VITAMIN-A POOR LIPOCYTES - A NOVEL DESMIN-NEGATIVE LIPOCYTE SUBPOPULATION, WHICH CAN BE ACTIVATED TO MYOFIBROBLASTS

Lipocytes have been classified as vitamin A-storing, desmin-positive c ells. In hepatic fibrogenesis, lipocytes transform into myofibroblasts , which express alpha-smooth muscle actin (alpha-SMA) and produce incr eased amounts of collagen. We isolated a population of vitamin A-poor lipocytes (VAPL) from normal rat liver and examined the morphological and biochemical differences between VAPL and vitamin A-replete lipocyt es (VARL). Desmin and (alpha-SMA expression were determined by Western blot in quiescent cells and in cells activated by culture on uncoated plastic. Both cell types were alpha-SMA-negative; however, in contras t to VARL, freshly isolated VAPL did not contain desmin. Desmin expres sion was induced in VAPL on activation. With time in culture, both VAP L and VARL expressed alpha-SMA and produced collagen, indicative of tr ansformation to myofibroblasts. Ferritin receptor expression was demon strated in cultured VARL after 1 day and in VAPL after 5 days, indicat ing that this is an early marker of lipocyte activation. After 7 days, VARL and VAPL were indistinguishable in terms of desmin, ferritin rec eptor expression, and collagen production. This study demonstrates the first isolation and characterization of two distinct quiescent subpop ulations of lipocytes from normal rat liver: desmin-negative VAPL and desmin-positive VARL. Both populations of cells can be activated to my ofibroblasts, the phenotype associated with hepatic fibrogenesis.