Integrin-FAK signaling rapidly and potently promotes mitochondrial function through STAT3
- Nishant P. Visavadiya†1,
- Matthew P. Keasey†1,
- Vladislav Razskazovskiy1,
- Kalpita Banerjee1,
- Cuihong Jia1,
- Chiharu Lovins1,
- Gary L. Wright1 and
- Theo Hagg1Email author
© The Author(s). 2016
Received: 2 September 2016
Accepted: 6 December 2016
Published: 15 December 2016
STAT3 is increasingly becoming known for its non-transcriptional regulation of mitochondrial bioenergetic function upon activation of its S727 residue (S727-STAT3). Lengthy mitochondrial dysfunction can lead to cell death. We tested whether an integrin-FAK-STAT3 signaling pathway we recently discovered regulates mitochondrial function and cell survival, and treatments thereof.
Cultured mouse brain bEnd5 endothelial cells were treated with integrin, FAK or STAT3 inhibitors, FAK siRNA, as well as integrin and STAT3 activators. STAT3 null cells were transfected with mutant STAT3 plasmids. Outcome measures included oxygen consumption rate for mitochondrial bioenergetics, Western blotting for protein phosphorylation, mitochondrial membrane potential for mitochondrial integrity, ROS production, and cell counts.
Vitronectin-dependent mitochondrial basal respiration, ATP production, and maximum reserve and respiratory capacities were suppressed within 4 h by RGD and αvβ3 integrin antagonist peptides. Conversely, integrin ligands vitronectin, laminin and fibronectin stimulated mitochondrial function. Pharmacological inhibition of FAK completely abolished mitochondrial function within 4 h while FAK siRNA treatments confirmed the specificity of FAK signaling. WT, but not S727A functionally dead mutant STAT3, rescued bioenergetics in cells made null for STAT3 using CRISPR-Cas9. STAT3 inhibition with stattic in whole cells rapidly reduced mitochondrial function and mitochondrial pS727-STAT3. Stattic treatment of isolated mitochondria did not reduce pS727 whereas more was detected upon phosphatase inhibition. This suggests that S727-STAT3 is activated in the cytoplasm and is short-lived upon translocation to the mitochondria. FAK inhibition reduced pS727-STAT3 within mitochondria and reduced mitochondrial function in a non-transcriptional manner, as shown by co-treatment with actinomycin. Treatment with the small molecule bryostatin-1 or hepatocyte growth factor (HGF), which indirectly activate S727-STAT3, preserved mitochondrial function during FAK inhibition, but failed in the presence of the STAT3 inhibitor. FAK inhibition induced loss of mitochondrial membrane potential, which was counteracted by bryostatin, and increased superoxide and hydrogen peroxide production. Bryostatin and HGF reduced the substantial cell death caused by FAK inhibition over a 24 h period.
These data suggest that extracellular matrix molecules promote STAT3-dependent mitochondrial function and cell survival through integrin-FAK signaling. We furthermore show a new treatment strategy for cell survival using S727-STAT3 activators.
KeywordsBioenergetics Cell death CRISPR ECM Endothelial cell Focal adhesion kinase Integrin Mitochondria Vitronectin STAT3
Integrins are heterodimer transmembrane receptors which bind ECM molecules to promote cell adhesion and initiate intracellular signaling that can lead to cell survival [1, 2]. Disruption of integrin binding can cause cell death, especially for cells attached to basement membranes , e.g., endothelial cells in the central nervous system (CNS). Among others, endothelial cells express αvβ3 integrins which contribute to their survival [4, 5]. Integrin signaling is important for normal endothelial cell function in maintaining the blood-brain-barrier (BBB) [6, 7], whose disruption by neural injury and stroke leads to disease progression . FAK is one of the major integrin signaling mediators and is activated via autophosphorylation on Y397  which can suppress apoptosis in endothelial cells .
Mitochondria not only play a vital role in energy production, particularly in the CNS , but also have emerged as a key stress-signaling hub within the cell . CNS endothelial cells have a very high mitochondrial mass compared to those of other organs , and mitochondrial function is important for maintaining the BBB and ATP-dependent trans-endothelial transport [13, 14]. Mitochondrial dysfunction after neurological insults plays a role in BBB breakdown and tissue degeneration [7, 15, 16]. Lengthy mitochondrial bioenergetic dysfunction leads to depletion of ATP, increased production of reactive oxygen/nitrogen species, calcium dysregulation, and release of pro-apoptotic proteins, leading to cell death [17, 18].
