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. 2014 May 8;7(3):747-61.
doi: 10.1016/j.celrep.2014.03.059. Epub 2014 Apr 24.

Regulated splicing of the α6 integrin cytoplasmic domain determines the fate of breast cancer stem cells

Hira Lal Goel  1 , Tatiana Gritsko  1 , Bryan Pursell  1 , Cheng Chang  1 , Leonard D Shultz  2 , Dale L Greiner  3 , Jens Henrik Norum  4 , Rune Toftgard  4 , Leslie M Shaw  1 , Arthur M Mercurio  5
Affiliations

Regulated splicing of the α6 integrin cytoplasmic domain determines the fate of breast cancer stem cells

Hira Lal Goel et al. Cell Rep. .

Abstract

Although the α6β1 integrin has been implicated in the function of breast and other cancer stem cells (CSCs), little is known about its regulation and relationship to mechanisms involved in the genesis of CSCs. We report that a CD44(high)/CD24(low) population, enriched for CSCs, is comprised of distinct epithelial and mesenchymal populations that differ in expression of the two α6 cytoplasmic domain splice variants: α6A and α6B. α6Bβ1 expression defines the mesenchymal population and is necessary for CSC function, a function that cannot be executed by α6A integrins. The generation of α6Bβ1 is tightly controlled and occurs as a consequence of an autocrine vascular endothelial growth factor (VEGF) signaling that culminates in the transcriptional repression of a key RNA-splicing factor. These data alter our understanding of how α6β1 contributes to breast cancer, and they resolve ambiguities regarding the use of total α6 (CD49f) expression as a biomarker for CSCs.

