Glioblastoma Cancer Stem Cells Evade Innate Immune Suppression of Self-Renewal through Reduced TLR4 Expression

Abstract

Tumors contain hostile inflammatory signals generated by aberrant proliferation, necrosis, and hypoxia. These signals are sensed and acted upon acutely by the Toll-like receptors (TLRs) to halt proliferation and activate an immune response. Despite the presence of TLR ligands within the microenvironment, tumors progress, and the mechanisms that permit this growth remain largely unknown. We report that self-renewing cancer stem cells (CSCs) in glioblastoma have low TLR4 expression that allows them to survive by disregarding inflammatory signals. Non-CSCs express high levels of TLR4 and respond to ligands. TLR4 signaling suppresses CSC properties by reducing retinoblastoma binding protein 5 (RBBP5), which is elevated in CSCs. RBBP5 activates core stem cell transcription factors, is necessary and sufficient for self-renewal, and is suppressed by TLR4 overexpression in CSCs. Our findings provide a mechanism through which CSCs persist in hostile environments because of an inability to respond to inflammatory signals.

Keywords: RBBP5; TLR4; cancer stem cells; glioblastoma; innate immunity; pluripotency transcription factors.

Figure 1. CSCs and Non-CSCs Respond Differentially to Innate Immune Stimulation and Express Differential Levels of TLR4

(A) CSCs (blue) and non-CSCs (black) were incubated with TLR ligands for 7 days, and proliferation relative to day 0 was assessed by CellTiter Glo assay and normalized to an untreated control. The following concentrations were used: LPS, 500 ng/mL; HMGB1, 1 μg/mL; CpG, 2 μM; Pam3C, 300 ng/mL; and Poly(I:C), 10 μg/mL. (B) The mRNA expression level of all known human TLRs was evaluated by qRT-PCR in CSCs and non-CSCs; data were normalized to the non-CSC group. (C and D) mRNA (C) and protein (D) levels of TLR4 were analyzed in CSCs and non-CSCs using qRT-PCR and western blot, respectively. Actin was used as an internal control for qRT-PCR and a loading control for western blotting. Each experiment was performed at least three times. Data are represented as mean ± SEM. *p < 0.05, **p < 0.01, and ***p < 0.001 as assayed by one-way ANOVA. See also Figure S1.

Figure 2. TLR4 Expression Negatively Correlates with Stemness

(A and B) Experimental paradigm to assess the effect of LPS on tumor growth in vivo (A). Mice (n = 7 per arm) were injected with cells from xenograft T3832. After 14 days, tumors were injected with LPS (150 or 300 μg), and tumor volume (B) was measured using electronic calipers 3 days later. Tumor volume at day 3 was normalized to the volume of the tumor at day 0 when LPS was injected. (C) Bulk tumors derived from patient specimens were sorted for TLR4 surface expression using fluorescence-activated cell sorting (FACS). The 20% of cells with the highest expression and the 20% of cells with the lowest expression were plated in a limiting dilution manner, and the number of wells containing spheres was counted after 10 days to generate stem cell frequencies using the online algorithm detailed in the STAR Methods. (D) CSCs were differentiated in DMEM containing 10% fetal bovine serum (FBS) for the indicated lengths of time, and protein levels of TLR4, SOX2, OCT4, NANOG, GFAP, and Actin were assessed by western blotting. Experiments in (C) and (D) were performed at least three times. Data are represented as mean ± SEM. ***p < 0.001 as assayed by one-way ANOVA. See also Figure S2.

Figure 3. TLR4 Overexpression Decreases Proliferation and Reduces CSC Maintenance

(A and B) Transient TLR4 overexpression in CSCs was measured at both the mRNA (A) and protein (B) levels using qRT-PCR and western blotting, respectively. Western blots were also stained for the pluripotency factors SOX2, NANOG, and OCT4. CSCs were nucleofected with pcDNA3-TLR4-YFP and used for experiments 3 days later. Actin was used as an internal control for qRT-PCR and a loading control for western blotting. (C) Proliferation over 7 days was assessed in CSCs derived from the shown specimens using CellTiter Glo after overexpression of TLR4. Growth was normalized to cells containing an empty vector. (D) CSCs overexpressing TLR4 were treated with LPS at 500 ng/mL, and proliferation was assessed for 7 days relative to cells containing a control vector. (E) Limiting dilution analysis was used to estimate stem cell frequencies in CSCs after nucleofection with control or TLR4 overexpression vectors. Cells were plated in a limiting dilution manner, and the number of wells containing spheres was counted after 10 days to generate stem cell frequencies using the online algorithm detailed in the STAR Methods. All experiments were performed at least three times. Data are represented as mean ± SEM. **p < 0.01 and ***p < 0.001 as assessed by one-way ANOVA. See also Figure S3.