Integrins can prevent apoptosis through FAK-AKT signaling [10, 19, 20], and inhibiting mitochondria-associated bit1 [20, 21], but have not been implicated in bioenergetic function. We recently discovered an integrin signaling pathway that inhibits CNTF expression, involving FAK, JNK and the S727 residue of the transcription factor STAT3 . Depending on phosphorylation of residues S727 or Y705, STAT3 can inhibit or promote nuclear gene expression . Recent seminal studies identified a non-transcriptional role of pS727-STAT3 in stimulating mitochondrial bioenergetic function through electron transport chain (ETC) complex I, II and V activity [24–26], probably not by binding directly , but by binding to prohibitin 1 . STAT3 can also reduce formation of the mitochondrial permeability transition pore, possibly by interacting with cyclophilin D , thus maintaining membrane potential necessary for bioenergetic function, as well as preventing release of cytochrome-c, which leads to apoptosis . STAT3 can also reduce ROS formation, possibly by improving complex I coupling, improving cell survival under ischemic conditions [30, 31]. It is unknown whether integrin or FAK signaling interacts with pS727-STAT3 to regulate mitochondrial function.
A few studies have identified mechanisms that activate S727-STAT3, including cytoplasmic PKC isoforms [32, 33] and the c-Met receptor acting via ERK and AKT [34, 35]. The small molecule bryostatin 1 activates PKC  whereas HGF activates c-Met . They are being developed for treating Alzheimer’s disease  and spinal cord injury , respectively. Stimulating S727-STAT3 more directly may be a treatment strategy that can circumvent pathologically disrupted integrin or FAK signaling.
The present study determined whether ECM-integrin-FAK-STAT3 signaling promotes mitochondrial function in cultured endothelial cells and whether treatments that activate STAT3 could rescue cells against the loss of FAK signaling.
The immortalized mouse brain endothelioma cell line (bEnd5) was created and characterized as a good model for CNS endothelial cells [39–41]. Culture medium components were from Gibco. The cells were grown in in T-75 Flask (Cat # CC7682-4175, CytoOne, Ocala, FL) in DMEM supplemented with 10% fetal calf serum, 3 mM glutamine, 100 units/ml penicillin and 100 μg/ml streptomycin, 1 mM sodium pyruvate, 1% non-essential amino acids. Cells were passaged every seven days with cells up to 35 passages being used for experiments. Before use, replated cells were maintained for 24 h except where noted specifically. The cells were counted in high resolution images obtained with a 10× objective, five fields per well, using ImageJ software (NIH), with 3–5 wells per condition.
Mitochondrial bioenergetics measurements
Mitochondrial bioenergetics measurements were made with an XF24 Extracellular Flux Analyzer (Seahorse Bioscience, North Billerica, MA). Cell density was optimized for recommended basal OCR ranges of 50–400 pMole/Min. For vitronectin (VTN) experiments, XF-24 culture plates were incubated with poly-d-lysine for 1 h (50 μg/ml; Sigma), washed 3× with autoclaved H2O, dried and coated with recombinant human VTN (0.5 μg/cm2, Cat#SRP3186, Sigma) for 1 h. BEnd5 cells were seeded at 100,000/well with or without RGDS (Arg-Gly-Asp-Ser peptide, 100 μg/ml, Cat#3498, Tocris, Bristol, UK) or P11 (His-Ser-Asp-Val-His-Lys-NH2 peptide, 10 μg/ml, Cat#4744, Tocris) peptides in serum-free medium supplemented with 2% B27 (Gibco cat# 10889038) for 4 h. We also tested the effects of VTN, EHS mouse laminin-111 (Cat#L2020, Sigma) and human plasma fibronectin (Cat#F3879, Sigma) as 50 μg/ml substrate with 10 μg/ml added to serum-free medium for 4 h. For the other experiments, 50,000 bEnd5 cells/well were seeded without substrate and grown for 44 h before 4 h incubation with FAK14 (5 or 10 μM, Cat#3414, Tocris) or stattic (5 or 10 μM, Cat#2798, Tocris) with or without bryostatin (50 nM, Cat#2383, Tocris) or HGF (500 ng/ml, Cat#315-23, PeproTech, Rocky Hill, NJ) or actinomycin D (0.3 μg/ml, Cat#1229, Tocris) treatment with or without 5 μM FAK14. After treatments, cells were washed once with XF base media supplemented with 2.5 mM glucose and 1 mM sodium pyruvate, and incubated for 1 h in a non-CO2 incubator. Before use, sensor cartridges were hydrated, loaded with oligomycin (Cat# 495455, Millipore), carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone (FCCP, Cat# 0453, Tocris, 1 μM as used by others [42, 43], antimycin A (Cat# A8654, Sigma), and calibrated according to manufacturer’s instructions. XF24 plates with cells were then loaded and mitochondrial respiration measurements performed according to a standard software protocol. The basal OCR, ATP production, maximum reserve and respiratory capacity were calculated as described , with averages calculated from five wells per condition in each individual experiment. Bioenergetics data are presented as a representative original trace (OCR raw data) while bar graphs were normalized to the average of controls for a set of experiments.
