Regulation of protein phosphorylation by triiodothyronine (T3) in neural cell cultures. Part I: Astrocytes.

Dissociated cells from 2-day-old rat cerebral hemispheres were cultured for 17 days in absence of thyroid hormones using conditions yielding mainly glial cells. Triiodothyronine (10(-8) M) was added for 0-72 h before the end of the incubation and [32P]phosphate was added for the last 4 h. Soluble (105,000 X g supernatant), particulate (105,000 X g pellet) and HMG (high mobility group; 0.75 M perchloric acid-soluble proteins) fractions were prepared and phosphorylated proteins in each fraction were analyzed by polyacrylamide gel electrophoresis. In the soluble fraction a protein (Mr = 19,000) incorporates less [32P]phosphate after only 4 h of T3 treatment. The maximal effect is attained after 7 h (-42%) and remains unchanged up until 72 h. In this fraction, the phosphorylation of some other proteins is increased but the maximal effect is observed 48 and 72 h after T3 administration. In the particulate fraction, exposure to T3 rapidly (4 h) increases the amount of a protein (Mr = 45,000) identified as beta-actin. Protein phosphorylation in this fraction is slightly, or not at all, affected by T3. In contrast, a rapid (between 4 and 7 h) increased phosphorylation of a 17 kDa protein in the HMG fraction is observed following T3 stimulation. This nuclear protein was further characterized as HMG 14. These results show that thyroid hormones can produce direct effects (not mediated by neurons) on the phosphorylation of specific proteins in cultured glial cells. Possible functional implications of the observed protein changes are discussed in this paper.