Abstract
Mesangial cells (MCs) grown without supplemental insulin (SI-MCs) express a quiescent phenotype and extracellular matrix (ECM) composition similar to MCs in vivo. In contrast, MCs routinely propagated in insulin (SI+MCs) are stimulated to proliferate, change their phenotype, and produce large amounts of collagens I and III. These effects of insulin may in part be mediated through cytoskeletal rearrangement. Differences in cytoskeletal arrangement were compared between SI-MCs and SI+MCs and 1 hr after addition of insulin (1 nM) or IGF-1 (100 nM) to SI-MCs. Cells were examined by light microscopy, electron microscopy, and immunostaining for specific cytoskeletal proteins and fibronectin. Insulin induced rapid rearrangement of stress fibers. Surface ruffling, actin aggregation, vimentin retraction, rearrangement of vinculin in focal adhesions, and fibronectin extraction were apparent. These direct effects of insulin on the SI-MC cytoskeleton occurred before insulin-induced changes in ECM composition. IGF-I induced cytoskeletal reorganization distinct from insulin. These observations demonstrate that insulin and IGF-I have unique effects on the MC cytoskeleton, which is turn may mediate secondary ligand effects on MCs.