CRISPR-Cas9 mediated knockdown of STAT3
CRISPR-Cas9 pre-cloned guide RNA cassettes targeting mouse STAT3 were purchased from Origene (Cat# KN316845) and knockdown performed according to  with some alterations. Briefly, 2 × 105 bEnd5 cells were seeded into 6-well plates and maintained for 24 h. Lipofectamine 3000 (Cat# L3000, Invitrogen) was used at 0.5% complexed with 5 μg plasmid (2.5 μg Cas9-gRNA with 2.5 μg Donor) in Opti-MEM. Transfections were performed according to standard procedures, with cells maintained for 24 h after transfection. Transfection medium was removed from cells the following day and replaced with fresh medium containing 2 μg/ml puromycin (Cat# A1113803) which caused 100% cell death in non-transfected control cells after 4 days. Cells resistant to puromycin treatment were grown in fresh medium and maintained until confluent (~7–14 days). PCR primers were designed against regions of the mouse STAT3 gene flanking the guide-RNA target sequence and were: Forward 5′-GGCCTTGACCTGTCTGTCAT -3′ and reverse 5′-TGTGCAGAGATCTCACCAAGT -3′ to generate an amplicon of 849 bp in a reaction with 100 ng of genomic DNA extracted from cells with the DNeasy Blood and Tissue kit (Qiagenssa, USA, Cat# 69506). PCR products were run on an agarose gel for purification and sequencing performed by the Molecular Core Facility at ETSU using the following primer 5′-GGCCTTGACCTGTCTGTCAT -3′. A point mutation was identified at the predicted Cas9-guide RNA cut site, resulting in a frame shift. Knockout of protein was confirmed by western blots.
Mitochondria were isolated as previously described . Briefly, cells were homogenized in ice-cold isolation buffer (215 mM mannitol, 75 mM sucrose, 0.1% bovine serum albumin, 20 mM HEPES, 1 mM EGTA; pH adjusted to 7.2 with KOH), followed by centrifugation, twice at 1,300 g for 3 min to obtain nuclear pellets. The supernatant was centrifuged at 13,000 g for 10 min to obtain mitochondrial pellets. The last supernatant was collected as the cytosolic fraction. The cold and the EGTA in the isolation buffer ensure that proteins remain phosphorylated and intact. Moreover, we have found that addition of protease and phosphatase inhibitors (Cat# P8340, Sigma), supplemented with 1 mM sodium orthovanadate, does not reveal any differences. The purity of sub-cellular fractions was confirmed using Western blotting for fraction-specific markers. For another experiment, mitochondria were treated with stattic (10 μM) or okadaic acid (1 μM; Cat# 5934, Cell Signaling) for 2 h in KCl-based respiration buffer (125 mM KCl, 2 mM MgCl2, 2.5 mM KH2PO4, 20 mM HEPES and 0.1% bovine serum albumin, pH 7.2) containing oxidative substrates pyruvate (5 mM) and malate (2.5 mM) before protein extraction.
