The extracellular matrix in hepatocellular carcinoma: Mechanisms and therapeutic vulnerability

Abstract

The tumor microenvironment (TME) is influenced by a "disorganized" extracellular matrix (ECM) that sensitizes cancer cells toward mechanical stress, signaling, and structural alterations. In hepatocellular carcinoma (HCC), lack of knowledge about key ECM proteins driving the TME refractory to targeted therapies poses a barrier to the identification of new therapeutic targets. Herein, we discuss the contributions of various ECM components that impact hepatocytes and their surrounding support network during tumorigenesis. In addition, the underpinnings by which ECM proteins transduce mechanical signals to the liver TME are detailed. Finally, in view of the bidirectional feedback between the ECM, transformed hepatocytes, and immune cells, we highlight the potential role of the ECM disorganization process in shaping responses to immune checkpoint inhibitors and targeted therapies. Our comprehensive characterization of these ECM components may provide a roadmap for innovative therapeutic approaches to restrain HCC.

Keywords: YAP/TAZ; agrin; chronic liver inflammation; collagen; extracellular matrix; fibrosis; glypican-3; hepatocellular carcinoma; immunotherapy; mechanotransduction; targeted therapies.

Graphical abstract

Figure 1

ECM regulating the “hallmarks of liver cancer” and its status in healthy liver, chronic disease conditions, and hepatocellular carcinoma (A) The liver ECM network as a molecular sink that harbors structural components, proteoglycans, and several growth factors. The impact of key ECM components (light blue boxes) known to regulate the specific cancer hallmark (reddish brown) are highlighted. (B) ECM in healthy liver tissues that act to maintain liver homeostasis by controlling the proliferation of hepatocytes and sustaining immune-suppressive functions. When chronic liver diseases develop, the ECM is altered by increases in collagen deposition, which enhance stiffness, elastin-derived peptides (EDPs), and PGs, as well as augments the influx of fibroblasts. This initiates an ECM corruption process, as manifested by a low immune response that impacts the hepatocytes, activates HSCs, and promotes abnormal vasculature leading to liver fibrosis. Increased ECM disorganization, in turn, increases the mechanosignaling and contractility of the hepatocytes, which leads to their neoplastic transformation. Cancerous hepatocytes further increase ECM disorganization, causing a vicious oncogenic feedforward loop accompanied by enhanced tissue rigidity, resistance to targeted therapies, and immune-escape mechanisms.

Figure 2

Disorganized ECM modulate mechanical checkpoints in the liver TME (A) A stiff ECM crosstalks with hepatocytes and HSCs, leading to increases in their rigidity, contractility, and geometrical confinement. Members of the disorganized ECM such as agrin, tenascin (TNCN), or osteopontin (OPN) activate their respective mechanosensory receptor integrins-lipoprotein related receptor-4 (Lrp4)-muscle-specific tyrosine kinase (MuSK), frizzled receptors (Fzds), or receptor tyrosine kinases (RTKs), respectively, to transmit biophysical signals. In response to stiff ECM, agrin stimulates integrin-FAK mechanosignaling and actomyosin-dependent RhoA activation, which potentiate the YAP/TAZ transcriptional program. TNCN induces nuclear β-catenin, and OPN via the integrin-EGFR pathway activates the MAPK-dependent transcription of genes that enhance tumorigenesis. (B) Secreted proteins from the disorganized ECM also impact other cell types in the liver TME. For instance, agrin, perlecan, and OPN stimulate the VEGF-VEGFR2 pathway in liver endothelial cells, which increases shear stress and angiogenesis that favors tumor growth. These impacts transformed hepatocytes that further remodel the ECM. Therefore, a bidirectional network establishes a physical continuum between the extrinsic “disorganized ECM” and intrinsic “oncogenic behavior.”

Figure 3

A disorganized liver ECM impacts immune surveillance machinery A low ECM complexity state within the matrix triggers tumor necrosis factor (TNF) and interferon γ (IFN-γ) to facilitate Kupffer cell and natural killer (NK) cell migration and recruitment within tumors for efficient phagocytosis and killing of cancer cells. Moreover, Kupffer cells stimulate regulatory T cells (Tregs) to recruit cytotoxic CD8⁺ T cells that are induced by increased TNF and IFN harbored within a compliant ECM. CD8⁺ T cells release perforin and granzyme to mediate tumor killing. With increased ECM disorganization, high collagen and elastin fiber deposition create a stiff matrix for trapping NK cells. Increased pro-angiogenic growth factors (VEGF, IL-8, and PDGFβ) and matrix metalloproteinases MMP9 and MMP14 generate abnormal vasculature and an EMT that favors tumor growth via TAM recruitment. Highly contractile CAFs also stimulate an EMT under the guidance of TGF-β and IL-6. Independently, a stiff ECM also induces PD-L1 via dendritic cells (DCs), TGF-β, and cyclooxygenase-2 (COX2), all of which suppress CD8⁺ T cell activity to generate a pro-tumorigenic immune response.