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Figures

Figure 1
Figure 1. Identification of distinct CD44highCD24low populations that differ in morphology and stem cell properties
A. MCF-10A ER/SRC cells were treated with 4-Hydroxy-Tamoxifen (TAM) and FACS analyzed using CD44, CD24 and α6 (GoH3) antibodies. B. The CD44high/CD24low population isolated from TAM-treated MCF-10A ER/SRC cells was sorted by FACS into two subpopulations based on expression of the α6 integrin subunit. Photomicrographs of these two subpopulations designated as EPTH (α6 high) and MES (α6 low) are shown. Scale bar = 100 μm C. Cell extracts from EPTH and MES cells were immunoblotted to assess expression of N-Cadherin, E-Cadherin, HIF-1α, vimentin, actin and VEGF-A (left panel). EPTH and MES cells were immunostained for CK8 and CK18 and the quantitation of positive cells is presented (Right panel). D. Expression of integrin α6, β1 and β4 mRNAs was quantified in EPTH and MES cells using qPCR. E. The CD44high/CD24low population from TAM treated MCF-10A ER/SRC cells was stained with PKH26 and cultured for 14 days. Cells were analyzed by FACS for PKH and integrin α6 expression. F. Expression of ALDH1A1 and BMI-1 mRNAs was quantified in EPTH and MES cells using qPCR. G The CD44high/CD24low population isolated from TAM-treated MCF-10A ER/SRC cells was treated with either taxol or Ethanol for 7 days and integrin α6 surface expression was analyzed by FACS. H. TAM- treated MCF-10A ER/SRC cells were FACS sorted into subpopulations based on expression of CD44 and CD24 and assayed for mammosphere formation (left panel). EPTH and MES cells were assayed for mammosphere formation (right panel). I. EPTH and MES cells (106 cells per mouse) were implanted in the mammary fat pads of NSG mice (n=12) and tumor formation was assessed by palpation. The curve comparison was done using Log-rank test (p=0.006).
Figure 2
Figure 2. α6B cytoplasmic variant expression determines tumor initiating potential in breast cancer cells
A. Cell extracts from EPTH and MES were immunoblotted for integrin α6A, α6B and tubulin. B. Cell extracts from breast cancer cell lines were immunoblotted for α6A (left blot) and α6B (right blot). C. Integrin α6-depleted SUM1315, MDA-MB-435 and MDA-MB-231 cells were stably transfected with either empty vector (EV), α6A or α6B. Transfectants were analyzed by immunoblotting using α6A, α6B or actin Abs. D. Mammospheres from SUM1315 transfectants (α6A or α6B) were passaged serially and the number of mammospheres is presented. E. Integrin α6A and α6B tranfectants of SUM1315, MDA-MB-435 and MDA-MB-231 cell lines were assessed for growth in soft agar. F. Integrin α6A and α6B tranfectants of SUM1315 (left panel) and MDA-MB-435 (right panel) cells were transplanted into mammary fat pads of NSG mice using 10-fold serial dilution. The formation of palpable tumors was used to evaluate tumor initiation. Data are presented as a log-log plot and frequency of stem cells is calculated by extreme limiting dilution analysis. Red, SUM1315-α6B (1/4,479) or MDA-MB-435-α6B (1/1,425); black, SUM1315-α6A (1/68,078) or MDA-MB-435-α6A (1/37,586). G. Expression of α6B was depleted in MDA-MB-231 cells using α6B-specific TALENs in combination with a donor plasmid containing a puromycin expression cassette, and expression of α6A, α6B and actin was assessed by immunoblotting. H. Mammospheres from the transfectants in G (Puro-alone or TALENs-pool) were passaged serially and the number of mammospheres is presented. I. Soft agar growth from the transfectants in G (Puro-alone, TALENs-pool, TALENs-C1 and TALENs-C2) is presented as the mean number of colonies in 20 fields.
Figure 3
Figure 3. ESRP1 promotes integrin α6A expression
A. Expression of ESRP1, ESRP2, RBFOX2 and MBNL1 mRNAs was quantified in EPTH and MES cells using qPCR. B. Expression of ESRP1 mRNA was quantified in breast cancer cell lines using qPCR. C. ESRP1 was down-regulated in MCF-10A cells and cell extracts were immunoblotted to assess the expression of integrin subunits α6A and α6B, ESRP1 and actin. D. The effect of ESRP1 on α6B and α6A expression, as assessed by immunoblotting (left panels), or on the ability of breast cancer cell lines to form colonies in soft agar (right panel). E. The effect of ESRP1 expression on ESRP1 (left panels), integrin α6A expression (middle panel) or integrin α6B expression (right panel) was analyzed using qPCR. F. The CD44high/CD24low population from TAM-treated MCF-10A ER/SRC cells was treated with either taxol or Ethanol for 7 days and expression of α6B, α6A and ESRP1 was analyzed by immunoblotting. G. The effect of ESRP1 expression on self-renewal ability was analyzed by serial passage of mammospheres.
Figure 4
Figure 4. ESRP1 and integrin α6A expression is low in triple-negative tumors and negatively correlated with integrin α6B and VEGF
