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. 2010 May 7;6(5):421-32.
doi: 10.1016/j.stem.2010.02.018.

Integrin alpha 6 regulates glioblastoma stem cells

Justin D Lathia  1 , Joseph Gallagher, John M Heddleston, Jialiang Wang, Christine E Eyler, Jennifer Macswords, Qiulian Wu, Amit Vasanji, Roger E McLendon, Anita B Hjelmeland, Jeremy N Rich
Affiliations

Integrin alpha 6 regulates glioblastoma stem cells

Justin D Lathia et al. Cell Stem Cell. .

Abstract

Cancer stem cells (CSCs) are a subpopulation of tumor cells suggested to be critical for tumor maintenance, metastasis, and therapeutic resistance. Prospective identification and targeting of CSCs are therefore priorities for the development of novel therapeutic paradigms. Although CSC enrichment has been achieved with cell surface proteins including CD133 (Prominin-1), the roles of current CSC markers in tumor maintenance remain unclear. We examined the glioblastoma stem cell (GSC) perivascular microenvironment in patient specimens to identify enrichment markers with a functional significance and identified integrin alpha6 as a candidate. Integrin alpha6 is coexpressed with conventional GSC markers and enriches for GSCs. Targeting integrin alpha6 in GSCs inhibits self-renewal, proliferation, and tumor formation capacity. Our results provide evidence that GSCs express high levels of integrin alpha6, which can serve not only as an enrichment marker but also as a promising antiglioblastoma therapy.

