Endocrine Abstracts

Thyroid hormone receptor a1 (TRa1) mediates the genomic action of thyroid hormone (T3). The biology of TRa1 in growth, differentiation, and development is well studied, but how TRa1 could crosstalk with key regulators involved in cancer development is largely unknown. In the present study, we used an epithelial cell line (NCM460D, referred to as 460D hereafter), established from normal human colon mucosal epithelium as a model to explore the interaction of TRa1 with the tumor suppressor p53. It is known that mutations in p53 promote progression of colorectal cancer. Using viral transduction, we prepared 460D cells stably expressing wild-type p53 (460D-WTp53) or mutant p53R248G (460D-MTp53). We further exogenously and stably expressed TRa1 into 460D-WTp53 (460D-WTp53-TRa1) and 460D-MTp53 cells (460D-MTp53-TRa1). Analysis showed that 460D-MTp53 cells grew faster than 460D-WTp53 cells in vitro. However, TRa1 inhibited cell proliferation in both 460D-MTp53 and 460D-WTp53 cells. Consistent with the in vitro findings, 460D-MTp53 cells induced larger xenograft tumors than 460D-WTp53 cells. Remarkably, TRa1 suppressed xenograft tumor growth induced by 460D-MTp53 or 460D-WTp53 cells. Immunohistochemical analysis (IHC) showed that Ki-67 positively stained tumor cells were in the order of 460D-WTp53 > 460D-WTp53-TRa1; 460D-MTp53>460D-MTp53-TRa1, indicating that TRa1 suppressed p53-induced tumor cell growth. IHC analysis indicated that cleaved capase-3 positively stained tumor cells were in the order of 460D-WTp53-TRa1>460D-WTp53; 460D-MTp53-TRa1>460D-MTp53, indicating that TRa1 induced apoptosis. To understand how TRa1 suppressed 460D-WTp53 and 460D-MTp53 functions, we first ascertained whether TRa1 could physically interact with WTp53 or MTp53. Using co-immunoprecipitation assay, we found that TRa1 physically associated with WTp53 or MTp53. The cyclin-dependent kinase inhibitor 1(the p21^Cip1/Waf1 gene) is known to be a WTp53 direct target gene, with p53 binding cis-regulatory elements on the promoter of the p21 gene. We found that the extent of the expression of the p21 mRNA was 460D-WTp53>460D-WTp53-TRa1. However, in 460D-MTp53 cells, there were no apparent changes in the expression of the p21 mRNA by TRa1. Consistent with mRNA expression, chromatin immunoprecipitation analysis of 460D-WTp53-TRa1 cells showed that binding of WTp53 to the p21 promoter was reduced ~50% when p53 was associated with TRa1. Taken together, our findings indicate that TRa1 is a novel negative regulator of p53 functions via suppression of p53 transcription. Considering reports that somatic TP53 mutations occur in 38-50% of human cancers, our studies raise the possibility that TRa1 could be a potential molecular target in human cancers.