Asporin Is a Fibroblast-Derived TGF-beta1 Inhibitor and a Tumor Suppressor Associated with Good Prognosis in Breast Cancer.

BACKGROUND: Breast cancer is a leading malignancy affecting the female population worldwide. Most morbidity is caused by metastases that remain incurable to date. TGF-beta1 has been identified as a key driving force behind metastatic breast cancer, with promising therapeutic implications. METHODS AND FINDINGS: Employing immunohistochemistry (IHC) analysis, we report, to our knowledge for the first time, that asporin is overexpressed in the stroma of most human breast cancers and is not expressed in normal breast tissue. In vitro, asporin is secreted by breast fibroblasts upon exposure to conditioned medium from some but not all human breast cancer cells. While hormone receptor (HR) positive cells cause strong asporin expression, triple-negative breast cancer (TNBC) cells suppress it. Further, our findings show that soluble IL-1beta, secreted by TNBC cells, is responsible for inhibiting asporin in normal and cancer-associated fibroblasts. Using recombinant protein, as well as a synthetic peptide fragment, we demonstrate the ability of asporin to inhibit TGF-beta1-mediated SMAD2 phosphorylation, epithelial to mesenchymal transition, and stemness in breast cancer cells. In two in vivo murine models of TNBC, we observed that tumors expressing asporin exhibit significantly reduced growth (2-fold; p = 0.01) and metastatic properties (3-fold; p = 0.045). A retrospective IHC study performed on human breast carcinoma (n = 180) demonstrates that asporin expression is lowest in TNBC and HER2+ tumors, while HR+ tumors have significantly higher asporin expression (4-fold; p = 0.001). Assessment of asporin expression and patient outcome (n = 60; 10-y follow-up) shows that low protein levels in the primary breast lesion significantly delineate patients with bad outcome regardless of the tumor HR status (area under the curve = 0.87; 95% CI 0.78-0.96; p = 0.0001). Survival analysis, based on gene expression (n = 375; 25-y follow-up), confirmed that low asporin levels are associated with a reduced likelihood of survival (hazard ratio = 0.58; 95% CI 0.37-0.91; p = 0.017). Although these data highlight the potential of asporin to serve as a prognostic marker, confirmation of the clinical value would require a prospective study on a much larger patient cohort. CONCLUSIONS: Our data show that asporin is a stroma-derived inhibitor of TGF-beta1 and a tumor suppressor in breast cancer. High asporin expression is significantly associated with less aggressive tumors, stratifying patients according to the clinical outcome. Future pre-clinical studies should consider options for increasing asporin expression in TNBC as a promising strategy for targeted therapy

Study of the expression of a Small Leucin-Rich Proteoglycan, Asporin, in normal human osteoblasts and regulation by breast cancer cells

Dottorato di Ricerca in Biochimica Cellulare ed attività dei Farmaci in Oncologia, Ciclo XXVII, a.a. 2014Asporin (ASPN) is an extracellular matrix protein that belongs to the Small Leucine Rich Repeat proteoglycan (SLRP) family. Asporin is abundantly expressed in the articular cartilage of individuals with osteoarthritis. In the context of osteoarthritis, several studies have shown that asporin regulates cartilage matrix gene expression and cartilage formation by modulating the transforming growth factor-β (TGF- β) signaling pathway. Asporin directly binds to TGF‐β and inhibits TGF-β-mediated expression of cartilage matrix genes. Previous studies in our laboratory, showed that Asporin inhibits TGF- β-1-mediated SMAD2 phosphorylation in breast cancer cells as well as migration and epithelial to mesenchymal transition in A549 human lung cancer cells. The present study was undertaken to investigate whether asporin secretion could indirectly mediate the ability of metastatic breast cancer cells to regulate osteoblastic differentiation. The Wnt antagonist sclerostin (SOST) is a potent inhibitor of bone formation. We considered the possibility that the balance between ASPN and SOST present in the ECM may create a specific environment favorable to aggressive breast cancer cell growth. Results: Breast cancer cells do not produce ASPN themselves but they regulate its expression in osteoblasts. Normal human osteoblasts have been cultured in presence of MCF7 and MDA-MB-231 serum-free conditioned medium. Immunoblot analysis and real time PCR, revealed a significant increase in ASPN expression and secretion in osteoblasts treated with MCF7-conditioned medium, while the opposite effect was observed with MDA-MB-231-conditioned medium. We investigated the role of MCF7 and MDAMB231 conditioned media in osteoblast differentiation and mineralization through alkaline phospatase and Runx2 expression. Our results showed the ability of MCF7 conditioned medium to induce the osteoblast differentiation and mineralization compared to the MDA-MB-231 conditioned medium treatment. Osteoblasts treated with MCF7 conditioned medium and challenged with recombinant SOST showed a significant reduction in their differentiation potential through the decrease of ASPN expression. Contrarily to non-metastatic MCF-7 breast cancer cells, MDA-MB-231 metastatic breast cancer cells inhibited the secretion of ASPN by osteoblasts through the overexpression of SOST. The result is the reduction of osteoblast differentiation and mineralization that can create a specific environment favorable to aggressive breast cancer cell growthUniversità degli Studi della Calabri

Asporin binds to TGF-β1 and inhibits its downstream signaling and function.

