Misexpression of wild-type and truncated isoforms of the high-mobility group I proteins HMGI-C and HMGI(Y) in uterine leiomyomas.

High-mobility group I (HMGI) proteins are architectural transcription factors expressed predominantly during embryonic development. Their genetic loci are the most frequent targets of chromosomal rearrangements in uterine leiomyomas and other benign tumors. It was therefore suggested that both HMGI genes are involved in the neoplastic transformation of benign tumors. By Western analysis we found that 16 of 33 uterine leiomyomas expressed high levels of HMGI-C or HMGI(Y) proteins, whereas they were not detected in the corresponding myometrium. Immunohistochemistry demonstrated that the expression of HMGI-C is restricted to leiomyoma smooth muscle cells but is not expressed in vascular smooth muscle cells or the connective tissue of the tumor. Northern blotting confirmed the protein expression data for HMGI-C, whereas HMGI(Y) mRNA and protein levels did not correlate, suggesting that posttranscriptional mechanisms are involved in the regulation of HMGI(Y) expression. Three of the uterine leiomyomas analyzed expressed HMGI-C gene products with altered molecular weight. Two of them were proved to consist of the entire DNA-binding domain but lacked sequences of the C-terminal acidic tail. Conversely, other tumors expressed HMGI-C or HMGI(Y) genes that were not affected by mutations of the coding region. Thus we identified uterine leiomyomas that expressed mutated HMGI-C, whereas other uterine leiomyomas expressed wild-type HMGI-C or HMGI(Y). On the basis of our data we assume that the enhanced expression of functionally active HMGI proteins, whether they are wild-type or not, is important for the pathogenesis of uterine leiomyomas.

Recent developments in molecular action of antihormones.

Antihormones are by definition antagonists of steroid hormone action. They interact with the ligand binding domains of steroid hormone receptors and competitively inhibit the action of the receptors by mechanisms that are not quite understood. In certain cases antihormones also exhibit agonistic activity especially in connection with certain naturally occurring receptor mutants. These observations together with findings of indiscriminate interaction of antihormones with several classes of steroid receptors have necessitated a search of more effective and reliable antihormones. Recent advances in the resolution of the crystal structure of the ligand binding domains of certain members of the steroid receptor family and identification of non-liganded activation of steroid receptors have produced considerable information that can be harnessed into a fruitful search for a new generation of antihormones.

Activation of transcription by progesterone receptor involves derepression of activation functions by a cofactor.

Hormone-induced progesterone receptors (PR) bound to response elements stimulate transcription initiation at target promoters through a mechanism that presumably involves cofactors or coactivators. To allow identification of such cofactors of transcriptional activation in a functional assay, we have established a reconstituted transcription system that is characterized by a specific loss of responsiveness to purified baculovirus-expressed wild type PR. In contrast to wild type PR, a C-terminally truncated PR mutant displayed strong activation potential in this system. As the purified recombinant full-length PR is capable of DNA binding, our results suggest that C-terminal sequences of PR mediate a cis-repression of N-terminal activation functions. Moreover, using this PR-nonresponsive transcription system, we identified and partially purified an activity from rat liver, termed COPRA (cofactor of PR activation), that restores transactivation by full-length PR. Characterization of COPRA revealed that this cofactor exhibits activator specificity and is not involved in basal transcription. We postulate that COPRA acts by relieving the repression of activation functions mediated by C-terminal sequences.

Identification of a transactivation function in the progesterone receptor that interacts with the TAFII110 subunit of the TFIID complex.

Transcriptional activation of target genes by the human progesterone receptor is thought to involve direct or indirect protein-protein interactions between the progesterone receptor and general transcription factors. A key role in transcription plays the general factors. A key role in transcription plays the general transcription factor TFIID, a multiprotein complex consisting of the TATA-binding protein and several tightly associated factors (TAFs). TAFs have been shown to be required for activated transcription and are, thus, potential targets of activator proteins. Using in vitro interaction assays, we could identify specific interactions between the progesterone receptor and the TATA-binding protein-associated factor dTAFII110. The dTAFII110 domain responsible for the interaction is distinct from that reported to suffice for binding to Sp1. Somewhat surprisingly, deletion analysis indicated that the previously identified activation functions 1 and 2 of the progesterone receptor are not required for this interaction but pointed to an important role of the DNA binding domain. In cotransfection experiments and an in vitro transcription assay, the DNA binding domain of the progesterone receptor displayed significant activation potential. These findings, taken together, suggest that an interaction between the progesterone receptor and TAFII110 may represent an important step in the mechanism of activation.