Soft matrices inhibit cell proliferation and inactivate the fibrotic phenotype of deep endometriotic stromal cells in vitro.

Can deep infiltrating endometriotic stromal cells (DES) sense changes in extracellular matrix (ECM) stiffness and respond to them? Soft matrices inhibit cell proliferation and inactivate the fibrotic phenotype of DES in vitro. Deep infiltrating endometriosis (DIE) is characterized histologically by dense fibrous tissue. Tissue stiffening is a hallmark of fibrosis. Studies show that matrix stiffness is involved in the progression of numerous diseases, including cancer and fibrosis. However, no studies to date have investigated whether tissue stiffening could influence cell behavior in DIE. Previous in vitro studies typically analyzed cells grown on rigid plastic or glass substrates with stiffness in the gigapascal (gPa) range, which is much stiffer than that occurring in vivo. To investigate how changes in ECM stiffness affect the behavior of DES, it is critical to model in vivo tissue compliance conditions in vitro. For this laboratory study, paired endometrial and endometriotic samples from 40 patients who had histological evidence of DIE and endometrial samples from 23 patients without endometriosis were analyzed (uterine fibroma: n = 10, tubal infertility: n = 13). All participants were 20-37 years old and had regular menstrual cycles of 26-32 days. The abundance of F-actin, alpha smooth muscle actin (αSMA), Ki67, and procollagen type I in DES and endometrial stromal cells (EES) on polyacrylamide gel substrates of varying stiffness (2, 4, 8, 16 and/or 30 kPa) was determined by immunofluorescence confocal microscopy. mRNA level of type I collagen, matrix metalloproteinase-1 (MMP-1), MMP-14 and cyclin D1 was measured by real-time PCR. The cellular proliferation index (CPI), assessed as the percentage of Ki67-positive cells among the total number of nuclei stained by 4',6-diamidino-2-phenylindole (DAPI) was determined. Increased matrix stiffness induced F-actin stress fiber formation in both EES and DES, whereas αSMA-containing stress fibers were induced only in DES. Furthermore, increased stiffness increased the CPI in both EES (16 or 30 kPa versus 2 kPa, P < 0.05) and DES (16 or 30 kPa versus 2, 4 or 8 kPa, P < 0.05). Increased stiffness increased the percentage of procollagen I-positive cells as well as mRNA levels of type I collagen in both EES and DES in a matrix stiffness-dependent manner (2, 8 and 30 kPa) (P < 0.05). Increased stiffness also increased MMP-14 mRNA levels in EES (30 versus 2 kPa, P < 0.05), but decreased MMP-1 mRNA levels in DES in a matrix stiffness-dependent manner (2, 8 and 30 kPa; P < 0.05). Treatment with transforming growth factor (TGF)-β1 further increased type I collagen mRNA levels in both EES and DES when compared with cells grown on a substrate of the same stiffness (2, 8 or 30 kPa, with versus without TGF-β1, P < 0.05). Treatment with TGF-β1 also increased MMP-1 (8 or 30 kPa, P < 0.05 versus no TGF-β1) and MMP-14 mRNA levels (2, 8 or 30 kPa, P < 0.05 versus no TGF-β1) in EES, but decreased MMP-1 mRNA levels (2, 8 or 30 kPa, P < 0.05 versus no TGF-β1) in DES. On a soft substrate (2 kPa), both EES and DES exhibited a small rounded morphology with diffuse labeling for F-actin. No F-actin-positive stress fibers were observed in either EES or DES grown on 2 kPa substrates. There were more Ki67-positive EES when grown on 2, 4 or 8 kPa compared with Ki67-positive DES (P < 0.05). A tremendous gap exists between the present in vitro model and in vivo deep endometriotic tissues. Cell culture systems that more closely mimic the cellular complexity typical of in vivo endometriotic tissues are required to develop novel strategies for treatment of DIE. A disadvantage of polyacrylamide is its cytotoxicity but in the two-dimensional culture models used here, where cells are seeded above the polyacrylamide gel, this should not have a major impact. Finally, the soft substrates we used in vitro (2 and 4 kPa) may represent the elasticity of the endometrium in vivo, however, currently there are no data regarding tissue stiffness in DIE in vivo. Hormonal suppressive therapy is not usually effective for treating DIE. Interrupting the mechanical interactions between endometriotic fibroblasts and aberrant ECM may be a novel strategy for treatment of DIE. This study was supported in part by Karl Storz Endoscopy & GmbH (Tuttlingen, Germany). No competing interests are declared.