Multiple ING1 and ING2 genes in Xenopus laevis and evidence for differential association of thyroid hormone receptors and ING proteins to their promoters

ING (INhibitor of Growth) tumor suppressor proteins are epigenetic factors involved in numerous cellular processes including apoptosis in species ranging from yeast to humans. We recently isolated ING1 and ING2 transcript variants in Xenopus laevis and showed that these transcripts were differentially regulated by thyroid hormone (TH) during postembryonic development. However, no information exists regarding ING gene structure and how it relates to these differential responses to TH. To further investigate the regulation of ING genes by TH, we isolated ING1 and ING2 gene sequences and demonstrated that there are at least duplicate genes for each. The relationship between transcript variants and their responsiveness to TH were examined through promoter sequence and chromatin immunoprecipitation analyses on tail homogenates. Both TH receptors (TRs) differentially associated with ING1 and ING2 promoter regions with increased recruitment in the presence of TH. This occurred irrespective of gene transcript level response to this hormone. However, differential recruitment of RNA polymerase II corresponded well to transcript levels. ING proteins consistently associated with their own gene promoters except in the region generating the TH-inducible xING1b5 transcript. In this case, a substantial recruitment of TRβ in the absence ING proteins occurred. These data establish the TH-dependent recruitment of transcription factors to ING promoter regions and suggest that differential TR recruitment in response to TH may not be a sufficient indicator for modulating the expression of ING in the tail.