Guggulsterone sensitizes glioblastoma cells to Sonic hedgehog inhibitor SANT-1 induced apoptosis in a Ras/NFjB dependent manner

Abstract

Since Shh pathway effector, Gli1, is overexpressed in gliomas, we investigated the effect of novel Shh inhibitor SANT-1 on glioma cell viability. Though SANT-1 failed to induce apoptosis, it reduced prolifer- ation of glioma stem-like cells. Apart from canonical Shh cascade, Gli1 is also induced by non-canonical pathways including NFjB. Therefore, a combinatorial strategy with Ras/NFjB inhibitor, Guggulsterone, was employed to enhance effectiveness of SANT-1. Guggulsterone inhibited Ras and NFjB activity and sensitized cells to SANT-1 induced apoptosis via intrinsic apoptotic mechanism. Inhibition of either Ras or NFjB activity was sufficient to sensitize cells to SANT-1. Guggulsterone induced ERK activation also contributed to Caspase-9 activation. Since SANT-1 and Guggulsterone differentially target stem-like and non-stem glioma cells respectively, this combination warrants investigation as an effective anti-gli- oma therapy.

1. Introduction

Sonic hedgehog pathway has been implicated in cancer forma- tion and progression [32]. Binding of Shh to its receptor Patched (Ptch1) de-represses Smoothened (Smo) which stabilizes the nu- clear accumulation of Gli to regulate transcription of Shh target genes [21]. Gli, a component of the Shh signaling pathway, is amplified in glioma [22] and Shh pathway inhibitor Cyclopamine decreases Gli1 expression and inhibits glioma cell growth [4]. Importantly, Cyclopamine inhibits glioma cancer stem-cell self-re- newal and tumorigenicity [10].

In addition to Gli-1, transcription factor NFjB is constitutively activated in GBM tumors that promotes their growth and survival; and inhibition of NFjB activity induces glioma cell apoptosis [37]. Given the ability of NFjB to regulate a number of genes associated with GBM progression, inhibition of its activity is considered a po- tential anti-glioma therapeutic strategy [42]. NF-jB activation in- duced Shh signaling promotes pancreatic cancer progression [30], and NF-jB/Shh axis promotes cancer cell proliferation and apopto- sis resistance [20]. We have reported Ras mediated NFjB transcrip- tional activation in glioma cells [45]. Co-operative interaction between Ras and Shh promotes proliferation of pancreatic cancer cells [29,33]. Moreover, oncogenic Ras-induced melanomas require sustained Shh–Gli signaling [48].

(Z)-Guggulsterone, a plant steroid, obtained from Commiphora mukul inhibits NFjB activation [15] and suppresses both constitu- tive and inducible STAT3 activation [1]. The anticancer activity of Guggulsterone has been shown in a xenograft model of prostate [55], skin [39] and human head and neck cancers [26]. Constitutive activation of STAT3 regulates the expression of anti-apoptotic genes in GBM [35] and STAT3 maintains constitutive NFjB activa- tion in tumors [25]. Importantly, STAT3 supports Ras dependent oncogenic transformation [15]. Targeting the elevated Ras activa- tion in GBMs [16] induces glioma cell death [6]. As a consequence, several approaches that target NFjB and STAT3 as anti-glioma therapy are being actively pursued [2]. Since Guggulsterone inhib- its both NFjB and STAT3 activation, its effect on glioma cell viabil- ity was investigated.

Blockade of Shh pathway sensitizes medulloblastoma to the pro-apoptotic agent lovastatin [3]. Cellular sterol levels correlate with diminished responsiveness to Shh signaling through Smo [11]. As Shh effectors act downstream of lovastatin to regulate cell death [3] and since Guggulsterone lowers cholesterol [53], we investigated whether Guggulsterone could potentiate the ability of Shh inhibitor to affect glioma cell viability. Shh pathway antag- onists Cyclopamine and SANT bind Smo, thereby repressing Gli [8]. Combination of HDAC inhibitor SAHA with SANT-1 additively sup- pressed the proliferative ability of pancreatic cancer cells [9]. Also, Shh inhibitor Cyclopamine showed additive and synergistic effects with TMZ [10]. We therefore investigated whether similar combi- natorial approach using Shh inhibitor SANT-1 and Guggulsterone may be effective in regulating glioma cell proliferation.

