Responsiveness to thyroid hormone is enhanced in rat hepatocytes cultured as spheroids compared with that in monolayers: Altered responsiveness to thyroid hormone possibly involves complex formed on thyroid hormone response elements

We previously reported that the expression of type I iodothyronine 5'-deiod inase (5'DI) gene was increased by 3,3,',5-triiodothyronine (T-3) in isolat ed rat hepatocytes when cultured as spherical aggregates (spheroids), where as this effect was greatly attenuated in conventional monolayer cultures. I n the current study, we examined whether the enhanced Tg responsiveness in spheroid cultures extends to other T-3-responsive genes. As observed for 5' DI, T-3 increased spot 14, malic enzyme and fibronectin messenger RNAs (mRN As) by fourfold to fivefold in spheroid cultures, while the effect in monol ayer cultures was blunted. This difference in T-3 responsiveness was also o bserved when T-3-responsive reporters consisting of the luciferase gene und er the control of triiodothyronine response element (TRE) were introduced i nto hepatocytes using a replication-defective adenovirus vector. These resu lts suggest that the factors required for T-3-dependent transcriptional act ivation are preserved in spheroid cultures and that they must exert their e ffect by interacting with TRE. Maximal binding capacity of nuclear T-3 rece ptor was not different between monolayer and spheroid cultures while the ex pression of retinoid X receptor-alpha (RXR alpha) mRNA was higher in sphero id cultures compared with that in monolayers. The difference in RXR alpha m RNA expression, together with enhanced proteolytic cleavage in monolayers t hat we demonstrated recently, may account for the difference in T-3 respons iveness between the two hepatocyte culture systems.