate versus lactate as fuel sources. Among the parameters measurable by FLIM, only the lifetime of protein-bound NAD(P)H (τ₂) was sensitive to these changes, in contrast to the optical redox ratio, mean NAD(P)H lifetime, free NAD(P)H lifetime, or the relative amount of free and protein-bound NAD(P)H. NAD(P)H τ₂ offers the ability to non-invasively quantify diversions of carbon away from the TCA cycle/ETC, which may support mechanisms of drug resistance.

Label-free imaging of redox status and collagen deposition showing metabolic differences in the heart

The heart has high metabolic demand to maintain function. The primary source of energy supply to support correct contractile muscle function differs between a fetus and an adult. In fetal life, ATP is primarily generated by glycolysis and lactate oxidation, whereas following birth, there is a shift towards a reliance on mitochondrial metabolism and fatty acid oxidation. This change in metabolic status is an adaptation to different fuel availability, oxygenation and growth patterns. In this study, we have employed 2-photon excitation fluorescence microscopy to define the relationship between two critical metabolic cofactors nicotinamide adenine dinucleotide(P)H and flavin adenine dinucleotide, effectively utilizing a redox ratio to differentiate between the metabolic status in fetal (proliferative) and adult (quiescent/hypertrophic) hearts. Two-photon imaging was also used to visually confirm the known increase in collagen deposition in the adult heart. The changes observed were consistent with a hypertrophic growth profile and greater availability of fatty acids in the adult heart, compared to the proliferative fetal heart. Two-photon excitation fluorescence microscopy is therefore a convenient imaging technology that enables the monitoring of striated muscle architecture and the metabolic status of heart tissue. This imaging technology can potentially be employed to visualize cardiac and other muscle pathologies.

Mapping metabolic changes by noninvasive, multiparametric, high-resolution imaging using endogenous contrast

Monitoring subcellular functional and structural changes associated with metabolism is essential for understanding healthy tissue development and the progression of numerous diseases, including cancer, diabetes, and cardiovascular and neurodegenerative disorders. Unfortunately, established methods for this purpose either are destructive or require the use of exogenous agents. Recent work has highlighted the potential of endogenous two-photon excited fluorescence (TPEF) as a method to monitor subtle metabolic changes; however, mechanistic understanding of the connections between the detected optical signal and the underlying metabolic pathways has been lacking. We present a quantitative approach to detecting both functional and structural metabolic biomarkers noninvasively, relying on endogenous TPEF from two coenzymes, NADH (reduced form of nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide). We perform multiparametric analysis of three optical biomarkers within intact, living cells and three-dimensional tissues: cellular redox state, NADH fluorescence lifetime, and mitochondrial clustering. We monitor the biomarkers in cells and tissues subjected to metabolic perturbations that trigger changes in distinct metabolic processes, including glycolysis and glutaminolysis, extrinsic and intrinsic mitochondrial uncoupling, and fatty acid oxidation and synthesis. We demonstrate that these optical biomarkers provide complementary insights into the underlying biological mechanisms. Thus, when used in combination, these biomarkers can serve as a valuable tool for sensitive, label-free identification of changes in specific metabolic pathways and characterization of the heterogeneity of the elicited responses with single-cell resolution.

Oxidative stress via inhibition of the mitochondrial electron transport and Nrf-2-mediated anti-oxidative response regulate the cytotoxic activity of plumbagin

Plumbagin, an anti-cancer agent, is toxic to cells of multiple species. We investigated if plumbagin targets conserved biochemical processes. Plumbagin induced DNA damage and apoptosis in cells of diverse mutational background with comparable potency. A 3-5 fold increase in intracellular oxygen radicals occurred in response to plumbagin. Neutralization of the reactive oxygen species by N-acetylcysteine blocked apoptosis, indicating a central role for oxidative stress in plumbagin-mediated cell death. Plumbagin docks in the ubiquinone binding sites (Q0 and Qi) of mitochondrial complexes I-III, the major sites for oxygen radicals. Plumbagin decreased oxygen consumption rate, ATP production and optical redox ratio (NAD(P)H/FAD) indicating interference with electron transport downstream of mitochondrial Complex II. Oxidative stress induced by plumbagin triggered an anti-oxidative response via activation of Nrf2. Plumbagin and the Nrf2 inhibitor, brusatol, synergized to inhibit cell proliferation. These data indicate that while inhibition of electron transport is the conserved mechanism responsible for plumbagin's chemotoxicity, activation of Nrf2 is the resulting anti-oxidative response that allows plumbagin to serve as a chemopreventive agent. This study provides the basis for designing potent and selective plumbagin analogs that can be coupled with suitable Nrf2 inhibitors for chemotherapy or administered as single agents to induce Nrf2-mediated chemoprevention.

