The effect of T3 on cell growth was studied in cultured GC cells, a thyroid hormone-sensitive rat pituitary cell line that produces GH. Cells grown in modified Joklik's Minimal Essential Medium with 15% normal serum had a mean doubling time of 27.1 ± 2.8 h, and this was not altered by the addition of T3 in concentrations up to 1 × 10−73 M. When hypothyroid (Tx) calf serum (T3, < 0.15 × 10-9 M) was substituted for normal serum [medium designed to simulate Tx conditions (Tx medium)], the growth rate was unchanged for 2 days and then decreased. After 4 days in Tx medium, the mean doubling time was 107.5 ± 19.2 h. The addition of T3 (5 × 10-8 M) to the Tx calf serum used in the medium (Tx-T3 medium) resulted in a detectable increase in growth rate after 2 days and thereafter an exponential growth rate similar to that of cells grown continuously in T3-supplemented medium. The respective growth rates in Tx and Tx-T3 media were not altered by the addition of rat GH (500 ng/ml). To define further the factors contributing to the growth response to T3, the duration of the cell cycle stages of GC cells in exponential growth in Tx or Tx-T3 medium was determined by measuring the percentage of labeled mitoses after 15-min exposure to [3H]thymidine. Compared to cells in Tx-T3 medium, cells in Tx medium exhibited an increase of 7.9-fold in the duration of Gi, of 2.2-fold in G2, and of 1.35-fold in the mitotic (M) stage. No change in the duration of the period of DNA synthesis was apparent. Examination of DNA profiles by flow cytofluorometry showed that the decrease in growth rate and prolongation of Gi, G3, and M stages of the cell cycle in Tx medium were not due to cell damage or abnormalities in DNA ploidy. Thus, T3 effected a 3.9-fold increase in growth rate due to a decrease in the G1, G2, and M stages of the cell cycle. The data suggest that growth rate changes occurred in the entire cell population and did not result from the development of degenerated or altered cells under Tx conditions. Cultured GC cells appear to be a good model for examination of thyroidal effects on the growth and development of mammalian cells.
ASJC Scopus subject areas