Contrasting effects of two alternative splicing forms of coactivator-associated arginine methyltransferase 1 on thyroid hormone receptor-mediated transcription in Xenopus laevis.

Thyroid hormone receptors (TRs) can repress or activate target genes depending on the absence or presence of thyroid hormone (T3), respectively. This hormone-dependent gene regulation is mediated by the recruitment of corepressors in the absence of T3 and coactivators in its presence. Many TR-interacting coactivators have been characterized in vitro. Among them is coactivator-associated arginine methyltransferase 1 (CARM1), which methylates histone H3. We are interested in investigating the role of CARM1 in TR-mediated gene expression in vivo during postembryonic development by using T3-dependent frog metamorphosis as a model. We first cloned the Xenopus laevis CARM1 and obtained two alternative splicing forms, CARM1a and CARM1b. Both isoforms are expressed throughout metamorphosis, supporting a role for these isoforms during the process. To investigate whether Xenopus CARM1s participate in gene regulation by TRs, transcriptional analysis was conducted in Xenopus oocyte, where the effects of cofactors can be studied in the context of chromatin in vivo. Surprisingly, overexpression of CARM1b had little effect on TR-mediated transcription, whereas CARM1a enhanced gene activation by liganded TR. Chromatin immunoprecipitation assays showed that both endogenous CARM1a and overexpressed CARM1a and b were recruited to the promoter by liganded TR. However, the binding of liganded TR to the target promoter was reduced when CARM1b was overexpressed, accompanied by a slight reduction in histone methylation at the promoter. These results suggest that CARM1 may play a role in TR-mediated transcriptional regulation during frog development and that its function is regulated by alternative splicing.