Pharmacological inhibition of Axl affects smooth muscle cell functions under oxidative stress

Abstract

We previously demonstrated that reactive oxygen species (ROS) activate Axl, a receptor tyrosine kinase, resulting in increased survival of rat aortic smooth muscle cells (RASMs). Our experiments in Axl knockout mice showed significant reduction in vascular pathologies. We hypothesize that selective pharmacological inhibitors of Axl could prove beneficial in treating vascular diseases associated with oxidative stress. We investigated a role for two novel compounds specific for Axl (R428 and R572) on ligand independent activation of Axl mediated cell survival and migration. Stimulation of RASMs with H2O2 for 5 min significantly increased Akt phosphorylation (p-Akt). Inhibition at 50% (IC50) of p-Akt was calculated at lower concentrations in R428 (100 nM) and R572 (10 nM) compared to Fc-Axl (2 μg/mL). Flow cytometry staining with Annexin V showed a 2-fold increase in apoptosis with R428 and R572 compared to Fc-Axl after H2O2, which was validated by concomitant increases in cleaved caspase-3. Pretreatment with R428 and R572 decreased cell migration by ~ 50% in response to 20% serum (similar to that after Fc-Axl). R428 and R572 decreased intracellular production of ROS in comparison to Fc-Axl. In conclusion, R428 and R572 are more potent inhibitors of ligand independent mediated Axl signaling compared to Fc-Axl in RASMs under oxidative stress.

1. Introduction

1.1. Axl-dependent pathway

Axl belongs to a family of receptor tyrosine kinases (RTKs) called the TAM family, which includes Tyro3, Axl, and Mer (Lemke and Rothlin, 2008). The endogenous ligand for Axl is growth arrest specific protein 6 (Gas6). Gas6/Axl system increases cell survival and proliferation primarily by activating the Akt signaling pathway (Cavet et al., 2010; Hasanbasic et al., 2004; Melaragno et al., 1998; Nagai et al., 2005). A study by Melaragno et al. (1998) was the first report demonstrating that Axl and Gas6 expression are regulated in response to vascular injury. Our recent findings in Axl knockout (Axl−/−) mice showed that Axl is an important regulator of vascular remodeling (Konishi et al., 2004; Korshunov et al., 2007; Korshunov et al., 2006). Vascular remodeling in response to injury is associated with increases in vascular smooth muscle cells (VSMCs) survival and growth. Axl’s anti-apoptotic effect on VSMCs is primarily dependent on activation of PI3 kinase and Akt (Melaragno et al., 2004). However, Axl can also activate ERK signaling pathways. In a model of diabetes both Axl expression and signaling were differentially regulated. Under conditions of low glucose activation of the lower molecular weight isoform of Axl (114 kDa) resulted in cell survival signaling via the PI3 kinase/Akt/mTOR pathway. In response to high glucose, however, the higher molecular weight isoform of Axl (140 kDa) was expressed and led to activation of ERK with a concomitant increase in VSMCs migration (Cavet et al., 2010).

1.2. Oxidative stress, vascular disease, and Axl activation

Oxidative stress is a well known contributor to the pathogenesis and progression of cardiovascular diseases including hypertension, diabetes and restenosis (Taniyama and Griendling, 2003). Reactive oxygen species (ROS), such as hydrogen peroxide (H2O2), can stimulate VSMCs proliferation, migration, and survival, which are integral processes in vascular diseases (Griendling and Ushio-Fukai, 1998). VSMCs growth and migration occur in response to ROS activation of multiple receptors, stimulated by platelet derived growth factor (PDGF) and angiontensin II (Ang II) (Rao and Berk, 1992; Seshiah et al., 2002; Sundaresan et al., 1995). Our group found that ROS can also activate Axl in VSMCs, which was independent of Gas6 (Konishi et al., 2004). Since ROS is a prominent factor in cardiovascular disease we proposed that Axl is regulated by oxidative stress in the vessel wall. Our in vivo studies in a mouse model of hypertension supported the role of Axl in vascular function under oxidative stress (Korshunov et al., 2007). Therefore, Axl mediated Akt signaling is an attractive target for treatment of vascular pathologies induced by ROS.