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Figures

Figure 1
Figure 1. Integrin α6 is expressed in human GBM cells and localized to the perivascular compartment
Immunostaining of GBM surgical biopsies (HP 08-0444, HP 06-033, HP 08-0445) demonstrates that integrin α6 (green) is expressed in the perivascular compartment (double stained with CD31 in red, A, B) and is co-expressed with CD133 (red, C, D). Blood vessels marked with an “*”, regions of interest marked with a white arrow, and enlarged regions of interest marked with yellow arrow and shown in A’-D’. All nuclei counterstained with Hoechst in blue. Scale bar represents 50 μm. Flow cytometry analysis (E) indicates that integrin α6 is expressed in a fraction of cells in primary surgical GBM biopsies 18 hours after isolation (CCF1585) or after short term culture (CCF1518) with varying overlap with CD133 expression.
Figure 2
Figure 2. Integrin α6 is co-expressed with CD133-positive GSCs
(A) Quantitative PCR from tumor xenografts (T3359, T4105, T4302, T4597, T0166) indicates that in CD133 enriched GSC enriched populations, integrin α6 is also highly expressed in comparison to CD133 depleted non-stem glioma cells (n = 3, +/− S.E.M; *, p < 0.05; **, p < 0.001). (B) Reverse transcription PCR analysis demonstrates that integrin α6 and the GSC marker Olig2 are highly expressed in GSCs isolated from T4302, T4597, and T0166 tumor xenografts. NC, negative control (no cDNA added). (C) Flow cytometry analysis indicates that integrin α6 expression overlaps with CD133 expression in tumor xenografts (T3691, T0322). (D) Immunostaining analysis from tumor xenograft cells (T3359) immediately after CD133 enrichment show co-expression of CD133 positive cells with integrin α6 (red) and GSC markers CD133 (green) and Olig2 (green, nuclei counterstained in blue with Hoechst). CD133 negative cells show limited GSC or integrin α6 marker expression. Scale bar, 10 μm.
Figure 3
Figure 3. Integrin α6 co-segregates with CD133-positive GSCs
Immunostaining analysis of cryosections from tumorspheres generated from GSC enriched populations (T3359, T3691, T4302) show the peripheral region (yellow arrow, enlarged inset in bottom right corner) which appears to correlate with high integrin α6 expression and the inner region (white arrow), which is low in integrin α6 expression. Photomicrographs show integrin α6 (green) is co-expressed with GSC markers Olig2 (red, A, D, E), CD133 (red, B, E, H), and nestin (red, C, F, I). All nuclei counterstained with Hoechst in blue. Scale bar, 10 μm.
Figure 4
Figure 4. Integrin α6 segregates with co-receptor integrin β1 and ligand, laminin
Photomicrographs from tumorspheres generated from GSC enriched populations (T3359, T3691, T4302) show integrin α6 (green) is expressed with co-receptor integrin β1 (red, A, B, C) and ligand laminin (red, C, D, E) in the peripheral region (yellow arrow), but not the inner region (white arrow). All nuclei counterstained with Hoechst in blue. Scale bar, 50 μm.
Figure 5
Figure 5. Selection of cells based on integrin α6 expression enriches for a population which display GSC properties
After GSC enrichment based on integrin α6 and CD133 expression, each cell population (black: CD133negative/integrin α6lo, green: CD133positive/integrin α6lo, red: CD133negative/integrin α6hi, blue: CD133positive/integrin α6hi) had a different growth profile. Cell titer assays demonstrated increased growth in integrin α6hi cells isolated from T0322 (A) or T3359 (B) cells. **, p < 0.01 with ANOVA comparison to CD133negative/integrin α6lo cells at the same timepoint; ###, p<0.001 with ANOVA comparison of CD133positive/integrin α6hi cells to either CD133positive/integrin α6lo or CD133negative/integrin α6hi cells at the same timepoint. In tumorsphere formation assays for T3359 (C) and T3691 (D) cells, selection for CD133 and integrin α6 increased the number of tumorspheres per well. ***, p < 0.001 with ANOVA comparison to CD133negative/integrin α6lo cells; ###, p<0.001 with ANOVA comparison of CD133positive/integrin α6hi cells to either CD133positive/integrin α6lo or CD133negative/integrin α6hi cells; && p < 0.01 and &&&, p<0.001 with ANOVA comparison to CD133positive/integrin α6lo cells. GBM cells (from tumor specimens T3359, T4302) were sorted at a single cell per well based on integrin α6 expression. Graphs (E, F) indicate that cells selected from the integrin α6hi high population (orange bar) formed tumorspheres at a significantly higher frequency than cells from the integrin α6lo population (black bar, ***, p < 0.001). (G, H) Kaplan-Meier survival curves demonstrate decreased survival when cells high in integrin α6 expression are transplanted into the right frontal lobe of immunocompromised mice (†, p < 0.067 for 1000 CCF1966 cells; **, p < 0.01 for 500 T08-0387 cells (p = 0.0051) with log-rank analysis of survival curves).
Figure 6
Figure 6. Integrin α6 knockdown results in a reduction in the GSC phenotype
Knockdown of integrin α6 using two separate lentiviral shRNA constructs results in a decreased cell proliferation profile as assessed by the cell titer assay in T3691 (A) and T4121 (B) xenograft tumor cells. ***, p<0.001 with ANOVA comparison to non-targeting shRNA at the same timepoint; ##, p<0,01 with ANOVA comparison of shRNA1 to shRNA2 at the same timepoint. (C) Quantification of the number of tumorspheres per well and representative pictures of tumorspheres demonstrates potential self-renewal was also impaired in GSC targeted with the lentivial shRNA constructs in T4302 cells. *, p < 0.05 with ANOVA comparison to non-targeting control. (D) EdU incorporation demonstrates a decrease in proliferative capacity T4302 cells using both shRNA constructs (*** p < 0.001 with ANOVA comparison to non-targeting control, ###, < 0.001 with ANOVA comparison to shRNA1). Knockdown of integrin α6 also increases cell death as assessed by a DNA fragmentation assay (E) on T4302 xenograft tumor cells (M1 = 1 kb ladder, Un = uninfected control, NT = non-targeting shRNA control, sh1=shRNA1, sh2=shRNA2, Ctrl = staurosporine control, M2 = 100 bp ladder). Cell death was also confirmed using a caspase 3/7 assay (F) on T4121 xenograft tumor cells (***, p < 0.001 with ANOVA comparison to non-targeting control; ###, p < 0.001 with ANOVA comparison to shRNA1). (G) Kaplan-Meier survival curve demonstrate increased survival when integrin α6 is targeted with shRNA in comparison to a non-targeting control. 1000 GSCs infected with integrin α6 shRNA targeting constructs or non-targeting control were intracranially transplanted into the right frontal lobe of immunocompromised mice. *, p < 0.05; **, p < 0.01 by log-rank analysis of survival curves. (H) Representative light micrograph showing H&E staining for a control non-targeting integrin α6 1000 cell tumor showing characteristic bilateral tumor location, migrating edges, and secondary tumors, insets displayed below.
Figure 7
Figure 7. Integrin α6 blocking antibody results in a reduction in the GSC phenotype
This decrease in GSC proliferation was also seen using a blocking antibody to integrin α6 in T3691 (A) and T4121 (B) xenograft tumor cells in a concentration dependent manner as compared to an isotype control antibody. **, p < 0.01 with ANOVA comparison to isotype control at the same timepoint. (C) Tumorsphere formation is also inhibited by addition of the blocking antibody in T3359 xenograft tumor cells as shown by quantification of the number of tumorspheres per well and in representative images. *, p < 0.01. (D) Kaplan-Meier survival curve demonstrating increased survival when GSCs are incubated with the blocking antibody. 1000 integrin α6 + /CD133 - GSCs incubated with either a control or integrin α6 blocking antibody for five days and intracranially transplanted into the right frontal lobe of immunocompromised mice. **, p < 0.01 with log-rank analysis of survival curves.
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