Induction of the lipocyte phenotype in murine hepatic stellate cells: reorganisation of the actin cytoskeleton

Hepatic stellate cells (HSCs) are intralobular connective tissue cells pres enting myofibroblast or lipocyte phenotypes. They participate in the homeos tasis of liver extracellular matrix, repair, regeneration and fibrosis unde r the former phenotype, and control retinol metabolism, storage and release under the latter one. Responding to systemic or local demands, they can co nvert into the required phenotype with deep modifications of their structur es. Using immunofluorescence microscopy and Western blots, we investigated the expression and organisation of actin filaments and of two actin-binding proteins, alpha -actinin and tropomyosin, in the cloned GRX cell line repr esentative of murine HSCs. GRX cells expressing the myofibroblast phenotype showed typical well-organised actin stress-fibres, anchored at the focal a dhesions located at the cell periphery. Retinol treatment induced active re organisation of the cytoskeleton. The major stress fibres were reduced in l ength, and frequently formed a polygonal meshwork. Subsequently, they fragm ented and generated diffuse or granular actin in the perinuclear area, a th in continuous layer around lipid droplets and, in fully converted lipocytes , a peripheral layer of thin actin fibres. alpha -Actinin and tropomyosin w ere present only in lipocytes, co-distributed with actin in a granular form . Since the cytoskeleton reorganisation preceded lipid accumulation, we con clude that the induction of the lipocyte phenotype represents a full reprog ramming of cell gene expression and function. We consider that both the lip ocyte and the myofibroblast phenotypes should be considered "activated stat es" of HSCs, each responding to specific physiological or pathological modi fications of liver functions.