1.3. Inhibitors of Axl

The most commonly used inhibitors of the Gas6/Axl pathway are warfarin and recombinant Fc-Axl. Both inhibitors interfere with Axl’s endogenous ligand, Gas6. Warfarin inhibits vitamin K dependent γ- carboxylation, which is a necessary modification required for Gas6 activation (Nakano et al., 1997). Fc-Axl is a recombinant protein which mimics the extracellular immunoglobulin binding domain of Axl thereby neutralizing Gas6 and preventing its interaction with Axl (Nagata et al., 1996). While warfarin and Fc-Axl inhibit ligand mediated Axl activation there is little evidence of their ability to block ligand independent Axl activation, specifically by ROS. The potential benefit of inhibiting Axl mediated signaling has been demonstrated by manipulating Axl expression both molecularly and with transgenic mice (Holland et al., 2005; Konishi et al., 2004; Korshunov et al., 2007; Korshunov et al., 2006). It has also recently been shown that a small molecule inhibitor of Axl, R428, developed by Rigel Pharmaceutical Inc. selectively blocks Axl mediated angiogen- esis and metastasis of breast cancer, emphasizing the therapeutic effects of inhibiting Axl (Holland et al., 2010). The goal of this study was to test the effectiveness of small molecule inhibitors, including R428, to block Axl mediated VSMCs functions under oxidative stress. We hypothesize that these small Axl inhibitors will be effective at blocking ligand independent Axl mediated VSMC cell survival.

2. Methods

2.1. Materials

Axl inhibitors R428, R572, R562, R570, and R624 were provided by Rigel Pharmaceuticals Inc. Chemical structure of these Axl inhibitors are patented: R428 (Holland et al., 2010); R562 (Goff et al., 2008a); R570 (Goff et al., 2008c); R572 (Goff et al., 2008c); and R624 (Goff et al., 2008b). Fc-Axl was purchased from R&D Systems (Minneapolis, MN). Primary antibodies to p-Akt (Ser-473), p-p38 and cleaved caspase-3 were purchased from Cell Signaling Technologies (Beverly, MA). Actin antibody was purchased from Santa Cruz Bio Technology (Santa Cruz, CA) and tubulin antibody was purchased from Sigma. Hydroethidium, 2′,7′-dichlorodihydrofluorescein diacetate (DCF-DA) was purchased from Molecular Probes, Invitrogen (Carlsbad, CA). FITC labeled Annexin V kit was purchased from Trevigen (Gaithersburg, MD). All other reagents and chemicals were from Sigma.

2.2. Cell culture

Rat aortic vascular smooth muscle cells (RASMs) were isolated from thoracic aortas of Sprague–Dawley rats and maintained in DMEM containing 4.5 g/L glucose supplemented with 10% fetal bovine serum (Invitrogen) and penicillin/streptomycin (Gibco) as previously described (Ishida et al., 1999). RASMs were used between passages 8 and 15.

2.3. Axl inhibitor selectivity

The order of compounds based on their biochemical inhibitory concentrations at 50% (IC50) for Axl is as follows (6–64 nM): R562 b R570 b R428 b R624 b R572. In addition, Rigel’s data indicate that R428 and R572 have the highest affinity for Axl over other members of the TAM family of receptors in several cell types (Holland et al., 2010).

2.4. IC50 for Axl inhibitors

RASMs were incubated in serum free DMEM containing increasing concentrations of Axl inhibitors (nM: 10, 30, 100, 300, and 1000) for 24 h. Cells were stimulated with 600 μM H2O2 for 5 min as previously described (Konishi et al., 2004) and then rinsed with phosphate buffered saline (PBS). Cells were lysed by addition of 1× lysis buffer (Cell Signaling Biotechnology) containing 0.1% protease inhibitor cocktail for 5 min on ice. Cells were scraped and lysates were agitated on a rocker for 20 min at 4 °C and centrifuged at 10,000 rpm at 4 °C for 10 min to remove cell debris. Protein concentration was determined by Bradford protein assay (BioRad).