Western blot analysis
Protein from whole cells was extracted using 1% RIPA lysis buffer (cat# R0278, Sigma) with standard protease and phosphatase inhibitors (Cat# P8340, Sigma). Proteins from subcellular fractions and whole cell lysate were separated by SDS–PAGE using Criterion 4–20% Tris–HCl (10–250 kD) gels (Bio-Rad, Hercules, CA). After transfer to PVDF membranes, and after washing in TBST buffer containing 0.1% Tween 20, and blocking in 5% milk in TBST, the primary antibodies were diluted in 5% milk in TBST and incubated at 4 °C for 18 h. Primary antibodies against pyruvate dehydrogenase (PDH, 1:2,500, Cat#3205), pS727-STAT3 (1:400, Cat#9134), STAT3 (1:1,000, Cat#12640), pY397-FAK (1:500, Cat#3283), FAK (1:1,000, Cat#3285), α-tubulin (1:2,000, Cat#2125) all from Cell Signaling Technology (Danvers, MA) and histone 3H3 (1:15,000, ab1791) and ATPase (1:5,000, ab14730) from Abcam (Cambridge, MA). After washing in TBST, the membranes were incubated in species- and isotype-specific HRP-conjugated secondary antibodies in TBST (1:2000, Cat#7074, Cat#7076, Cell Signaling). Chemiluminescence (ECL, Cat# 34080, ThermoFisher) was used to reveal immunoreactive protein bands, detected by X-ray film or imaged on a Licor Odyssey FC (Lincoln, NE) for 10 min.
Quantitative capillary ProteinSimple westerns
For one of the FAK inhibition experiments, quantitative analysis of protein expression was performed according to the ProteinSimple protocol guide with reagents of a kit (Cat#, SM-W004 and DM-001, ProteinSimple), except where noted. Briefly, cell lysates were diluted to 0.2 μg/μl with 0.1× sample buffer supplemented with 1× fluorescent molecular weight markers and 40 mM DTT for a 5 μl reaction (1 μg protein/reaction). Samples were heated at 95 °C for 5 min before loading into 24 single designated wells of a pre-filled plate along with blocking reagent, primary antibodies (1:25, pS727-STAT3, 1:50, tSTAT3, or 1:500, α-tubulin and ATPase) in antibody diluent, with pSTAT3 and tubulin/ATPase or STAT3 and tubulin/ATPase mixed together for detection within the same capillary), anti-rabbit HRP conjugated secondary antibody, luminol-peroxide mix to generate chemiluminescence and washing buffer. Plates were loaded into the automated ProteinSimple “Wes” for electrophoresis and fluorescence imaged in real time by a CCD camera for immunodetection in the capillary system at default settings: Electrophoresis, 375 volts, 25 min; blocking, 5 min; primary antibody, 30 min; secondary antibody, 30 min. Data was analyzed using Compass software (ProteinSimple) and expressed as peak intensity or synthetic bands. Quantification was performed by normalizing areas under protein peaks to α-tubulin loading control.
siRNA and STAT3 plasmid transfections
SiRNAs against mouse FAK (Cat# L-041099, Dharmacon) or a non-targeting negative control (Cat# D-001810-10, Dharmacon) were transfected into bEnd5 cells using lipofectamine-2000 (Cat #11668, Invitrogen) 24 h after plating cells in Seahorse plates at 12,500 cells/well, with 50 nM siRNA and 0.5% Lipofectamine according to manufacturer’s protocol. A second transfection was performed 24 h later (48 h after plating) due to FAK’s long protein half-life  and bioenergetics assessed 5 days later. Transfection efficiency was >80% as visualized by fluorescent microscopy using siGLO RNA (Cat #D-001630-02, Dharmacon). Mutant S727A STAT3 expression plasmid was obtained from Addgene (Cat# 8708) while wild type mouse STAT3 was PCR amplified from cDNA and directionally cloned into a pcDNA 3.1(+) expression vector using BamHI and EcoRI restriction sites. bEnd5 cells were transfected with 500 ng plasmid per well using lipofectamine 3000 (Cat# L3000, Invitrogen) and maintained for 24 h prior to Sea Horse analysis.