2. Materials and methods

2.1. Cell culture and treatment

Glioblastoma cell lines A172, U87MG, T98G and astroglial SVG-p12 cell line were obtained from American Type Culture Collection (ATCC, Manassas, VA, USA) and cultured in DMEM supplemented with 10% fetal bovine serum. On attaining semi-confluence, cells were switched to serum free media (SFM) and after 12 h, cells were treated with Guggulsterone or SANT-1 (in Dimethyl sulphoxide, DMSO), either alone or in combination, in SFM for 24 h. DMSO treated cells were used as controls. Free floating colonies of stem-like cells obtained by growing U87MG in Pro-N (proliferating neurospheres) media as described earlier [44], were cultured with different combinations of Guggulsterone and SANT-1 (Tocris Biosciences, Bris- tol, UK). The number and size of the spheres were calculated and cells were subse- quently processed for Western blot analysis. All reagents were purchased from Sigma (St. Louis, MO) unless otherwise stated.

2.2. Determination of cell viability

Viability of cells treated with different combinations of Guggulsterone and SANT-1 in the presence and absence of Pan-Caspase, Caspase-3, 8, and 9 inhibitor or ERK inhibitor U0126 for 24 h in 96-well plates was assessed using the MTS assay (Promega, Madison, WI, USA) as described [24]. Values were expressed as a percent- age relative to those obtained in controls.

2.3. Western blot and immunoprecipitation

Protein was isolated from whole cell lysates and nuclear extracts from cells treated with different combinations of Guggulsterone and SANT-1, or similarly trea- ted stem like cells and Western blot was performed as described [45]. The following antibodies were used – NFjB p65, Gli-1, Cytochrome c, Bax, c23, Cyclin D1, IQGAP-1, cMyc, p27 (Abcam, Cambridge, UK), p21 (BD Biosciences, San Diego, CA, USA), pERK1/2, ERK1/2, pSTAT3 (Tyr 705), STAT3 (Cell Signaling, Danvers, MA, USA), Nestin (Dako, Glostrup, Denmark). Antibodies were purchased from Santa Cruz Biotech- nology (Santa Cruz, CA, USA) unless otherwise mentioned. Secondary antibodies were purchased from Vector Laboratories (Burlingame, CA, USA). The blots were stripped and re-probed with anti-b-actin (Sigma) to determine equivalent loading as described [45]. Immunoprecipitation was performed with Fas antibody to deter- mine its association with FADD to form DISC as described previously [13].

2.4. Assay of Caspase-3, 8 and 9 activities

The Colorimetric Assay kits for Caspases-3, 8 and 9 (Abcam) were used to deter- mine the enzymatic activity of Caspases in cells treated with different combinations of Guggulsterone or SANT-1 as per the manufacturer’s instructions.

2.5. Transfections and luciferase assay

Cells transfected with 0.3 lg of DN-IjB or FADD-DN construct were treated with different combination of Guggulsterone and SANT-1 as described [45]. In experiments with DN-constructs, control transfection using the appropriate empty vectors for each construct was employed. Reporter assay in cells transfected with NFjB or Gli-1 luciferase constructs, and treated with different combinations of Gug- gulsterone or SANT-1 was performed using Lipofectamine 2000 (Life Technologies- Invitrogen, Carlsbad, CA, USA) as described [45]. NFjB constructs were obtained from Clontech (Madison, WI, USA). The Gli-1 luciferase reporter and FADD-DN constructs were gift from Ariel Ruiz i Altaba University of Geneva Medical School and Anne-Odlie Hueber, Hungary.

2.6. Measurement of Ras activity

The Ras activity was performed using a commercially available Ras activation assay kit purchased from Upstate Biotechnology (Millipore, Temecula, CA, USA), as described previously [12].

2.7. Flow cytometry assay of mitochondrial mediated apoptosis

Mitochondrial membrane potential changes were assayed with MitoLight® mitochondrial apoptosis detection kit (Millipore). Cells treated with SANT-1 or Gug- gulsterone or both were treated with 50 ll of pre-diluted MitoLight solution (900 ll of water, 1 ll of MitoLight dye, and 100 ll of 10× incubation buffer) for 30 min according to the manufacturer’s instructions, and mitochondrial membrane poten- tial in these cells were then analyzed by flow cytometry (Becton, Dickinson, Inc., Franklin Lakes, NJ). Results were analyzed Cell Quest pro software on FACS Calibur (Becton Dickinson).

