Thyroid hormone-induced cell proliferation in GC cells is mediated by changes in G1 cyclin/cyclin-dependent kinase levels and activity

The thyroid hormone, 3,3', 5-triiodo-L-thyronine (T-3), is essential for gr owth and regulation of metabolic functions. The biological activities of T- 3 are mediated by its interaction with the thyroid hormone nuclear receptor s (TRs). The mechanism by which TRs mediate cell growth is unknown. We foun d that T-3 stimulated cell growth in GC cells by shortening the doubling ti me approximately S-fold. Flow cytometric analysis indicated that the growth stimulatory effect was mainly due to shortening of G1 phase accompanied by increases in S and G2/M: phases of the cell cycle. These changes correlate d with T-3-induced increases in messenger RNA and protein levels of two key regulators of G1 progression, cyclins D1 and E, as well as cdk2. Furthermo re, the kinase activities associated with cyclin D1 and E were activated up to 4-fold by T-3, which led to increased phosphorylation of the retinoblas toma protein (Rb), the driving force in G1 to S cell cycle progression. The se results show for the first time that the growth promoting effect of T-3 in GC cells is mediated, at least in part, by increases in cyclin/cdk activ ities and the phosphorylation state of Rb. The functional link of T-3 to Rb has important implications for the understanding of the biology of normal and cancer cells.