2.5. Immunoblots

Equal amounts of protein (10–30 µg) were separated on SDS-PAGE gels and transferred to nitrocellulose membrane (HybondTM EDL). Membranes were incubated with appropriate antibody dilutions. Membrane bound antibodies were visualized with Odyssey (Li-Cor Odyssey) using secondary antibodies conjugated to Li-Cor sensitive fluours. Phosphorylated Akt (p-Akt) was normalized to actin by densitometry. The ratios of p-Akt to actin for each inhibitor concentration were compared to the control (DMSO).

2.6. Apoptosis

RASMs were serum starved with R428 and R572 for 24 h and then stimulated with H2O2 (100 μM) for 6 h. Cells were then processed for flow cytometry using FITC-labeled Annexin V kit as described by vendor. Flow cytometry was performed using a 6-color BD FACSCantoTM II (BD Biosciences) and data were analyzed with FlowJo software.Cleaved caspase-3 was measured as described above for immuno- blots. RASMs stimulated for 6 h with 100 μM H2O2 were used for cleaved caspase-3.

2.7. Measurement of ROS

Hydroethidium, 2′,7′-dichlorodihydrofluorescein diacetate (DCF- DA) was used to measure ROS. RASMs were serum starved for 24 h with Axl inhibitors, trypsinized and resuspended at approximately 5×106 cells/mL in 3% serum-PBS (FSB) solution containing DCF-DA (20 μM) for 30 min in a light-protected humidified chamber at 37 °C. Cells were centrifuged for 5 min at 1000 rpm at 4 °C and resuspended in FSB. RASMs were stimulated with H2O2 (600 μM) for 5 min at 37 °C. Cells were rinsed with FSB, resuspended in 0.5 mL FSB and processed for FACS analyses.

2.8. Boyden chamber assay

RASMs migration was measured using a Boyden chamber assay as described (Satoh et al., 2008). Control medium (0% serum DMEM) or serum containing DMEM (20% serum) was placed in the lower chamber. A collagen-coated polyvinylpyrrolidone-free polycarbonate membrane was placed over the bottom wells. RASMs that had been incubated in serum free media for 24 h were suspended in DMEM containing 0.1% bovine serum albumin with either control (DMSO), R428 (100 nM) or R572 (10 nM) at 37 °C for 30 min. Treated cells at ~ 8000/well were seeded into the upper chamber. Cells adhering to the lower side of the collagen-coated membrane were fixed and stained using Diff Quick Staining Kit (Fisher Scientific) after 4 h incubation at 37 °C in 95% O2 and 5% CO2. The number of migrated cells was quantified using ImagePro software (Satoh et al., 2008).

2.9. Statistical analysis

Results are reported as mean±SEM. IC50 values were calculated using a computer algorithm that assumes a linear curve between two points around 50% of the maximum response as we described previously (Chen et al., 2007). Statistical tests were done with JMP software for MacIntosh. Comparison for two groups was performed using Student’s t-test. Differences between 3 or more groups were analyzed by means of a repeated-measures one-way ANOVA. Significance was taken at pb 0.05.