2.8. Flow cytometric analysis of DNA content

FACS analysis was performed to determine the effect of Guggulsterone or SANT- 1 or both on cell cycle progression of glioma cells, using Cell Quest program on FACS Calibur (Becton Dickinson) as described [46]. Results were analyzed using Cell Quest pro software.

2.9. Colony formation in soft agar

The soft agar colony formation ability of cells treated with different combina- tions of SANT-1 and Guggulsterone was performed using CytoSelect™ 96-Well Cell Transformation Assay kit (Cell Biolabs, Inc.), as described previously [13].

2.10. Statistical analysis

All comparisons between groups were performed using two-tailed paired Stu- dent’s t-test. All P-values less than 0.05 were taken as significant.

3. Results

3.1. SANT-1 has no effect on glioma cell viability but potentiates Guggulsterone induced apoptosis

As Gli, a component of the Shh signaling pathway, is aberrantly expressed in gliomas [22], we investigated whether treatment of glioma cells with Shh inhibitor SANT-1 could induce glioma cell apoptosis. Treatment with different concentrations of SANT-1 had no effect on cell viability (Fig. 1a). Besides Shh, aberrant acti- vation of Ras and NFjB occurs in glioma. We, therefore, investi- gated the effect of NFjB inhibitor Guggulsterone on glioma cell viability. Guggulsterone induced glioma cell death in a dose depen- dent manner (Fig. 1b). We next investigated whether simultaneous
inhibition of Shh and Ras–NFjB axis through co-treatment of gli- oma cells with SANT-1 and Guggulsterone – inhibitors of Shh and NFjB respectively could induce glioma cell death. To determine the synergism between Guggulsterone and SANT-1, cell death in- duced upon treatment with different doses (10–60 lm) of these compounds, either alone or in combination was evaluated. Death induced by the combination of Guggulsterone and SANT-1 was sig- nificantly greater than the additive value calculated from cell death in response to the drugs when treated individually. This suggests that Guggulsterone and SANT-1 act synergistically to induce gli- oma cell death (Fig. 1b). As death induced by 30–50 lM concentration of Guggulsterone were comparable, 30 lM concentration was used in the subsequent experiments. While a ~20% reduction in viability was observed upon treatment with 30 lM of Guggulster- one (Fig. 1c), co-treatment with SANT-1 resulted in 45–55% de- crease in viability as compared to control (Fig. 1c). While SANT-1 alone has no effect on glioma cell viability, it potentiated the ability of Guggulsterone to induce cell death. To further confirm Gugguls- terone and SANT-1 induced glioma cell death, TUNEL assay was performed. The 15–20% increase in TUNEL positive cells observed upon Guggulsterone treatment, was further elevated to ~40% in the presence of SANT-1 (Fig. 1d). Moreover treatment with either SANT-1 or GS had no effect on astroglial cell line SVG p12, an insig- nificant decrease in viability was observed upon treatment with a combination of both. This indicated that SANT-1 and GS combina-
tion decreases the viability of glioma cells without affecting nor- mal astrocytes (Supplementary Fig. 1).

3.2. Apoptosis induced by SANT-1 and Guggulsterone co-treatment involves Caspase-3 activation

Treatment with SANT-1 has no significant effect on Caspase-3 activity. However, the ~1.7-fold increase in Caspase-3 activity ob- served upon Guggulsterone treatment was further increased to 6–8 folds in the presence of SANT-1 (Fig. 1e). To confirm the role of Cas- pase-3 in Guggulsterone and SANT-1 induced death, the viability of cells treated with SANT-1 or Guggulsterone or both was deter- mined in the presence and absence of Pan-Caspase and Caspase- 3 inhibitor. Treatment with Pan-Caspase inhibitor and Caspase-3 inhibitor significantly rescued glioma cells from Guggulsterone and SANT-1 induced apoptosis (Fig. 1f).

