Modulation of Thyroid Hormone-Dependent Gene Expression in Xenopus laevis by INhibitor of Growth

INhibitor of Growth (ING) proteins belong to a large family of plant homeodomain finger-containing proteins important in epigenetic regulation and carcinogenesis. We have previously shown that ING1 and ING2 expression is regulated by thyroid hormone (TH) during metamorphosis of the Xenopus laevis tadpole. The present study investigates the possibility that ING proteins modulate TH action. Tadpoles expressing a Xenopus ING2 transgene (Trans.sub.ING2) were significantly smaller than tadpoles not expressing the transgene (Trans.sub.GFP). When exposed to 10 nM 3,5,3'-triiodothyronine (T.sub.3 ), premetamorphic Trans.sub.ING2 tadpoles exhibited a greater reduction in tail, head, and brain areas, and a protrusion of the lower jaw than T.sub.3 -treated Trans.sub.GFP tadpoles. Quantitative real time polymerase chain reaction (QPCR) demonstrated elevated TH receptor [beta] (TR[beta]) and TH/bZIP transcript levels in Trans.sub.ING2 tadpole tails compared to Trans.sub.GFP tadpoles while TR[alpha] mRNAs were unaffected. In contrast, no difference in TR[alpha], TR[beta] or insulin-like growth factor (IGF2) mRNA abundance was observed in the brain between Trans.sub.ING2 and Trans.sub.GFP tadpoles. All of these transcripts, except for TR[alpha] mRNA in the brain, were inducible by the hormone in both tissues. Oocyte transcription assays indicated that ING proteins enhanced TR-dependent, T.sub.3 -induced TR[beta] gene promoter activity. Examination of endogenous T.sub.3 -responsive promoters (TR[beta] and TH/bZIP) in the tail by chromatin immunoprecipitation assays showed that ING proteins were recruited to TRE-containing regions in T.sub.3 -dependent and independent ways, respectively. Moreover, ING and TR proteins coimmunoprecipitated from tail protein homogenates derived from metamorphic climax animals. We show for the first time that ING proteins modulate TH-dependent responses, thus revealing a novel role for ING proteins in hormone signaling. This has important implications for understanding hormone influenced disease states and suggests that the induction of ING proteins may facilitate TR function during metamorphosis in a tissue-specific manner.