3. Results

3.1. IC50 of Axl inhibitors using p-Akt as indicator of Axl activation in RASMs

Akt is the predominant signaling molecule activated upon stimulation of Axl in RASMs (Konishi et al., 2004). H2O2 (600 μM) treatment for 5 min resulted in a significant increase in p-Akt under control conditions (DMSO). Dose-response curves using a range of concentrations from 10 nM to 1 μM were generated for five Axl inhibitors: R428, R572, R562, R570, and R624 (Fig. 1A). IC50s were determined by stimulating RASMs with H2O2 (Fig. 1A). R562, R570 and R624 had minimal or nearly no effect on p-Akt at the highest concentrations following H2O2 treatment (Fig. 1A). In contrast, p-Akt was decreased in a dose-dependent manner in the presence of R428 and R572 (Fig. 1A and B). We calculated IC50s based on p-Akt for R428 (100 nM) and R572 (10 nM). Represen- tative immunoblots for R428 and R572 for p-Akt are shown in Fig. 1B. We demonstrate two of the five concentrations closest to the calculated IC50s for R428 and R572 (Fig. 1B). We also tested the effect of Fc-Axl (2 μg/mL) on p-Akt. As previously reported (Konishi et al., 2004), Fc-Axl decreased Axl mediated p-Akt upon H2O2 treatment (Fig. 1B). However, R428 and R572 blocked Akt- dependent pathway to a greater extent than Fc-Axl (Fig. 1B). Therefore, we identified the two most potent inhibitors of Axl among five tested in VSMCs and thus selected R428 and R572 for further investigation.

Receptor tyrosine kinase stimulation can also result in the activation of ERK pathways, including p38 (Ushio-Fukai et al., 2001). We found that there was no effect of either R428 or R572 on p-p38 (Fig. 1B) which is consistent with the previous finding that p38 activation was not altered in response to H2O2 in aortae from Axl−/− mice (Konishi et al., 2004). We also performed a flow cytometry assay for cell cycle progression (staining with propidium iodide), which demonstrated that neither R428 nor R572 affected RASMs proliferation (not shown). This is consistent with a recent finding demonstrating that R428 does not affect cell proliferation in cancer (Holland et al., 2010). Together, these data suggest that inhibition of Axl with R428 or R572 has no effect on the proliferative state of VSMCs. Rather, it is more likely that Axl affects cell survival in VSMCs as suggested by our previous findings (Konishi et al., 2004).

3.2. R428 and R572 increase apoptosis in RASMs

Recent data showed that 100 μM H2O2 for 4 h caused slight increases in VSMCs apoptosis (Kato et al., 2009). To investigate the effect of the Axl inhibitors on apoptosis we performed Annexin V FITC and propidium iodide double-staining. Representative double- stained flow cytometry charts of RASMs are shown in Fig. 2: under basal unstimulated conditions (Fig. 2A), control conditions with DMSO (Fig. 2B), and following incubation with R428 (100 nM; Fig. 2C) and R572 (100 nM; Fig. 2D). The lower-right quadrant of each flow cytometry chart represents the apoptotic population of RASMs, while the upper-right quadrant represents necrotic RASMs (Fig. 2). We tested various doses of H2O2 (100 μM, 600 μM and 1 mM) and time courses (5 min, 6 h, and 24 h; data not shown). We experimentally chose a dose of 100 μM and time of incubation (6 h) of H2O2 that did not dramatically increase apoptosis and does not induce necrosis in RASMs (compare Fig. 2B vs. 2A). We also tested whether or not the Axl inhibitors alone would affect apoptosis and found that there were no increases above basal levels (not shown). Therefore, neither H2O2 nor treatment alone with Axl inhibitors result in toxicity of VSMCs, as was reported in animal experiments for R428 (Holland et al., 2010). Quantification of flow cytometry demonstrated that R428 and R572 similarly increased apoptosis, which was 3 times greater compared to the DMSO control (Fig. 2E). In contrast, Fc-Axl did not potentiate VSMCs apoptosis under such oxidative conditions (Fig. 2E). Of note, neither R428 nor R572 had an effect on cell necrosis in comparison to control conditions in RASMs (Fig.
2F).

As a second method of apoptosis detection we measured cleaved caspase-3 in RASMs (Fig. 3). Inhibition of Axl with R428 (100 nM) and R572 (100 nM) increased cleaved caspase-3 in response to H2O2 (100 μM, 6 h) compared to Fc-Axl, which had no detectable increase in cleaved caspase-3 compared to control (DMSO; Fig. 3). It should be noted that R572 had a greater effect on increasing cleaved caspase-3 in response to H2O2 (Fig. 3). These results are consistent with our Annexin V flow cytometry data (Fig. 2). Because cleavage of caspase-3 may occur in a time dependent manner we also tested the effect of the inhibitors upon 600 μM H2O2 after 5 min. Our results for the 5 min and 6 h time points were very similar (compare right and left panels in Fig. 3). Taken together we showed that R428 and R572 have greater pro-apoptotic effects compared to Fc-Axl in RASMs under oxidative stress by using two methods of detection of apoptosis.