Label-free imaging of healthy and infarcted fetal sheep hearts by two-photon microscopy

Coronary heart disease is one of the largest causes of death worldwide, making this a significant health care issue. A critical problem for the adult human heart is that it does not undergo effective repair in response to damage, leaving patients with a poor prognosis. Unlike the adult, fetal hearts have the ability to repair after myocardial damage. Using two-photon microscopy, we have visualised the morphological and metabolic changes following myocardial infarction in sheep fetuses, to characterise response to cardiac injury in a mammalian model. Following myocardial infarction, fetal hearts showed no significant increase in collagen deposition in the region of the infarction, when compared to either the surrounding tissue or shams. In contrast, metabolic activity (i. e. NAD(P)H and FAD) was significantly reduced in the region of myocardial infarction, when compared to either the surrounding tissue or sham hearts. For comparison, we also imaged two hearts from preadolescent sheep (sham and myocardial infarction) and showed highly ordered collagen deposition with decreased metabolic activity within the infarcted area. Therefore, two-photon imaging had the capacity to image both morphological and metabolic changes in response to myocardial infarction and showed differences in the response with age. Picture: Two-photon imaging of myocardial infarction (b and d) enabled the visualisation of increased collagen (blue; Em=431 nm) and changes in other tissue autofluorescence (green; Em=489-606 nm) in fetal (a and b) and preadolescent (c and d) hearts, compared to shams (a and c). The excitation wavelength was 840 nm. Scale bars: 10 μm.

Label-free monitoring of ambient oxygenation and redox conditions using the photodynamics of flavin compounds and transient state (TRAST) spectroscopy

Transient state (TRAST) monitoring can determine population dynamics of long-lived, dark transient states of fluorescent molecules, detecting only the average fluorescence intensity from a sample, when subject to different excitation pulse trains. Like Fluorescence Correlation Spectroscopy (FCS), TRAST unites the detection sensitivity of fluorescence with the environmental sensitivity of long-lived non-fluorescent states, but does not rely on detection of stochastic fluorescence fluctuations from individual molecules. Relaxed requirements on noise suppression, detection quantum yield and time-resolution of the instrument, as well as on fluorescence brightness of the molecules studied, make TRAST broadly applicable, opening also for investigations based on less bright, auto-fluorescent molecules. In this work, we applied TRAST to study the transient state population dynamics within the auto-fluorescent coenzymes flavin adenine dinucleotide (FAD) and flavin-mononucleotide (FMN). From the experimental TRAST data, we defined state models, and determined rate parameters for triplet state and redox transitions within FMN and FAD, stacking and un-stacking rates of external redox active quenching agents and by the adenine moiety of FAD itself. TRAST experiments were found to be well capable to resolve these transitions in FMN and FAD, and to track how the transitions are influenced by ambient oxygenation and redox conditions. This work demonstrates that TRAST provides a useful tool to follow local oxygenation and redox conditions via FMN and FAD fluorescence, and forms the basis for measurements on flavo-proteins and of redox and metabolic conditions in more complex environments, such as in live cells.