A. Frozen clinical specimens from TPN (n=20) and non=TPN (n=36) breast tumors were used to compare ESRP1, integrin α6A, integrin α6B and VEGF-165 by qPCR. B. Graph depicts the fold change in mRNA levels between the TPN and non-TPN specimens shown in A. C. Correlation graphing reveals a significant positive correlation between α6A and ESRP1 expression (left panel), a significant negative correlation between VEGF-165 and ESRP1 expression (middle panel) and a significant negative correlation between α6B and ESRP1 expression (right panel). D. Frozen clinical specimens from TPN (n=17) and non=TPN (n=18) breast tumors were used to compare expression of integrin α6B by immunoblotting. E. Densitometric analysis of the immunoblot shown in D. F. Correlation graphing reveals a significant negative correlation in the expression of integrin α6B and ESRP1.
Figure 5
Figure 5. VEGF/NRP2/GLI1 signaling represses ESRP1 expression and promotes integrin α6Bβ1-mediated self-renewal
A. VEGF-A, NRP1 and NRP2 mRNA expression was quantified by qPCR in breast cancer cell lines expressing integrin α6A or α6B variants. B. EPTH and MES populations were analyzed for VEGF-A, NRP1, NRP2 or GLI1 mRNA expression using qPCR. C. SUM1315-α6B tranfectants were depleted of VEGF and NRP using either VEGF siRNA or NRP inhibitors and these cells were assayed for their ability to be passaged serially as mammospheres. The number of mammospheres observed at passage 4 is shown. D. VEGF was depleted in SUM1315 cells and the expression of VEGF, GLI1, ESRP1, α6A, α6B and actin was assessed by immunoblotting. E. Cell extracts from SUM1315 transfectants (EV, α6A or α6B) were immunoblotted with Abs specific to GLI1, ESRP1 and actin. F. Expression of α6B was depleted in MDA-MB-231 cells using α6B-specific TALENs, and expression of GLI1 and actin was assessed by immunoblotting. G. Cell extracts from SUM1315 cells expressing either GFPsh or GLI1sh were immunoblotted using Abs to GLI1, ESRP1, α6A, α6B, BMI1 and actin (left panels). MCF-10A cells were transfected with GLI1 and analyzed for expression of GLI1, α6A, α6B, ESRP1 and actin (right panel). H. A significant negative correlation between GLI1 and ESRP1 expression was observed in breast cancer patient cohorts. I. Expression of ESRP1, α6A and α6B is compared between MMTV-PyV-MT and MMTV-GLI1 mouse tumor models.
Figure 6
Figure 6. BMI1 suppresses ESRP1 and controls α6Bβ1-mediated self-renewal
A. Extracts from α6A or α6B expressing breast cancer cells were immunoblotted with Abs to GLI1, BMI1 and actin. B. MCF-7 cells expressing either empty vector (EV), α6A or α6B were analyzed for expression of α6A, α6B, BMI1 and actin. C. BMI1 was expressed in MCF-10A cells and its effect on α6A, ESRP1 and α6B expression was assessed by qPCR. D. Down-regulation of BMI1 or GLI1 significantly increased ESRP1 and α6A, but reduced α6B mRNA levels. E. Binding of BMI1 on the ESRP1 promoter was analyzed using ChIP. Primers P2 and P3, which span the region -1422 to -1802 upstream of the transcription start site of ESRP1 show significant binding of BMI1. F. Effect of BMI1 down-regulation on the self-renewal potential of α6B transfectants is shown at mammosphere passages 3 and 4. G. A significant negative correlation between expression of BMI1 and ESRP1 was observed in breast cancer patient cohorts.
Figure 7
Figure 7. Integrin α6Bβ1 is at the nexus of a feed forward VEGF signaling loop that sustains self-renewal and tumor initiation
A. FACS profile of MDA-MB-231 cells stably expressing GFP under control of the VEGF promoter is shown. B. Freshly sorted VEGF-high or VEGF-low populations were used to measure colony formation in soft agar. C. VEGF-high or VEGF-low cells were implanted in the mammary fat pad of NSG mice (n=8) and tumor formation was assessed by palpation. The curve comparison was done using Log-rank test (p=0.008). D. The effect of VEGF expression on self-renewal ability was analyzed by serial passage of mammospheres. E. Expression of key signaling proteins in VEGF-high and VEGF-low cells was quantified by qPCR. F. Expression of VEGF, α6A and α6B in VEGF-high and -low populations was assessed by immunoblotting. VEGF-high cells express high levels of α6B and low levels of α6A compared to the VEGF-low population. G. ESRP1 expression in the VEGF-high population increases α6A mRNA expression as measured by qPCR. H. Effect of ESRP1 expression on the self-renewal ability of VEGF-high cells was analyzed by serial passaging of mammospheres. I. Effect of ESRP1 expression in VEGF-high cells on tumor onset was analyzed by transplanting transfectants (105 cells/mouse) in mammary fat pads of NSG mice (n=8). The curve comparison was done using Log-rank test (p=0.02).
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