3.3. Effects of R428 and R572 on migration in RASMs

VSMCs migration is very important in vascular pathology, especially under increased oxidative conditions. Axl activation can result in increased cell migration and angiogenesis (Fridell et al., 1998). Our group recently reported that inhibition of Axl using Fc-Axl reduced migration of RASMs (Cavet et al., 2010). Pre-incubation of RASMs with R428 (100 nM) and R572 (10 nM) decreased cell migration compared to DMSO in response to 20% serum (Fig. 4). This is supported by a recent finding demonstrating that R428 significantly reduced tumor angiognesis, neovascularization and cell invasion in models of breast cancer metastasis (Holland et al., 2010). Thus, pharmacological inhibition of Axl using R428 and R572 reduces VSMCs migration.

3.4. Intracellular ROS production

Oxidative stress can induce increases in intracellular ROS produc- tion in VSMCs (Brunt et al., 2006). We tested the effect of the Axl inhibitors on ROS production in RASMs in response to H2O2 using DCF-FA staining (Fig. 5). A flow cytometry overlay for DCF-FA staining is shown in Fig. 5A and is quantified in Fig. 5B. In response to 600 μM H2O2 for 5 min there was a significant increase in intracellular ROS (arrow; H2O2) in control (DMSO; thick black line, Fig. 5A) compared to basal levels (thin black line, Fig. 5A). Fc-Axl significantly augmented ROS production compared to control (dark gray line, Fig. 5A). The effects of the inhibitors varied compared to control with R428 significantly reducing ROS below DMSO (dotted line, Fig. 5A). R572 resulted in levels comparable to DMSO (dashed line, Fig. 5A). Most importantly, both R428 and R572 produced significantly less ROS compared to Fc-Axl (Fig. 5B). There were no increases in ROS production after preincubation of the inhibitors alone (not shown). These data suggest that small molecules that inhibit Axl are more effective at limiting intracellular ROS production than Fc-Axl in RASMs.

4. Discussion

We have demonstrated that two out of five tested small molecules that inhibit Axl, namely R428 and R572, are effective at blocking Axl mediated survival and migration in VSMCs. Specifically, R428 and R572 decreased activation of Akt induced by oxidative stress, while p- p38 was not affected by these compounds in RASMs (Fig. 1). Using two methods of detection of apoptosis we showed that R428 and R572 have greater pro-apoptotic effects compared to Fc-Axl in RASMs under oxidative stress (Figs. 2, 3). R572 and R428 prevented VSMCs migration in response to serum (Fig. 4). Finally, we demonstrated that R428 and R572 decrease intracellular production of ROS compared to Fc-Axl (Fig. 5). These findings suggest that the studied compounds could be effective inhibitors of Axl mediated VSMCs responses, especially during chronic oxidative stress.

4.1. Axl signaling in vascular diseases

The Gas6/Axl pathway has been shown to be important in several models of cardiovascular disease (Melaragno et al., 1998). Gas6 and RTKs play a key role in thrombosis via activation of PI3 kinase and phosphorylation of β3 integrin, which promote platelet aggregation (Angelillo-Scherrer et al., 2005). Axl and Gas6 expression increased in response to carotid balloon injury (Melaragno et al., 1998). These increases in Gas6 and Axl expression were also found upon activation of G-protein coupled receptors by thrombin and Ang II in VSMCs. It has also been shown that in both human and mouse models of atherosclerosis that Gas6 expression is increased in endothelial cells, VSMCs, and macrophages (Tjwa et al., 2009). It was recently found that increased plasma levels of Gas6 correlated with diagnosed cardiovascular risk factors and was a likely predictor of future cardiovascular events (Jiang et al., 2009).

