The role of the myofibroblast in tumor stroma remodeling

Abstract

Since its first description in wound granulation tissue, the myofibroblast has been recognized to be a key actor in the epithelial-mesenchymal cross-talk that plays a crucial role in many physiological and pathological situations, such as regulation of prostate development, ventilation-perfusion in lung alveoli or organ fibrosis. The presence of myofibroblasts in the stroma reaction to epithelial tumors is well established and many data are accumulating which suggest that the stroma compartment is an active participant in tumor onset and/or evolution. In this review we summarize the evidence in favor of this concept, the main mechanisms that regulate myofibroblast differentiation and function, as well as the biophysical and biochemical factors possibly involved in epithelial-stroma interactions, using liver carcinoma as main model, in view of achieving a better understanding of tumor progression mechanisms and of tools directed toward stroma as eventual therapeutic target.

Figure 1. The myofibroblast in the tumor stroma. Myofibroblasts associated with the tumor stroma can be activated from a variety of different progenitor cells, including locally residing fibroblasts, epithelial and endothelial cells via epithelial-to-mesenchymal transition, pericytes and bone marrow-derived circulating fibrocytes and mesenchymal stem cells (MSCs). In the intact tissue, the precursor cells are stress-shielded by a functional extracellular matrix (ECM); they do not develop contractile features and cell-matrix adhesions. Upon injury and loss of tissue homeostasis inflammatory signals activate stromal cells to spread and remodel the initially soft ECM. The gradual increase in ECM stiffness permits the formation of contractile microfilament bundles of cytoplasmic actins (stress fibers) that characterize the proto-myofibroblast. Transforming growth factor-β1 (TGFβ1) in conjunction with the stiff ECM stimulates proto-myofibroblasts to express and incorporate α-smooth muscle actin (α-SMA) into stress fibers. The force generated by α-SMA-containing stress fibers leads to further ECM contraction, thereby establishing a mechanical feedback loop. The chemical and mechanical environment created by proto-myofibroblasts and differentiated myofibroblasts support epithelial cell transformation, invasion and tumor growth.

Figure 2. Integrin-mediated TGFβ1 activation by cell contraction. Structural and force spectroscopy data suggest that the transforming growth factor-β1 (TGFβ1) and latent TGFβ1 binding protein-1 (LTBP-1) binding domains of the latency-associated peptide (LAP) act as a sensor in a mechanical model of integrin-mediated TGFβ1 activation. Mechanical stretch, applied through cell integrins will open the latent TGFβ1 complex to release TGFβ1. Flexible domains in the LAP that are prone to unfolding lie within a domain that has been coined “straitjacket.” In the context of a poorly organized and compliant matrix and when cells develop only low contractile activity, the lack of sufficient mechanical tension will prevent the integrin-mediated conformational changes that are required to activate TGFβ1 from the latent complex. Conversely, on a stiff matrix the transmission of contractile forces via integrins to the LAP will favor unfolding of the straitjacket region, resulting in TGFβ1 release. Reproduced with permission.

Figure 3. Hepatocellular carcinoma and cholangiocarcinoma. Gross appearances of a typical case of hepatocellular carcinoma undergoing on cirrhosis (A) and of a typical case of intrahepatic cholangiocarcinoma showing important hyaline changes (B). The matrix of the tumor stroma is scanty in the hepatocellular carcinoma (C) while in the center of the intrahepatic cholangiocarcinoma, the tumor stroma is fibrous and the tumor cells are inconspicuous (D) (Masson’s trichrome histochemistry).

Figure 4. Expression of α-SMA and of periostin in the cholangiocarcinoma tissue and in rat granulation tissue. In the cholangiocarcinoma tissue, there is a strong staining for α-smooth muscle actin (α-SMA) in the stromal cells within the tumor (A). A similar distribution is seen with periostin staining (B). Merge image for the two proteins (C) shows that α-SMA and periostin are colocalized in stromal cells, though periostin (green) has a slightly wider distribution than α-SMA (red) as it is both intracellular in myofibroblasts and deposited in the matrix. (D) In the rat granulation tissue, a strong expression for periostin is observed in myofibroblasts, underlining the similarity between the stroma reaction and the granulation tissue.