Actin-binding proteins as molecular targets to modulate hepatic stellate cell proliferation. Focus on "MARCKS actin-binding capacity mediates actin filament assembly during mitosis in human hepatic stellate cells".

HEPATIC STELLATE CELLS (HSCs) are liver resident mesenchymal cells that play a crucial role in liver fibrosis, as they are a major source for excessive deposition of extracellular matrix (ECM) proteins, of which type I collagen predominates (1). In the normal liver, HSCs reside in the space of Disse and are the major storage sites of vitamin A. Following chronic injury, HSCs activate or transdifferentiate into myofibroblast-like cells, acquiring contractile, proinflammatory, and fibrogenic properties. Activated HSCs migrate, proliferate, and accumulate at the sites of tissue repair, secreting large amounts of ECM and regulating its degradation. Drugs aimed at inhibiting HSC proliferation are currently considered as a promising antifibrotic strategy in the management of patients with chronic liver disease. The acquisition of mitogenic and motogenic properties by activated HSCs is tightly dependent on a dynamic and extensive reorganization of the actin cytoskeleton (2). Actin, a highly conserved protein, constitutes an essential component of the cytoskeleton in most eukaryotic cells and exists in two principal forms, globular, monomeric (G) and filamentous polymeric (F). Actin filaments regulate major cellular functions, including the generation and maintenance of cell morphology and polarity, endocytosis and intracellular trafficking, contractility, motility, and cell division. The assembly of actin filaments and their organization into functional networks is regulated by a large number of actin-binding proteins (ABPs). ABPs stabilize and cross-link actin filaments through the control of specific signaling pathways. There are about 48 different types of ABPs (9). They include myosins, monomer binders (e.g., profillin), cross-linkers (e.g., filamin), bundlers (fasci and actinin), stabilizers (e.g., tropomyosin), anchors to membrane (e.g., annexin, catenin), signalers (e.g., vasodilator-stimulated phosphoprotein or VASP), etc. Despite the important role of actin in the acquisition of fibrogenic properties by activated HSCs, little is known about actin biology and the presence and functions of ABPs in this cell type. Previous studies have investigated the role of myristoylated alanine-rich kinase C substrate (MARCKS), a well-characterized ABP, in some biological properties of HSCs. Activation of HSCs results in overexpression of MARCKS, which modulates platelet-derived growth factor (PDGF)-induced HSC motility (7). MARCKS and the modulation of PDGF -receptors are functionally associated, such that the activation of protein kinase C (PKC)ε is indispensable for PDGF-BB-induced MARCKS phosphorylation and cell migration. MARCKS is a member of a small family of PKC substrates that are widely expressed in different cell types and tissues (3). The protein is localized in the plasma membrane and is an actin filament cross-linking protein. As a consequence of these membraneand actin-binding capacities, MARCKS is thought to have a role in processes that depend on membrane turnover and cytoskeletal remodeling, including cell motility, phagocytosis, membrane trafficking, and mitogenesis. The function of MARCKS in the reorganization of the local actin cytoskeleton is regulated by phosphorylation at a specific phosphorylation site domain (PSD). This binding site also interacts with PKC, calmodulin, and phosphatidylserine and sequesters phosphatidylinositol 4,5-bisphosphate (PIP2) (10). Overall, the MARCKS PSD allows the selective targeting and regulation of numerous signal transduction complexes that result in a local transient softening and remodeling of the actin cytoskeleton (5). Therefore, MARCKS appears to be an appealing target site for therapeutic intervention in conditions associated by abnormal actin activity. In this issue of American Journal of Physiology-Cell Physiology, Rombouts et al. (8) provide convincing evidence supporting a role for MARCKS and actin in regulating mitosis in human HSCs. By using confocal analysis, the authors showed that localization of phosphorylated MARCKS varies in distinct phase of mitosis to act as a centrosome protein in conjunction with Aurora B kinase (AUBK), a chromosomal marker and mitotic regulator. The finding that AUBK and phosphoMARCKS accumulate together with actin during cytokinesis suggests the association of phospho-MARCKS with cleavage furrow formation. To test this hypothesis, the authors manipulated MARCKS gene expression. Thus, MARKS gene silencing resulted in loss of F-actin as well as inhibition of proliferation of human HSCs. In addition, by studying the cell cycle, the authors showed that MARCKS is a key regulator of mitosis. Increased cell cycle time in MARCKS-depleted cells was due to abnormal cell morphology and an aberrant cytokinesis. Altogether, these results suggest that MARCKS along with actin plays an important role in the regulation of mitosis in human HSCs (Fig. 1). Although this study provides further insight into the important role of ABP in regulating key biological properties of activated HSCs, there are many unanswered questions that future studies will need to address: 1) which are the main ABPs expressed in activated HSCs and what are their relative roles in regulating different biological properties of these cells? 2) What are the cellular signals that regulate the expression and phosphorylation of ABPs, including MARKS? Can we moduAddress for reprint requests and other correspondence: R. Bataller, Depts. of Medicine and Nutrition, 2209 McGavran-Greenberg, Univ. of North Carolina at Chapel Hill, Chapel Hill, NC 27599-7461 (e-mail: [email protected]). Am J Physiol Cell Physiol 303: C355–C356, 2012; doi:10.1152/ajpcell.00192.2012. Editorial Focus