3.3. Guggulsterone has no effect on DISC formation or Caspase-8 activation

Death Inducing Signaling Complex (DISC) resulting from Fas– FADD interaction stimulates Caspase-8 activation to induce apop- tosis through Caspase-3 activation. While Guggulsterone increased FADD expression both in the presence and absence of SANT-1, no change in Fas levels was observed (Supplementary Fig. 2a). Despite elevated FADD levels, Guggulsterone failed to increase Fas–FADD interaction and subsequent DISC formation (Supplementary Fig. 2b). Treatment with Guggulsterone or SANT-1 either alone or in combination had no effect on Caspase-8 activation (Supplemen- tary Fig. 2c). Moreover, transfection with DN-FADD failed to rescue glioma cells from Guggulsterone induced apoptosis both in the presence and absence of SANT-1 (Supplementary Fig. 2d). This indicated that the combination of Guggulsterone and SANT-1 in- duces glioma cell death in a FADD and Caspase-8 independent manner.

3.4. Guggulsterone and SANT-1 induces mitochondrial mediated apoptosis through Caspase-9 activation

While Caspase-3 functions to initiate apoptotic damage, Cas- pase-9 amplifies the apoptotic cascade [54]. Since Guggulsterone and SANT-1 mediated glioma cell death was Caspase-8 indepen- dent, we determined Caspase-9 activity in cells treated with dif- ferent combinations of SANT-1 and Guggulsterone. SANT-1 had no effect on Caspase-9 activation. However, Guggulsterone in- duced ~2-fold increase in Caspase-9 activation was further ele- vated to 4–6 folds in the presence of SANT-1 (Fig. 2a). To investigate the role of this elevated Caspase-9 activation in Gug- gulsterone and SANT-1 induced death, the viability of cells trea- ted with SANT-1 or Guggulsterone or both in the presence of Caspase-9 inhibitor was determined. Guggulsterone and SANT-1 induced apoptosis was rescued in cells treated with Caspase-9 inhibitor (Fig. 2b).

As Caspase-9 is the initiator Caspase associated with mitochon- drial pathway of apoptosis [28], we determined the involvement of mitochondria in Guggulsterone and SANT-1 induced death. Disrup- tion of mitochondrial transmembrane potential was determined with MitoLight (™) Apoptosis Detection Kit. While SANT-1 had no effect on mitochondrial transmembrane potential, Guggulster- one increased MitoLight fluorescence significantly. Flow cytomet- ric analysis by MitoLight revealed a significant increase in mitochondria mediated apoptosis in Guggulsterone and SANT-1 treated cells (Fig. 2c). This was accompanied by increase in cyto- chrome c and Bax levels in Guggulsterone treated cells both in the presence and absence of SANT-1 (Fig. 2d).

3.5. Guggulsterone inhibits NFjB pathway while SANT-1 affects Gli1 in glioma cells

Guggulsterone suppresses NFjB activation [19,47]. Since inhibi- tion of NFjB activation sensitizes glioma cells to chemotherapeu- tics [17], we evaluated the status of NFjB in cells treated with Guggulsterone both in the presence and absence of SANT-1. Treat- ment with Guggulsterone resulted in a decreased p65 NFjB levels both in the presence and absence of SANT-1 (Fig. 3a). IjBa is phos- phorylated by IjKa/b which leads to IKBa degradation and in- creased nuclear import of NFjB (p65) [18]. Guggulsterone mediated decreased pIjKa/b level in glioma cells (Fig. 3a) was accompanied by increase in IjBa levels and decrease in nuclear NFjB levels (Fig. 3a). The decrease in NFjB level was accompanied by decrease in its transcriptional activity (Fig. 3b). While treatment with Guggulsterone significantly decreased NFjB activity both in the presence and absence of SANT-1, the latter alone had no significant effect on pIjKa/b, IjBa and NFjB expression (Fig. 3a) or its activity (Fig. 3b).On the other hand, SANT-1 significantly decreased Gli-1 expression (Fig. 3a) as well as Gli-1 transcriptional activity (Fig. 3c) both in the presence and absence of Guggulsterone. A slight but insignif- icant decrease in Gli-1 activity was also observed in Guggulsterone treated cells (Fig. 3c).

3.6. Abrogation of NFjB activation sensitizes glioma cells to SANT-1 mediated apoptosis

SANT-1 neither had any effect on NFjB activation nor did it interfere with the ability of Guggulsterone to down-regulate NFjB activity. However, SANT-1 potentiated the death inducing ability of Guggulsterone. We therefore determined whether NFjB inhibition in Guggulsterone treated cells could have sensitized glioma cells to Guggulsterone and SANT-1 induced death. Interestingly, a 30% de- crease in cell viability was observed in SANT-1 treated cells trans- fected with DN-IjB (Fig. 3d). This suggested that elevated NFjB activation in glioma cells confers resistance to Shh inhibitor in- duced apoptosis.

