Remodeling of the extracellular matrix (ECM) is a key hallmark of cancer progression. A critical component of ECM remodeling is the assembly of the glycoprotein fibronectin (FN) into insoluble fibrils, which provide a scaffold for invading vascular endothelial cells and escaping cancer cells, as well as a framework for collagen deposition and oncogenic cytokine tethering. FN fibril assembly is induced by Transforming Growth Factor-β1 (TGF-β1), which was originally identified for its role in malignant transformation. Addition of exogenous TGF-β1 drives FN fibril assembly while also upregulating endogenous TGF-β1 expression and autocrine signaling. In the current study, we sought to determine if autocrine TGF-β1 signaling plays a role in FN fibril formation in either MCF10A mammary epithelial cells, which behave similarly to healthy epithelia, or malignant MDA- MB-231 breast cancer cells. Our results show two interesting findings: first, malignant MDA-MB- 231 cells assemble less FN into fibrils, despite expressing and secreting more soluble FN; second, autocrine TGF-β1 signaling is required for FN fibril formation in MCF10A epithelial cells, even in the presence of exogenous, active TGF-β1. This suggests that autocrine TGF-β1 is signaling through distinct pathways from active exogenous TGF-β1. We hypothesized that this signaling was mediated by interactions between the TGF-β1 latency associated peptide (LAP) and α_v integrins; indeed, incubating MCF10As with soluble LAP, even in the absence of the active TGF-β1 ligand, partially recovered FN fibril assembly. Taken together, these data suggests that autocrine TGF-β1 plays a critical role in FN fibril assembly, and this interaction is mediated by LAP-integrin signaling.

细胞外基质（ECM）的重塑是癌症进展的关键标志。ECM 重塑的一个关键组成部分是将糖蛋白纤连蛋白 (FN) 组装成不溶性原纤维，为入侵血管内皮细胞和逃逸癌细胞提供支架，以及胶原蛋白沉积和致癌细胞因子束缚的框架。FN 原纤维组装由转化生长因子-β1 (TGF-β1) 诱导，最初因其在恶性转化中的作用而被鉴定。添加外源性 TGF-β1 可驱动 FN 原纤维组装，同时上调内源性 TGF-β1 表达和自分泌信号传导。在当前的研究中，我们试图确定自分泌 TGF-β1 信号传导是否在 MCF10A 乳腺上皮细胞（其行为与健康上皮相似）或恶性 MDA-MB-231 乳腺癌细胞中的 FN 原纤维形成中发挥作用。我们的结果显示了两个有趣的发现：首先，恶性 MDA-MB-231 细胞将较少的 FN 组装成原纤维，尽管表达和分泌更多的可溶性 FN；其次，即使存在外源性活性 TGF-β1，MCF10A 上皮细胞中 FN 原纤维的形成也需要自分泌 TGF-β1 信号传导。这表明自分泌 TGF-β1 通过与活性外源 TGF-β1 不同的途径发出信号。我们假设这种信号传导是由 TGF-β1 潜伏相关肽 (LAP) 和 α _v整合素之间的相互作用介导的；事实上，即使在没有活性 TGF-β1 配体的情况下，将 MCF10As 与可溶性 LAP 一起孵育，也能部分恢复 FN 原纤维组装。综上所述，这些数据表明自分泌 TGF-β1 在 FN 原纤维组装中发挥着关键作用，并且这种相互作用是由 LAP 整合素信号传导介导的。