42 Hepatoma lines are affected similarly, with increased proliferation and chemotherapeutic resistance when grown on increasingly stiff polyacrylamide gels. This effect is mediated by the Fak, Erk, Pkb/Akt, and Stat3 pathways, primarily downstream of integrin β1 signaling.43 Stromal stiffness also increases activation of stellate cells44 and portal fibroblasts,45 creating a positive feedback loop that continues to promote fibrosis. Stromal stiffness is regulated in part by matrix metalloproteinases PD-0332991 ic50 (MMPs) and their inhibitors, but MMPs can regulate cell proliferation independently of their effects on stromal stiffness. Although MMPs degrade the

stroma, they paradoxically increase liver growth,46 HSC proliferation,47 and tumor progression.48 MMPs might also liberate sequestered growth factors (see next section). Alternatively, the production of reactive

oxygen species in response to MMP activity may overcome the loss of stromal stiffness by promoting genomic instability. This type of reactive oxygen species induction is reportedly downstream of an alternatively spliced form of Rac1, which is induced after mammary epithelial exposure to MMP-3.49, 50 A third possibility JQ1 is that MMP induction of reactive oxygen species leads to enhanced stellate cell activation, also through a Rac1-mediated mechanism.51 Growth factors are sequestered by the ECM and signal in an autocrine or paracrine manner to nearby cells.52, 53 Initial work focused on fibroblast growth factor (FGF) sequestration in the ECM,54 but many other cytokines are passively sequestered, including ligands from the FGF, TGF, BMP, Wnt, and interleukin families. MMPs can both activate and inhibit growth factor signaling: they Carnitine palmitoyltransferase II liberate growth factors from the ECM, but can also remove the extracellular receptors by cleavage at the cell surface. Other signaling factors are actively recruited to the ECM by regulatory carrier proteins.

For example, TGF-β signaling is highly dependent on ECM interactions. TGF-β is directly recruited to the ECM by latent TGF-β binding proteins (LTBPs), which have affinity for both TGF-β and ECM fibrils. When bound to LTBP, TGF-β is unable to signal. This suggests that accumulation of ECM would lead to increased proliferation and decreased apoptosis, because TGF-β signaling would be suppressed. However, LTBPs contain multiple proteinase sensitive sites, and cleavage of those sites by MMPs leads to the release of TGF-β.55 In the setting of inflammation or increased migratory potential, elevated MMP activity can liberate sequestered TGF-β. Fibrotic ECM, containing more sequestered TGF-β, would release greater amounts of the cytokine. This could antagonize oncogenesis by inhibiting proliferation and promoting apoptosis. The nature of ECM-cytokine interactions may change depending on the particular cytokine, duration, and cellular context of each interaction.