Gas6 ligand mediated activation of Axl is now well characterized. However, Axl can also be activated in a ligand independent manner (Hafizi and Dahlback, 2006). For instance, Axl activation and endothelial cell survival were increased via functional association with β3-integrin complex in response to shear stress (D’Arcangelo et al., 2006). Additionally, oxidative stress can stimulate Axl resulting in downstream activation of Akt with a subsequent increase in VSMCs survival (Konishi et al., 2004). We showed that in Axl−/− mice carotid intima-media thickening was significantly reduced which correlated with a decrease in p-Akt (Konishi et al., 2004). Vascular remodeling is marked by increases in VSMCs survival and migration. Our data demonstrate that inhibition of Axl results in decreased p-Akt (Fig. 1), which presumably leads to increased cell apoptosis (Figs. 2, 3). Furthermore, we demonstrated that Axl inhibition decreased VSMCs migration (Fig. 4). Therefore, interfering with Axl dependent signaling in VSMCs could potentially alter two major cellular processes essential for progression of pathological vascular wall remodeling.

4.2. ROS-mediated cell survival in vascular pathology and regulation of Axl

Both low and high levels of ROS can positively and negatively affect cell survival (Taniyama and Griendling, 2003). Thus, there is a paradoxical effect of ROS in VSMCs functions. Oxidative stress, even at low levels, can activate protein tyrosine kinases resulting in cell growth (Berk, 1999; Irani, 2000; Paravicini and Touyz, 2006; Rao, 1996). This most likely occurs via increases in tyrosine phosphory- lation as has been demonstrated downstream of agonist induced activation of epidermal growth factor receptor (EGFR), PDGF and fibroblast growth factor (FGF) (Chen et al., 2001; Frank et al., 2003; Rao, 1997; Saito et al., 2002; Wang et al., 2000). More interestingly, ROS activation of tyrosine kinases can occur in a ligand independent manner, as has been shown for Src kinase. However, data are emerging for ROS mediated direct activation of RTKs as well. For example, it was previously demonstrated in VSMCs that H2O2 induced ligand independent activation of RTKs, specifically EGFR, resulted in an increase in ERK and Ras signaling (Rao, 1996). Additionally, intracellular ROS generated in response to Ang II stimulation results in the transactivation of the EGFR and this effect is abrogated by antioxidants (Ushio-Fukai et al., 2001). These studies were highly suggestive of ROS direct activation of RTK’s as a protective mechanism against oxidative stress. Importantly, these studies indicated that ERK was the predominant cell signaling molecule activated in response to short-term low levels of oxidative stress. In contrast, we showed that Akt it preferentially activated in response to higher levels of ROS and mediated by Axl stimulation. Furthermore, the Axl inhibitors effectively decreased Akt signaling. The preferential effect of Akt activation is consistent with a recently published study (Holland et al., 2010) demonstrating that the Axl inhibitors, specifically R428, exhibited N 100 fold selectivity for Axl over EGFR, insulin receptor, PDGFR and tyrosine kinase of other subfamilies (133 kinases). Thus we would suggest that Axl is important in mediating cell survival and that other RTK’s, such as EGFR, are involved in cell proliferation.

Our previous data suggest that Axl is one of the most important

RTK’s that regulate survival in response to ROS in vitro in RASMs and in hypertension in vivo (Konishi et al., 2004; Korshunov et al., 2006). However, in the presence of the selective Axl inhibitors H2O2 mediated increases in VSMCs apoptosis were significantly amplified (Fig. 2). Additionally, we showed that H2O2 induces intracellular ROS production and that the small molecule Axl inhibitors prevented further increases in intracellular ROS. In fact R428 significantly decreased the H2O2-mediated increase in ROS in RASMs (Fig. 5). This finding suggests that oxidative stress increases intracellular ROS, which in turn activate RTK’s, specifically Axl. Furthermore, we emphasize the fundamental differences between selective Axl inhibitors and Fc-Axl in response to oxidative stress, since Fc-Axl resulted in further increases in intracellular ROS production.