CDCP1 drives triple-negative breast cancer metastasis through reduction of lipid-droplet abundance and stimulation of fatty acid oxidation

Triple-negative breast cancer (TNBC) is notoriously aggressive with high metastatic potential, which has recently been linked to high rates of fatty acid oxidation (FAO). Here we report the mechanism of lipid metabolism dysregulation in TNBC through the prometastatic protein, CUB-domain containing protein 1 (CDCP1). We show that a "low-lipid" phenotype is characteristic of breast cancer cells compared with normal breast epithelial cells and negatively correlates with invasiveness in 3D culture. Using coherent anti-Stokes Raman scattering and two-photon excited fluorescence microscopy, we show that CDCP1 depletes lipids from cytoplasmic lipid droplets (LDs) through reduced acyl-CoA production and increased lipid utilization in the mitochondria through FAO, fueling oxidative phosphorylation. These findings are supported by CDCP1's interaction with and inhibition of acyl CoA-synthetase ligase (ACSL) activity. Importantly, CDCP1 knockdown increases LD abundance and reduces TNBC 2D migration in vitro, which can be partially rescued by the ACSL inhibitor, Triacsin C. Furthermore, CDCP1 knockdown reduced 3D invasion, which can be rescued by ACSL3 co-knockdown. In vivo, inhibiting CDCP1 activity with an engineered blocking fragment (extracellular portion of cleaved CDCP1) lead to increased LD abundance in primary tumors, decreased metastasis, and increased ACSL activity in two animal models of TNBC. Finally, TNBC lung metastases have lower LD abundance than their corresponding primary tumors, indicating that LD abundance in primary tumor might serve as a prognostic marker for metastatic potential. Our studies have important implications for the development of TNBC therapeutics to specifically block CDCP1-driven FAO and oxidative phosphorylation, which contribute to TNBC migration and metastasis.

Multicolor two-photon imaging of endogenous fluorophores in living tissues by wavelength mixing

Two-photon imaging of endogenous fluorescence can provide physiological and metabolic information from intact tissues. However, simultaneous imaging of multiple intrinsic fluorophores, such as nicotinamide adenine dinucleotide(phosphate) (NAD(P)H), flavin adenine dinucleotide (FAD) and retinoids in living systems is generally hampered by sequential multi-wavelength excitation resulting in motion artifacts. Here, we report on efficient and simultaneous multicolor two-photon excitation of endogenous fluorophores with absorption spectra spanning the 750-1040 nm range, using wavelength mixing. By using two synchronized pulse trains at 760 and 1041 nm, an additional equivalent two-photon excitation wavelength at 879 nm is generated, and achieves simultaneous excitation of blue, green and red intrinsic fluorophores. This method permits an efficient simultaneous imaging of the metabolic coenzymes NADH and FAD to be implemented with perfect image co-registration, overcoming the difficulties associated with differences in absorption spectra and disparity in concentration. We demonstrate ratiometric redox imaging free of motion artifacts and simultaneous two-photon fluorescence lifetime imaging (FLIM) of NADH and FAD in living tissues. The lifetime gradients of NADH and FAD associated with different cellular metabolic and differentiation states in reconstructed human skin and in the germline of live C. Elegans are thus simultaneously measured. Finally, we present multicolor imaging of endogenous fluorophores and second harmonic generation (SHG) signals during the early stages of Zebrafish embryo development, evidencing fluorescence spectral changes associated with development.

Optical redox ratio identifies metastatic potential-dependent changes in breast cancer cell metabolism

The development of prognostic indicators of breast cancer metastatic risk could reduce the number of patients receiving chemotherapy for tumors with low metastatic potential. Recent evidence points to a critical role for cell metabolism in driving breast cancer metastasis. Endogenous fluorescence intensity of nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FAD) can provide a label-free method for assessing cell metabolism. We report the optical redox ratio of FAD/(FAD + NADH) of four isogenic triple-negative breast cancer cell lines with varying metastatic potential. Under normoxic conditions, the redox ratio increases with increasing metastatic potential (168FARN>4T07>4T1), indicating a shift to more oxidative metabolism in cells capable of metastasis. Reoxygenation following acute hypoxia increased the redox ratio by 43 ± 9% and 33 ± 4% in the 4T1 and 4T07 cells, respectively; in contrast, the redox ratio decreased 14 ± 7% in the non-metastatic 67NR cell line. These results demonstrate that the optical redox ratio is sensitive to the metabolic adaptability of breast cancer cells with high metastatic potential and could potentially be used to measure dynamic functional changes that are indicative of invasive or metastatic potential.