CYTOSKELETAL ALTERATIONS IN HUMAN FETAL ASTROCYTES INDUCED BY INTERLEUKIN-1-BETA

Previous studies in this and other laboratories have shown that interl eukin-1 beta (IL-1 beta) is a selective and potent activator of human astrocytes with respect to induction of cytokines and hematopoietic gr owth factors. To study the effect of recombinant human IL-1 beta (rhlL -1 beta) on astrocyte morphology, glial fibrillary acidic protein (GFA P) and vimentin expression, and actin organization, we conducted a sys tematic survey using dissociated human fetal astrocyte cultures. Withi n hours of stimulation with IL-1 beta, the majority of astrocytes conv erted from flat, polygonal cells to small, contracted, highly branched cells. This change in morphology was more striking when serum was eli minated from the medium. Complete dissolution of filamentous actin occ urred simultaneously with the change in cell shape, as demonstrated by fluorescein-phalloidin binding. These ''activated'' astrocytes displa yed intense GFAP and vimentin immunoreactivity in the small perikarya and processes. in contrast, the large, flat astrocytes in control cult ures showed diffuse pale immunoreactivity for GFAP and vimentin. To qu antify the changes in GFAP and vimentin content with IL-1 beta stimula tion, densitometric analyses of northern and western blots were perfor med. Northern blot analysis of IL-1 beta-stimulated astrocytes reveale d a transient, marked decrease in steady-state levels of mRNA for GFAP , vimentin, and microtubule-associated protein 4. The decrease in mRNA levels was evident by 4-8 h and fell to the lowest level at 16-24 h ( 80-98% decrease by densitometry) with partial recovery by 72 h. By imm unoblotting, a significant decrease in both GFAP and vimentin protein content was observed after IL-1 beta stimulation. Furthermore, metabol ic labeling studies revealed an almost total loss of GFAP synthesis fo llowing stimulation with IL-1 beta for 16 h. These observations are co nsistent with the idea that increases in immunoreactivity were related to factors such as redistribution of epitope, rather than increases i n total protein content. We hypothesize that in IL-1 beta-stimulated a strocytes, synthesis of other proteins, e.g., inflammatory cytokines, occurs at the expense of structural proteins and that the decrease in content of cytoskeletal proteins may reflect an ''activated'' state of astrocytes.