3.7. Guggulsterone decreases Ras activity while SANT-1 has no effect

Ras regulates NFjB transcriptional activity [14,31]. Heightened Ras activation occurs in GBM [16] and we have shown Ras depen- dent NFjB activation in glioma cells [45]. As Guggulsterone inhib- ited NFjB activation in glioma cells, we determined the effect of Guggulsterone on Ras activity in glioma cells. While treatment with SANT-1 had no affect on Ras activation, Guggulsterone decreased Ras activity, both in presence and absence of SANT-1 (Fig. 4a). However, total Ras level remain unaffected by Gugguls- terone and SANT-1 treatment (Fig. 4a). Thus, Guggulsterone med- iated apoptosis is accompanied by dual abrogation of Ras/NFjB activation in glioma cells.

3.8. Guggulsterone mediated decrease in RAS activity sensitizes glioma cells to SANT-1

As increased apoptosis was observed in Guggulsterone and SANT-1 treated cells with decreased Ras activity and since Ras activ- ity was unaffected by SANT-1 alone, we investigated whether Ras plays any role in regulating the responsiveness of glioma cells to- wards Shh inhibitor, SANT-1. Over-expression of DN-Ras protein (RasN17) sensitized glioma cells to SANT-1 induced apoptosis (Fig. 4b). This indicated the involvement of Ras in regulating Shh in- duced apoptosis inducing signals. The importance of Ras in the reg- ulation of apoptosis in SANT-1 and Guggulsterone co-treated cells was further confirmed in RasV12 transfected glioma cells. Overexpression of wild type Ras protein by RasV12 vector significantly pre- vented SANT-1 and Guggulsterone induced death (Fig. 4c).

3.9. Guggulsterone induces glioma cell death in an ERK dependent manner

As ERK1/2 – a downstream effector of Ras is known to regulate gli- oma cell survival, we determined phosphorylated ERK levels in Gug- gulsterone treated cells with decreased Ras activity. Unexpectedly, an increase in ERK phosphorylation concomitant with down-regula- tion of Ras activity was observed upon Guggulsterone treatment. This increase in ERK phosphorylation was unaffected by SANT-1 (Fig. 5a). As ERK activation inhibits Jak–STAT pathway [43], and since Guggulsterone inhibits STAT3 activation we determined the status of STAT3 phosphorylation in these cells. A decrease in phosphorylated STAT3 level was observed in Guggulsterone treated cells both in the presence and absence of SANT-1 (Fig. 5a).

Ras independent ERK activation has been reported [41] and we have shown that ERK activation sensitizes glioma cells to Miltefo- sine induced apoptosis [50]. To determine the role of elevated ERK phosphorylation in Guggulsterone mediated apoptosis, the viabil- ity of glioma cells treated with different combinations of Gugguls- terone and SANT-1 was determined in the presence and absence of MEK/ERK inhibitors. MEK inhibitor, U0126, inhibited ERK phos- phorylation and significantly rescued glioma cells from GS and SANT-1 induced apoptosis (Fig. 5b). Similar results were obtained by another MEK inhibitor PD 0325901 (Data not shown). Thus, ele- vated ERK activation mediates the pro-apoptotic effect of Guggulsterone.

3.10. ERK regulates Caspase-9 activation in SANT-1 and Guggulsterone co-treated glioma cells

ERK activation involved in Icaritin induced apoptosis of human endometrial cancer cells is accompanied by increased Caspase 3 and 9 activation [51]. As co-treatment with Guggulsterone and SANT-1 induced apoptosis in a Caspase-9 dependent manner, the involvement of ERK phosphorylation in the regulation of Cas- pase-9 activation was determined. Increased Caspase-9 activation observed upon Guggulsterone and SANT-1 co-treatment was abro- gated in the presence of ERK inhibitor. This confirmed that Gug- gulsterone and SANT-1 mediated increase in Caspase-9 activation is ERK-dependent (Fig. 5c).

