Interaction of the putative androgen receptor-specific coactivator ARA70/ELE1α with multiple steroid receptors and identification of an internally deleted ELE1β isoform

Steroid-regulated gene transcription requires the coordinate physical and functional interaction of hormone receptors, basal transcription factors, and transcriptional coactivators. In this context ARA(70), previously called RFG and ELE1, has been described as a putative coactivator that specifically enhances the activity of the androgen receptor (AR) but not that of the glucocorticoid receptor (GR), the progesterone receptor, or the estrogen receptor (ER). Here we describe the cloning of the cDNA for ELE1/ARA(70) by RT-PCR from RNA derived from different cell lines (HeLa, DU-145, and LNCaP). In accordance with the previously described sequence, we obtained a 1845-bp PCR product for the HeLa and the LNCaP RNA. Starting from T-47D RNA, however, an 860-bp PCR product was obtained. This shorter variant results from an internal 985-bp deletion and is called ELE1 beta; accordingly, the longer isoform is referred to as ELE1 alpha. The deduced amino acid sequence of ELE1 alpha, but not that of ELE1 beta, differs at specific positions from the one previously published by others, suggesting that these two proteins are encoded by different nonallelic genes. ELE1 alpha is expressed in the three prostate-derived cell lines examined (PC-3, DU-145, and LNCaP), and this expression is not altered by androgen treatment. Of all rat tissues examined, ELE1 alpha expression is highest in the testis. This is also the only tissue in which we could demonstrate ELE1 beta expression. Both ELE1 alpha and ELE1 beta interact in vitro with the AR, but also with the GR and the ER, in a ligand-independent way. Overexpression of either ELE1 isoform in DU-145, HeLa, or COS cells had only minor effects on the transcriptional activity of the human AR. ELE1 alpha has no intrinsic transcription activation domain or histone acetyltransferase activity, but it does interact with another histone acetyltransferase, p/CAF, and the basal transcription factor TFIIB. The interaction with the AR occurs through the ligand-binding domain and involves the region corresponding to the predicted helix 3. Mutation in this domain of leucine 712 to arginine greatly reduces the affinity of the AR for ELE1 alpha but has only moderate effects on its transcriptional activity. Taken together, we have identified two isoforms of the putative coactivator ARA(70)/ELE1 that may act as a bridging factor between steroid receptors and components of the transcription initiation complex but which lack some fundamental properties of a classic nuclear receptor coactivator. Further experiments will be required to highlight the in vivo role of ELE1 in nuclear receptor functioning.