4.3. Therapeutic potential of increases in apoptosis in vascular pathology and pharmacological inhibition of Axl

Increases in VSMCs apoptosis have been shown to be beneficial in the treatment of cardiovascular diseases. Some of the strongest support for targeting VSMCs apoptosis derives from studies in spontaneously hypertensive rats (SHR). An ability of AT1R blockers, ACE inhibitors and calcium channel blockers to induce VSMCs apoptosis may explain their beneficial effects in SHR compared to β- blockers or diuretics (deBlois et al., 1997). It was proposed that AT1R inhibition in VSMCs mediated apoptosis via cleavage of caspase-3, especially in early phases of hypertension (Marchand et al., 2003;
Sharifi and Schiffrin, 1998). We showed that Axl−/− mice exhibited decreases in blood pressure, reduced remodeling and increased cell apoptosis in hypertension (Korshunov et al., 2007). Taken together, targeting Axl for pharmacological manipulation could have therapeu- tic benefits in vascular diseases.

Currently, the only available Axl inhibitors are warfarin and recombinant Fc-Axl. Both compounds, however, do not directly block Axl. Rather, they indirectly inhibit Axl signaling by interfering with Axl’s endogenous ligand, Gas6. Warfarin inhibits γ-carboxylation of Gas6 and Fc-Axl binds to and neutralizes Gas6 preventing its binding to and activation of Axl (Nagata et al., 1996). However, neither of these inhibitors is particularly advantageous. Although warfarin has been shown to inhibit cell proliferation in the kidney (Yanagita et al., 1999) it did not have a significant effect on inhibiting neointima formation in response to balloon injury in rats (Konishi et al., 2004). Furthermore, warfarin is highly non-specific since it universally prevents γ-carboxylation (Nakano et al., 1997). While Fc-Axl is specific for Gas6, in order for it to be effective in vivo it requires direct interaction with Gas6 residing in the vessel wall. There is some evidence that Fc-Axl is effective in vivo in inhibiting messangial cell proliferation (Yanagita et al., 2001) but there are no data suggesting the effectiveness of Fc-Axl in the vasculature. An inherent disadvan- tage to both warfarin and Fc-Axl is that these inhibitors are designed to specifically block Gas6 ligand dependent activation of Axl. Thus, ligand independent activation of Axl in VSMCs, such as with ROS, may not be blocked by these inhibitors. In fact, Fc-Axl only partially inhibits Axl phosphorylation in response to H2O2 (Konishi et al., 2004). We speculate that Fc-Axl could only prevent Axl activation mediated by Gas6 and not by H2O2. Thus, neither warfarin nor Fc-Axl is a preferential therapeutic inhibitor of Axl in response to oxidative stress.

4.4. Conclusions

We found that R428 and R572 were most effective at preventing Axl mediated activation of Akt in VSMCs. Most importantly we demonstrated that these compounds specifically blocked oxidative stress induced by Axl activation. In addition these small Axl inhibitors, in contrast to Fc-Axl, prevented further increases in Axl mediated ROS production. Therefore, these inhibitors are favorable compared to Fc- Axl, which is structurally larger and is required to be used at higher concentrations. However, it is still unclear how ROS activate Axl. We have recently demonstrated that increases in Axl signaling pathways and ROS production in response to oxidative stress involve Axl interaction with glutathiolated myosin heavy chain IIB (Cavet et al., 2010). We proposed that this interaction is mediated through the NAD(P)H oxidase (Cavet et al., 2010). In our future studies we will explore the role of NAD(P)H oxidase in ROS mediated activation of Axl and the mechanisms by which these small Axl inhibitors interfere with pro-survival signaling pathways. Based on our findings we think that R428 and R572 may potentially block multiple mechanisms of Axl receptor activation. In summary, pharmacological inhibition of ligand independent activation Bemcentinib of Axl in response to oxidative stress may lead to therapeutic advantages in treating vascular diseases.