Transcriptional Regulation by a Naturally Occurring Truncated Rat Estrogen Receptor (ER), Truncated ER Product-1 (TERP-1)

Abstract

Truncated estrogen receptor product-1 (TERP-1) is a naturally occurring rat estrogen receptor (ER) variant transcribed from a unique start site and containing a unique 5'-untranslated region fused to exons 5-8 of ERalpha. TERP-1 is detected only in the pituitary, and TERP-1 mRNA levels are highly regulated during the estrous cycle, exceeding those of the full-length ERalpha on proestrus. These data suggest that TERP-1 may play a role in estrogen- regulated feedback in the pituitary. We examined the ability of TERP-1 to modulate gene transcription in transiently transfected ER-negative (Cos-1) and ER-positive pituitary (alphaT3 and GH3) cell lines. In Cos-1 cells transiently cotransfected with TERP-1 and either ERalpha or ERbeta, low levels of TERP-1 (ratios of < 1:1 with ER) enhanced transcription of model promoters containing estrogen response elements by an average of 3- to 4-fold above that seen with ER alone. At higher concentrations of TERP-1 (> 1:1 with ER) transcription was inhibited. TERP-1 also had a biphasic action on transcription in the alphaT3 and GH3 pituitary cell lines, although the stimulatory action was less pronounced. TERP-1 actions were dependent on ligand-activated ER as TERP-1 did not bind estradiol in transfected Cos-1 cells or in vitro, and estrogen antagonists prevented the stimulatory effects of TERP-1. Coimmunoprecipitation studies suggest that TERP-1 does not bind with high affinity to the full-length ERalpha. However, TERP-1 may compete with ER for binding sites of receptor cofactors because steroid receptor coactivator-1 (SRC-1) rescued the inhibitory actions of TERP-1. The ability of TERP-1 to both enhance and inhibit ER-dependent promoter activity suggests that TERP-1 may play a physiological role in estrogen feedback in the rat pituitary.

Truncated Estrogen Receptor Product-1 Stimulates Estrogen Receptor α Transcriptional Activity by Titration of Repressor Proteins

“…Our previous studies showed that the TERP:ER␣ ratio influences ER␣ transcriptional activity in a biphasic manner in transient transfection experiments (28). For TERP suppressive effects on ERE-containing promoters, TERP dimerization with ER␣ or ER␤ is required (27,34).…”

“…TERP may be considered an orphan receptor, in that it is structurally similar to a known nuclear receptor (ER␣) but is inefficient or unable to bind ligand (26,28). At least two orphan nuclear receptors, short heterodimer partner (SHP) and DAX-1, have also been shown to modulate ER␣ activity (42,43).…”

“…Transcription-Our laboratory has observed previously that TERP modulated ER␣ transcriptional activity biphasically (28). High concentrations of TERP were suppressive for ER activity through inactive dimer formation with ER␣ that disrupted ER␣ binding to an ERE.…”

“…In vitro translated TERP is detected as a pair of doublets at 22-24 kDa, which likely represent translation from two methionines, at amino acids 393 and 408 in exon 5 (28,32). The larger protein is detected preferentially in transfected cells and in normal pituitaries (11,28,29,31,32).…”

“…The resulting protein lacks the DBD and does not bind ligand efficiently. TERP has no independent transcriptional activity on EREcontaining promoters but modulates ER transcriptional activity biphasically (27,28). In transient transfection assays, TERP stimulates ER␣ activity at low concentrations, where the ratio of transfected TERP to ER␣ is less than 1:1, but inhibits ER␣ activity at high concentrations, where the ratio of TERP to ER␣ is greater than 1:1 (28).…”