For confocal imaging, bEnd5 cells were seeded at 105 cells/35 mm clear bottom culture dishes (P35G-1.5–14-c, MarTek Corp, Ashland, MA). After 24 h, cells were treated with FAK14 (5 or 10 μM) for 4 h, followed by 30 min incubation with 20 nM tetramethylrhodamine methyl and ethylesters, which accumulate within active mitochondria in a potentiometric fashion (TMRM, Cat#T668, Invitrogen) and 20 μM Hoechst 33342 (Cat # 62249, ThermoFisher Scientific) dyes. The cells were imaged for TMRM red fluorescence (544/590 nm) and nuclear Hoechst blue fluorescence (460/490 nm) using a Leica SP8 confocal microscope.
For spectrofluorometry, bEnd5 cells were grown for 24 h at 10,000/well in 96 wells (Cat#3997, Corning) and treated with FAK14 for 4 h. Afterwards, mitochondrial membrane potential was estimated by incubation with 25 nM TMRM and 300 nM DAPI (Dilactate, Invitrogen), to normalize for the number of cells, for 30 min at 37 °C. Fluorescence (TMRM: 544/590, DAPI 360/450 nm excitation/emission) was measured with a plate reader. O2 •¯ formation was measured by incubation in 10 μM 2-,7-dichlorodihydrofluorescein diacetate (DCFH2-DA, Cat# D6883, Sigma) and 5 U/ml horseradish peroxidase (HRP, CatP8375, Sigma) as described before . Oxidized DCF fluorescence (485/520 nm) was measured after 10 min at 37 °C. H2O2 production was measured using 1 μM Amplex Red (Cat#A36006, Invitrogen) and 0.25 U/ml HRP at 30 °C as described . Formation of fluorescent resorufin from Amplex Red was measured (530/590 nm).
Statistical analyses were performed with Students t-test for two samples or with a One Way Analysis of Variance (ANOVA) followed by a Bonferroni post-hoc test using GraphPad Software (La Jolla, CA). The number of experiments indicated in the text are independent experiments.
Integrin antagonists inhibit mitochondrial function
FAK inhibition causes mitochondrial dysfunction
S727-STAT3 inhibition causes mitochondrial dysfunction
Stattic (6-nitrobenzo[b]thiophene 1,1-dioxide, MW = 211) is a small molecule inhibitor of STAT3, selectively blocking STAT3 SH2 domains at an IC50 of 5 μM, inhibiting STAT3 activation, dimerization and nuclear translocation . It has high specificity against STAT3 compared to STAT1, and other transcription factors (c-Myc, Max, Jun) and leads to reduced Y705 and S727 STAT3 (but not JAK1, JAK2, c-Src, AKT, JNK or ERK) phosphorylation [29, 51, 52]. Stattic caused mitochondrial dysfunction in bEnd5 cells as early as 2 h (Fig. 3d). Most of the pS727-STAT3 was present in the nucleus and mitochondria, as shown by sub-cellular fractionation (Fig. 3e), consistent with other studies . Stattic treatment of bEnd5 cells caused a reduction in mitochondrial pS727-STAT3 within 4 h (Fig. 3f).
To identify where S727-STAT3 is phosphorylated and dephosphorylated, freshly isolated bEND5 mitochondria were treated for 2 h with stattic or the serine/threonine phosphatase inhibitor, okadaic acid. Stattic did not have an effect, suggesting that STAT3 is phosphorylated in the cytoplasm before being imported, as others have found . Okadaic acid increased pS727-STAT3 (Fig. 3g). A higher molecular weight pS727 band was visible after treatment with okadaic acid in a position where a faint band of total STAT3 could be seen after longer exposure (not shown).
FAK inhibition reduces mitochondrial S727-STAT3
S727-STAT3 activators rescue mitochondrial function and cells against FAK inhibition
BEnd5 cell survival was assessed in regular tissue culture plates. FAK14 dose- and time-dependently caused loss of bEnd5 cells (Fig. 6a-e), up to more than 50% (Fig. 6d, e), and may have reduced proliferation between 4 and 24 h (controls in Fig. 6b vs. c and d vs. e). No cell loss was observed in the bioenergetics experiments (Fig. 2), probably due to the different culture conditions. Bryostatin and HGF promoted bEnd5 cell survival caused by FAK14 treatment at both 4 h (Fig. 6a, b, d) and 24 h (Fig. 6c, e).