3.11. Decreased NFjB and STAT3 activation in SANT-1 and Guggulsterone co-treated glioma cells is ERK independent

Though ERK pathway is independent from the NFjB anti-apop- totic pathway [52], ERK activation has been shown to trigger NF- jB activation mediated glioma cell death [5]. As Guggulsterone mediated decrease in NFjB and STAT3 activation was concurrent with increased ERK phosphorylation, the role of ERK in regulating NFjB and STAT3 activation in Guggulsterone treated cells was investigated. Guggulsterone mediated decrease in NFjB activity (Fig. 5d) and STAT3 (Fig. 5e) phosphorylation remained unaffected upon ERK inhibition.

3.12. Guggulsterone affects cell cycle regulation

We next investigated whether SANT-1 and Guggulsterone med- iated apoptosis is concomitant with altered expression of molecules associated with cell cycle progression. SANT-1 had no effect on p21 and p27 expression. However, an increase in p21 and p27 levels was observed in Guggulsterone treated cells both in the presence and absence of SANT-1 (Fig. 6a). Both Guggulster- one and SANT-1 significantly reduced cyclin D1 expression (Fig. 6a). Since Guggulsterone induces cell cycle arrest [56], FACS analysis was performed to determine the cell cycle profile of Gug- gulsterone treated A172 glioma cells in the presence and absence of SANT-1. Results indicated a ~4%, 5%, 9% and 14% cells at G2/M phase of cell cycle in untreated and cells treated with SANT-1, Gug- gulsterone, and Guggulsterone + SANT-1 combination, respec- tively. Though SANT-1 had no significant effect on cell cycle progression it increased Guggulsterone induced cell cycle arrest at G2/M phase (Fig. 6b). Similar results were obtained in T98G cells (data not shown).

3.13. Guggulsterone reduces colony forming ability of glioma cells

The ability of glioma cells, treated with different combinations of Guggulsterone and SANT-1, to grow in an anchorage-indepen- dent manner was assessed by the colony formation in soft agar, which is an in vitro tumorigenicity assay. While Guggulsterone reduced the colony forming ability of glioma cells in soft agar, SANT-1 had no significant effect (Fig. 6c). However, SANT-1 further enhanced the ability of Guggulsterone to inhibit the colony formation (Fig. 6c).

3.14. SANT-1 decreases the sphere forming ability of glioma stem-like cells

Targeting Shh pathway by Cyclopamine directly affects glioma stem like cells [38]. We determined whether despite its inability to induce apoptosis in adherent glioma cultures, SANT-1 could af- fect the sphere forming ability of glioma-stem-like cells obtained from U87MG cells. SANT-1 abrogated the number and size of free floating glioma stem-like spheres both in the presence and absence of Guggulsterone. Though Guggulsterone also reduced sphere for- mation, the decrease was insignificant (Fig. 7a–c). This ability of SANT-1 to reduce the proliferation of glioma stem-like cells was accompanied by decrease in Gli-1 levels. SANT-1 also decreased the levels of Nestin and IQGAP-1, markers of glioma stem-like cells (Fig. 7d). SANT-1 decreased cyclin D1 expression but had no effect on cMyc which is associated with cell cycle progression of cancer stem cells. However, the combination of SANT-1 and Guggulster- one greatly reduced cMyc expression (Fig. 7e). These findings indi- cate that SANT-1 affects the survival of stem-like cells while Guggulsterone largely affects differentiated non-stem like population.

4. Discussion

Though inhibition of Shh signaling by SANT-1 had no effect on glioma cell viability, co-treatment with Guggulsterone sensitized cells to SANT-1 mediated cell death. Previous studies indicate that interference with Shh signaling can be effective in inhibiting gli- oma cell proliferation only through a combinatorial strategy that simultaneously blocks both Shh and Ras pathways [48]. Gugguls- terone is an antagonist for farnesoid X receptor (FXR) [53], and this ability to inhibit FXR activation contributes to its cholesterol low- ering activity. Ras farnesylation is essential for its biological activ- ity [23], and we have previously shown that farnesyltransferase inhibitor (FTI) manumycin decreases glioma cell proliferation [13]. The ability of FTIs to affect NFjB activation suggested depen- dence of NFjB signaling on Ras [49]. As we have previously shown that Ras regulates NFjB activation in glioma cells [45], we investi- gated whether FTI, Guggulsterone, can sensitize glioma cells to Shh inhibitor, SANT-1, by modulating Ras and NF-jB activation in gli- oma cells.

