Alterations in chondrocyte cytoskeletal architecture during phenotypic modulation by retinoic acid and dihydrocytochalasin B-induced reexpression

The differentiated phenotype of rabbit articular chondrocytes was modulated in primary culture by treatment with 1 microgram/ml retinoic acid (RA) and reexpressed in secondary culture by treatment with the microfilament-disruptive drug dihydrocytochalasin B (DHCB) in the absence of RA. Because the effective dose of DHCB (3 microM) did not elicit detectable cell rounding or retraction, the nature and extent of microfilament modification responsible for induction of reexpression was evaluated. The network of microfilament stress fibers detected with rhodamine-labeled phalloidin in primary control chondrocytes was altered by RA to a "cobblestone" pattern of circularly oriented fibers at the cell periphery. Subsequent treatment with DHCB resulted in rapid changes in this pattern before overt reexpression. Stress fibers decreased in number and were reoriented. Parallel arrays of long fibers that traversed the cell were evident, in addition to fiber fragments and focal condensations of staining. Immunofluorescent staining of intermediate filaments revealed a marked decrease in complexity and intensity during RA treatment but no change during reexpression. An extended microtubular architecture was present throughout the study. These results clearly identify microfilaments as the principal affected cytoskeletal element and demonstrate that their modification, rather than complete disruption, is sufficient for reexpression. The specificity of DHCB and the reorientation of these filaments before reexpression of the differentiated phenotype suggests a causative role in the mechanism of reexpression.