Synergistic action of thyroid hormone, insulin and hydrocortisone on astrocyte differentiation

We report here on the synergistic regulation of astrocyte development by 3 hormones: thyroid hormone (TH), insulin, and hydrocortisone (HC). Their effect, in a defined serum-free media, on astrocyte morphology, on glia fibrillary acidic protein (GFAP) immunostaining pattern, and on glutamine synthetase (GS) was investigated. TH transformed the flat, polygonal astrocytes into process-bearing cells. This effect was accentuated by insulin, which by itself had no effect on astrocyte morphology. The morphological transformations were accompanied by changes in the pattern of GFAP immunostaining which indicated a more organized and directed cytoskeleton arrangement in the TH-insulin treated cultures. Over 95% of the cells in the culture expressed GFAP. All 3 hormones regulated GS levels. TH increased GS levels by 50% and insulin raised its levels by 3-fold. While having no effect on astrocyte morphology, HC increased GS levels by 3.7-fold in both the hormone-free and insulin-supplemented medium. HC acted synergistically with insulin in its action on GS bringing about a 12-fold increase in the enzyme activity. In contrast, TH did not interact with insulin and was additive with HC in its action on GS. The continuous presence of insulin and TH was required to maintain their morphological and GS effect, suggesting that these hormones might not only be important for astrocyte differentiation, but later on for astrocyte function as well. Since astrocytes interact with and affect neurons and oligodendrocytes, the findings reported here might have bearing on the development and function of these other brain cells as well.