FAK inhibition causes mitochondrial depolarization and ROS production
The present study demonstrates that ECM–integrin-FAK signaling regulates mitochondrial bioenergetics via pS727-STAT3. This adds an important component to what was known about integrin signaling functions. ECM-integrin binding is important for cell adhesion and regulates actin organization, cell movement and cell cycle control . Integrins can play an important role in cell survival, especially of cells that are attached to basement membranes, such as endothelial cells which undergo anoikis when attachment is disrupted . Disrupted integrin signaling can induce apoptosis by activating the pro-apoptotic Bad and Bax or reducing anti-apoptotic Bcl-2 [3, 58]. RGDS peptide can trigger pro-apoptotic induction and activation of caspase 8 and 9 in human endothelial cells . The P11 antagonist of αvβ3 integrin can cause apoptosis of human umbilical vein endothelial cells by up-regulating p53 expression resulting in caspase activation . Moreover, lengthy low bioenergetics leads to pathological mitochondrial pore formation and cytochrome-C release thus initiating apoptotic death [17, 18, 61]. Our data suggest that the integrin-FAK-STAT3 pathway is involved in suppressing such cell death mechanisms by maintaining mitochondrial function and integrity. This is consistent with the finding that fibronectin knockdown induces apoptosis in rat mesangial cells in a mitochondria-dependent manner, mainly as a result of cytochrome c release and downstream caspase-3 and −9 activation . Conversely, laminin and fibronectin protect pancreatic cancer cells from death by mechanisms involving the inhibition of both mitochondrial depolarization and caspase activity . Previously, we have shown that an αvβ3 agonist peptide can rescue endothelial cells after contusive spinal cord injury in adult mice .
Integrin-FAK signaling also can regulate mitochondrial biogenesis and morphology. FAK inactivation in cardiomyocytes causes structural abnormalities in mitochondria . Moreover, FAK interacts with mitochondrial transcriptional cascades and enhances mechanical stress-induced mitochondrial biogenesis . It remains to be determined how the FAK-STAT3 pathway affects biogenesis and morphology.
Most of our experiments were conducted with VTN as an integrin ligand. Its effects on mitochondrial function were mediated at least by αvβ3 integrin as shown by the inhibitory effects of the P11 peptide. Fibronectin and laminin also had a stimulating effect, consistent with their role in promoting endothelial cell survival [3, 67, 68], whereas RGD inhibition reduced bioenergetic function. Other ECM molecules also utilize RGD integrins and it remains to be determined whether additional basement membrane or ECM molecules  are involved in regulating mitochondrial function. Vitronectin is present in the basement membrane but also at high concentrations in the blood [70, 71]. It will be interesting to determine the role of plasma VTN on mitochondrial function or whether integrin activation is predominantly ab-luminal, luminal, or both in the vasculature. FAK may be predominantly clustered with the VTN receptor on the ab-luminal side of endothelial cells, where they would make contact with the basement membrane . If so, it is possible that pathological VTN leakage from the blood would promote survival of endothelial cells.
Our data suggest that FAK activates S727-STAT3. It remains to be determined which pathways downstream of FAK might be involved, but could include the serine kinases JNK1  or ERK . The latter is also activated by HGF . The bryostatin results suggest that one or more of the PKC kinases can activate STAT3 even under reduced FAK signaling conditions. FAK can also be activated by growth factor receptors and G-protein-linked receptors [74, 75] raising the possibility that they too could promote to mitochondrial function. The ECM may be involved in orchestrating these miscellaneous extracellular regulators. Our acute pharmacological approach with FAK14 and stattic showed a remarkably complete cessation of mitochondrial bioenergetic function within 4 h. A mitochondrial role for STAT3 has been convincingly demonstrated with STAT3 S727A mutants suppressing bioenergetic function by ~50-70% . Our results, validate these findings by confirming that expression of STAT3 S727A mutants cannot increase bioenergetic function in STAT3 knockout cells using CRISPR-Cas9, whereas wild type STAT3 controls can. Other pathways that regulate STAT3 pS727 activation might also be involved under more physiological conditions. For example, CNTF can promote mitochondrial function through NFKB expression and activity in dorsal root ganglion neurons . It remains to be determined whether this effect is fully transcription-dependent or might also involve S727-STAT3 activation.
