Tri-iodothyronine (T3) and dexamethasone interact to modulate osteoprogenitor cell differentiation in fetal rat calvaria cell cultures

We investigated the role of 3,5,3′-tri-iodothyronine (T3) in regulating differentiation of osteoprogenitor cells and also studied the effects of the glucocorticoid hormone dexamethasone (Dex) on the T3-induced effects on osteoprogenitor populations. This was done by determining the effects of either hormone alone, or of combinations of the two hormones, on the number of bone nodules formed in long-term cultures of rat calvaria cells. In this system, Dex has been shown to increase bone nodule formation, the maximal effective dose being 10 nM (Bellows et al. Endocrinology 121: 1985–1992; 1987). In standard culture medium containing 15% fetal bovine serum (FBS), low concentrations of T3 (0.001 – 0.1 nM) had no effect on the number of bone nodules, while higher concentrations of 1–100 nM inhibited. However, in culture medium containing 10 nM Dex, the lower concentrations of T3 markedly increased the number of nodules. Short term pulse experiments with these low concentrations of T3 in the presence of Dex indicated that stimulation of nodule formation occurred only when T3 was present prior to confluency. Higher concentrations of T3 (1–100 nM) decreased nodule number whether or not Dex was added. We then cultured cells in medium containing FBS from which T3 and T4 were removed by treatment with AG-1χ-10 resin. In both + or - Dex conditions, bone nodule formation was increased 1.5 to 2-fold in T3, T4-depleted medium when compared with cultures maintained in standard culture medium. In T3, T4- depleted medium, Dex increased bone nodule formation dose dependently with maximum effect at 10 nM Dex, essentially similar to the effects of Dex in standard medium. Similar to the T3 response in media containing nondepleted FBS, addition of T3 to cultures in medium containing T3, T4-depleted FBS decreased the number of nodules in a dose-dependent manner. These results show that in standard culture medium, T3 inhibits osteoprogenitor cell differentiation in the absence of Dex, but that in the presence of an optimal concentration of Dex, low concentrations of T3 stimulate osteoprogenitor cell differentiation. Also, the results of the experiments where Dex was added to T3, T4-depleted medium suggest that inhibitory factors other than T3, T4 may have been removed in the depleted serum.