Investigating the resistance mechanism of glima cells to Shh inhibitor, SANT-1, revealed that disruption of Ras and NFjB activa- tion by Guggulsterone sensitizes cells to SANT-1. This ability of Guggulsterone to inhibit both Ras and NFjB activation lessens the dependence of SANT-1 on these two signaling events to execute its pro-apoptotoic properties. It was the concurrent inhibi- tion of Shh and Ras pathway that prevented glioma cell prolifera- tion as reported previously [29]. Our studies indicate that resistant of glioma cells to Shh inhibitors could be overcome by inhibition of Ras–NFjB signaling (Fig. 8).

Although ERK stimulates Gli transcriptional activity [36], in- creased ERK activation in SANT-1 and Guggulsterone treated gli- oma cells was concurrent with decreased Gli-1 expression and transcriptional activity. This increased ERK activation in the pres- ence of Shh inhibitor could occur via pathway stimulated by Shh-ligands that operates independently of pathways requiring Smo; as Cyclopamine increases ERK activation independent of Smo [7]. Ras independent activation of Erk pathway have been pre- viously reported [41]. Though Ras mediated down-regulation of Fas renders cells resistant to Fas mediated apoptosis [34], abroga- tion of Ras activity upon Guggulsterone and SANT-1 co-treatment failed to induce Fas–FADD mediated DISC formation and Cas- pase-8 activation. Since Fas mediated apoptosis is dependent on inhibition of Shh induced ERK activation [27], it is likely that ele- vated ERK activation in Guggulsterone and SANT-1 co-treated cell prevents Fas mediated death execution through Caspase-8. How- ever, SANT-1 and Guggulsterone induce cell death in a Caspase-9 dependent manner, involving mitochondria.

Targeting Shh pathway by Cyclopamine results in incomplete tumor regression, since only glioma stem like cells are directly af- fected by Cyclopamine [38]. It was, therefore, interesting to ob- serve decreased proliferation of glioma stem-like spheres upon SANT-1 treatment. Though treatment with Guggulsterone reduced glioma stem-like cell proliferation, the decrease was insignificant as compared to SANT-1 mediated changes. As Shh inhibition criti- cally targets glioma stem-like cells, Shh inhibitor treatment possi- bly spares the differentiated glioma cells. This could have resulted in the decreased sensitivity of glioma cells used in this study, to SANT-1. This study suggests that the combination of Guggulster- one and SANT-1 can be used effectively to induce apoptosis in both stem like and non-stem differentiated population of glioma cells.

Treatment with guggulipid (Guggulsterone-containing nutraceutical) decreased tumor growth and potentiated the effects of cetuximab (currently available HNSCC therapy), in in vivo xeno- graft model of HNSCC [26]. Though guggulipid has been shown to cross blood brain barrier (BBB) [40], the ability of GS to cross BBB is largely unknown. Given the potential of GS to sensitize gli- oma cells to Shh inhibitor SANT-1, further studies demonstrating the effectiveness of this combination in in vivo glioma model war- rant investigation.

Our findings indicate that the resistance of glioma cells to Shh inhibitor SANT-1 induced apoptosis can be overcome by concur- rent inhibition of both Ras and NFjB signaling. It is the simulta- neous inhibition of Ras and NFjB by Guggulsterone that triggers the anti-proliferative effect of Shh inhibitor. Because the abroga- tion of Ras and NFjB signaling is required to regulate the respon- siveness of glioma cells to Shh inhibitors, it implies that targeting one pathway in the intricate web of signaling cascades runs the risk of failure in glioma therapy. Therefore better understanding of signaling circuitries involved in glioma tumorigenesis will en- able effective designing of treatment strategies either through combination therapy or by multi-targeted inhibitors.

Moreover, the differential ability of SANT-1 and Guggulsterone to target distinct glioma cell populations could be further exploited as a potent anti-glioma combination therapy over the existing con- ventional therapies that largely target one particular cell popula- tion. This combinatorial strategy holds promise for the treatment of GBMs as it could enhance efficacy of both Guggulsterone and SANT-1. Importantly, this work provides insights regarding the pathways that can be targeted to complement Shh inhibition in glioblastoma treatment.