The mechanisms and roles of STAT3 within the mitochondria are not well-understood. Our results with actinomycin are consistent with the known non-transcriptional nature of the role of STAT3 in mitochondrial function [24, 26], and support a non-transcriptional effect of FAK inhibition on mitochondria. In an apparent contradiction, in keratinocytes, S727-STAT3 can repress transcription of mitochondrial ETC genes, but no functional measures were reported . Our finding that pS727-STAT3 is mainly found in the mitochondrial and nuclear fractions, but less in the cytoplasm, is consistent with the findings that STAT3 translocates upon S727 phosphorylation in the cytoplasm . Moreover, treatment of isolated mitochondria with stattic, known to reduce phosphorylation and translocation , did not reduce mitochondrial pS727-STAT3. This suggests that the effects of integrin or FAK inhibition are mediated by a reduced translocation to the mitochondria, which may be dependent on chaperones such as HSPs [78, 79]. We also find for the first time that dephosphorylation of S727-STAT3 occurs within the mitochondria, as isolated mitochondria treated for 2 h with the serine/threonine phosphatase inhibitor, okadaic acid, had more pS727-STAT3. The rapidity suggests that mitochondrial pS727-STAT3 is short-lived and its constant import dependent on external stimuli which activate FAK signaling. This would represent a mechanism for rapid adjustments to cellular energy demands. Okadaic acid is known to inhibit PP2A (and PP1) which has been associated with pS727-STAT3 dephosphorylation in the cytoplasm . PP2A has been found in mitochondria  and has been linked to mitochondrial bioenergetics by promoting mitochondrial division through Drp1 dephosphorylation .
We detected an ~100 kDa pS727-STAT3 in okadaic acid-treated or freshly isolated mitochondria. This form may also have been found in avian neurons upon CNTF stimulation, showing a similar short-lived nature and dependency on persistent signaling . A higher molecular weight STAT3 can also be detected in Cos1 cells (Fig. 4 in  and splenocytes (Fig. 3 in ). Our results suggest that only a very minor fraction of mitochondrial STAT3 exists in this form and might be newly imported STAT3 which is post-translationally modified.
Mitochondrial STAT3 is thought to maintain optimal oxidative phosphorylation, by interacting with ETC complexes I, II, and V, and inhibiting formation of the mitochondrial permeability transition pore which would lead to loss of mitochondrial membrane potential [26, 29]. Our results suggest that ligand-dependent integrin-FAK activation of S727-STAT3 is an important mechanism which maintains a physiological mitochondrial function. Together, our data support the idea that the integrin-FAK signaling pathway, through STAT3, prevents the loss of mitochondrial membrane potential and decreased ATP synthesis, which are key mediators of mitochondrial dysfunction and subsequent triggers for release of pro-apoptotic factors and cell death .
Our results also suggest that this pathway promotes the potential antioxidant role of STAT3. In this scenario, STAT3 binds to complex I iron sulfur clusters with its cysteines being oxidized to serve as electron donors, thus reducing ROS generation [30, 31]. Indeed, STAT3 can reduce ROS formation in cardiomyocytes , preventing excessive ROS levels which can lead to cell death . Integrins and ECM molecules have been found to reduce [86, 87] or promote [88, 89] ROS production. It will be interesting to test whether pS727-STAT3 is differentially regulated under those conditions or in those cells. Mitochondrial ROS are also involved in physiological processes, including proliferation, differentiation and migration . It remains to be determined whether the integrin-STAT3 pathway plays a role in these physiological functions. Our finding that bryostatin did not reduce superoxide and H2O2 formation caused by FAK inhibition may explain why it was not fully effective in rescuing mitochondrial function or cell survival, despite restoring normal pS727-STAT3 levels. This raises the possibility that FAK can regulate mitochondrial function through other pathways.