<p>(A) Western blot analysis of phospho-SMAD2 (p-SMAD2) and SMAD2 total protein extracts from MDA-MB-468 breast cancer cells treated for 15 min with TGF-β1 and/or human recombinant asporin (Rec. ASPN). (B) Western blot analysis of p-SMAD2 in total protein extracts from MDA-MB-468 breast cancer cells treated with TGF-β1 and/or asporin peptide corresponding to the 159–205 amino acid region (ASPNpep.). (C) Western blot analysis of p-SMAD2 and SMAD2 in total protein extracts from EpRAS cells treated for 15 min with TGF-β1 (5 ng/ml) and/or asporin peptide. (D) EMT induction in EpRAS cells in the presence of TGF-β1 and/or asporin peptide. EMT was monitored both at the phenotype level (upper panel) and using Western blot evaluation of VIM expression in total protein extracts from EpRAS cells (lower panel). (A–D): HSC70 was used as loading control. (E) Transwell migration assay of EpRAS cells pretreated with TGF-β1 (5 ng/ml) and/or asporin peptide (10 μg/ml). (F) Quantification of the CSC population in EpRAS cells following TGF-β1 and/or asporin peptide treatment. (E and F): The data are presented as mean ± SD. All panels: statistical significance was calculated using the Student’s <i>t</i>-test (as described in the <a href="http://www.plosmedicine.org/article/info:doi/10.1371/journal.pmed.1001871#sec010" target="_blank">Methods</a> section). Western blots show representative data of three independent experiments.</p

Asporin is produced by breast fibroblasts in response to conditioned medium from breast cancer cells.

<p>(A) Western blot of total cell extracts (upper panel) and qRT-PCR analysis for asporin expression (lower panel) in breast cancer cell lines and NBFs incubated for 48 h with CM collected from a panel of breast cancer cells. (B) Western blot of total cell extracts (upper panel) and qRT-PCR analysis of asporin expression (lower panel) in non-cancerous epithelial breast cell line MCF-10A cells and NBFs incubated for 48 h with CM collected from MCF-10A. Fibroblasts treated with MCF-7 CM were used as the positive control for asporin expression induction. (C) Validation of NBFs and CAFs isolated from patient material. MCF-7 and MDA-MB-231 cells were used as epithelial controls. (D) Western blot analysis of asporin expression in total cell extracts of CAFs obtained from three different patients and treated with the CM of breast cancer cell lines. (A and B): The data are presented as mean ± SD. All panels: HSC70 was used as loading control; Western blots show representative data of three independent experiments.</p

Asporin is overexpressed in breast cancer tissues.

<p>(A) Tissue-specific pattern of mRNA expression of asporin (ASPN), biglycan (BGN), and decorin (DCN). Source: BioGPS (<a href="http://biogps.org/" target="_blank">http://biogps.org</a>). The data are presented as mean ± standard deviation (SD). (B) Representative IHC staining of asporin expression in ductal carcinoma and adjacent non-tumoral breast tissue (left panel) and normal breast tissue obtained from patients undergoing mammary reduction surgery (right panel). Asporin is almost exclusively expressed in breast cancer lesions, while a very low signal is detectable in the adjacent non-tumoral regions. Normal breast tissues are negative. Images of representative fields were taken at 100× and 400× magnification. (C) Western blot analysis of asporin expression in tumoral breast cancer tissues (T) and the adjacent normal counterpart (AdN) of six ductal adenocarcinoma patients. Ponceau red staining was used as loading control.</p

Co-injection of cancer cells and fibroblasts overexpressing asporin reduces primary breast cancer tumor growth and lung metastasis formation in vivo.

<p>(A) Western blot analysis of asporin expression in CM of MDA-MB-468 cells and in NBF stable clones used for subcutaneous injection in mice. Ponceau red is shown as loading control. (B) Bioluminescence imaging of control and asporin-expressing xenografts at day 28 after tumor engraftment. The color scale indicates the fluorescent intensity. (C) The volume (in cubic millimeters) of primary tumors measured weekly (from day 7 onwards). The data are presented as mean ± standard error of the mean (SEM) (<i>n</i> = 10 for each group). Statistical significance was calculated using Student’s <i>t</i>-test (**0.01 < <i>p</i> < 0.001; ***0.001 < <i>p</i> < 0.0001). (D) Human-specific Alu-PCR performed on genomic DNA isolated from dissected lungs was used to detect human cancer cells. The data are presented as mean ± SD. (E) Western blot analysis of asporin expression in mice primary tumors monitored for several weeks. HSC70 was used as loading control.</p