Figure 4. TLR4 Overexpression Compromises Stemness by Repressing the Transcription Factor RBBP5

(A) The expression of the shown transcription factors was analyzed in CSCs overexpressing TLR4 using qRT-PCR and normalized to cells containing a control vector. CSCs were nucleofected with pcDNA3-TLR4-YFP and used for experiments 3 days later. Actin was used as an internal control. (B) Protein levels of RBBP5 were analyzed in both CSCs and non-CSCs using western blot. Actin was used as a loading control. (C) A reporter expressing luciferase under the control of the RBBP5 promoter region was used to measure RBBP5 expression after transient TLR4 overexpression in CSCs. CSCs were nucleofected with pcDNA3-TLR4-YFP and used for experiments 3 days later. Luciferase levels were normalized to those of control (NT) cells. (D) Levels of RBBP5 and GFAP were determined by western blot in CSCs expressing both control and TLR4-overexpression vectors. Actin was used as a loading control. (E) CSCs from the shown specimens were differentiated by incubating with DMEM with 10% FBS for the indicated times, and protein levels of RBBP5 were determined by western blot. Actin was used as a loading control. All experiments were performed at least three times. Data are represented as mean ± SEM. *p < 0.05, **p < 0.01, and ***p < 0.001 as assayed by one-way ANOVA.

Figure 5. Targeting RBBP5 Mimics TLR4 Overexpression in CSCs

(A) Validation of two shRNA constructs targeting RBBP5 at the protein level compared with non-targeting vector (NT). Western blots were also stained for the pluripotency factors SOX2, NANOG, and OCT4. CSCs were infected with lentivirus containing one of two different RBBP5 shRNA constructs, selected for stable expression, and assayed within five passages for all experiments. Actin was used as a loading control. (B) The proliferation of CSCs stably expressing RBBP5 shRNA was measured and compared with an NT vector. The growth of the cells over 7 days as determined by CellTiter Glo was normalized to the growth of NT cells. (C and D) Limiting dilution analysis of the effect of RBBP5 knockdown on two different specimens. Cells were plated in a limiting dilution manner, and the number of wells containing spheres was counted after 10 days to generate stem cell frequencies using the online algorithm detailed in the STAR Methods. (E) Mice (n = 7 for KD, n = 6 for NT) were intracranially injected with 1000 T4121 NT or RBBP5-knockdown CSCs, and the time until endpoint was recorded. Kaplan-Meier survival plots are shown, and the log rank p value for significance between groups is shown next to the survival curve. (F) Proliferation was analyzed after RBBP5 overexpression over 7 days in CSCs. Growth was assessed using CellTiter Glo and normalized to that of cells containing an empty vector. Cells were infected with lentivirus, selected for stable expression, and used within five passages for all experiments. (G) Limiting dilution analyses of both CSCs and non-CSCs overexpressing RBBP5 compared with a control vector. Cells were plated in a limiting dilution manner, and the number of wells containing spheres was counted after 10 days to generate stem cell frequencies using the online algorithm detailed in the STAR Methods. All in vitro experiments were performed at least three times. Data are represented as mean ± SEM. **p < 0.01 and ***p < 0.001 as assayed by one-way ANOVA. See also Figure S4.

Figure 6. TBK1 Acts as a Signaling Intermediate between TLR4 and RBBP5

(A) Non-CSCs were treated with LPS (500 ng/mL) or HMGB1 (1 μg/mL) for 1 day, and the phosphorylation of TBK1 was analyzed by western blot with a p-TBK1-specific antibody. Total protein was used as a loading control. (B) Proliferation of non-CSCs treated with LPS (500 ng/mL) or HMGB1 (1 μg/mL) in the presence or absence of TBK1 inhibitor (BX795; 100 nM) for 1 day and normalized to untreated controls. (C) Non-CSCs were treated with LPS (500 ng/mL) with and without the addition of TBK1 inhibitor (BX795; 100 nM) for 1 day, and the mRNA levels of RBBP5 were measured and normalized to the control group. Actin was used as an internal control. (D) TLR4 was transiently overexpressed in CSCs using nucleofection of pcDNA3-TLR4-YFP, and phosphorylation and levels of TBK1 and IRAK1 were analyzed by western blotting and compared with control vector. Total protein was used as a loading control. (E) Reduced TLR4 expression in cancer stem cells enables them to persist in hostile environments. Working model showing the signaling axis where TLR4 activation leads to the phosphorylation of TBK1, which suppresses RBBP5. RBBP5 in turn regulates stem cell genes and is critical for CSC maintenance. In non-CSCs (left), TLR4 expression allows for the activation of TBK1 in the presence of appropriate TLR4 ligands, which consequently inhibits RBBP5, decreasing stem cell gene expression. In contrast (right), the lack of TLR4 expression in CSCs suppresses the inhibition of RBBP5 and promotes the activation of self-renewal programs with consequences on proliferation, growth, and tumor initiation. All experiments were performed at least three times. Data are represented as mean ± SEM. *p < 0.05 and **p < 0.01 as assayed by one-way ANOVA. See also Figure S5.