Our study also explored a potential therapeutic strategy for endothelial cells with dysfunctional integrin signaling such as is seen after traumatic detachment, by circumventing integrin-FAK signaling through direct STAT3 activation. Our results are proof of principle that this can be achieved, also because we have seen similar decreases in bioenergetics, pS727-STAT3, and mitochondrial membrane potential after FAK inhibitor treatment in primary endothelial cells derived from adult mouse brain (Visavadiya et al., unpublished results). However, although it remains to be seen whether this therapeutic approach can be optimized and applied to animal models of disease. This is a reasonable expectation because bryostatin is being explored as a treatment for Alzheimer’s disease  and HGF is in clinical trials for spinal cord injury . This new therapeutic strategy could have implications for disorders where endothelial cells are disturbed, e.g., to maintain proper BBB integrity during neurological or other insults. FAK inhibitors are in clinical trials for solid tumors [91, 92] due to their inhibition of angiogenesis and cell adhesion [49, 93]. Our results suggest that mitochondrial dysfunction caused by such agents could also explain some of the effects. In apparent contrast to our proposed effects of FAK inhibition on bioenergetics function and ROS, mitochondrial ROS could trigger gastric cancer progression by stimulating cancer cell migration via the β5-integrin induction . Also, mitochondrial bioenergetic dysfunction caused by mitochondrial DNA mutations leads to increased motility and migration in cytoplasmic hybrid cells “cybrid cells” which harbor A3243T mutation in the leucine transfer RNA gene through a mechanism involving ECM molecules . It remains to be determined whether the integrin-FAK-STAT3 pathway plays different roles in different cell types, and whether it is absent in cancer cells which defeat anoikis to metastasize.
In conclusion, our data suggest that integrin-FAK signaling maintains mitochondrial bioenergetic function by maintaining membrane potential and ETC function, thus reducing ROS formation, via pS727-STAT3. Further, we propose a treatment strategy that can circumvent dysfunction of integrin signaling which may help to develop new neuroprotective treatments. Our finding also provides a platform to investigate fundamental mechanisms of how ligand binding to integrins modulate physiological mitochondrial bioenergetic function in cells.
v-Akt murine thymoma viral oncogene/protein kinase B (PKB)
One way analysis of variance
Adenosine triphosphate synthase
B-cell lymphoma 2
Mouse brain endothelioma cell line
Bcl-2 inhibitor of transcription
CRISPR associated protein 9
Central nervous system
Ciliary neurotrophic factor
Clustered regularly interspaced short palindromic repeats
Dulbecco’s modified eagle medium
Ethylene glycol-bis(β-aminoethyl ether)-N,N,N’,N’-tetraacetic acid
Extracellular signal-regulated kinases
Electron transport chain
FAK14 (10 μM)
FAK14 (5 μM)
Focal adhesion kinase
Carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone
- H2O2 :
Hepatocyte growth factor
Heat shock proteins
c-Jun N-terminal kinases
- KH2PO4 :
Potassium phosphate monobasic
- MgCl2 :
Nuclear factor kappa B
- O2 .- :
Oxygen consumption rate
PF573228 (10 μM)
PF573228 (20 μM)
Protein kinase C
Phosphorylated serine 727 residue of STAT3
Phosphorylated tyrosine 397 residue of FAK
Reactive oxygen species
Stattic (5 μM)
Serine 727 residue of STAT3
Sodium dodecyl sulfate-polyacrylamide gel electrophoresis
Small interfering RNA for FAK
Small interfering RNA
Signal transducer and activator of transcription 3
Tris-buffered saline tween 20
Tetramethylrhodamine, methyl ester
Total signal transducer and activator of transcription 3
- αvβ3 :
alpha v beta 3
- ΔΨm :
We are grateful for the technical support by Aruna Visavadiya, Ying Li, and Rhesa Dykes.
This work was supported by NIH grant NS45734 and ETSU medical school funds.
Availability of data and materials
All data generated or analyzed during this study are included in this published article.
NPV, MPK, VR, KB, CL, CJ and TH performed experiments and analyzed and interpreted data. MPK and TH conceived the overall idea. NPV, MPK and TH wrote the paper. GLW provided intellectual input as well as technical guidance for the Seahorse experiments. All authors read and approved the manuscript.
The authors declare that they have no competing interests.
Consent for publication
Ethics approval and consent to participate
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