GFI1 as a novel prognostic and therapeutic factor for AML/MDS.

Genetic and epigenetic aberrations contribute to the initiation and progression of acute myeloid leukemia (AML). GFI1, a zinc-finger transcriptional repressor, exerts its function by recruiting histone deacetylases to target genes. We present data that low expression of GFI1 is associated with an inferior prognosis of AML patients. To elucidate the mechanism behind this, we generated a humanized mouse strain with reduced GFI1 expression (GFI1-KD). Here we show that AML development induced by onco-fusion proteins such as MLL-AF9 or NUP98-HOXD13 is accelerated in mice with low human GFI1 expression. Leukemic cells from animals that express low levels of GFI1 show increased H3K9 acetylation compared to leukemic cells from mice with normal human GFI1 expression, resulting in the upregulation of genes involved in leukemogenesis. We investigated a new epigenetic therapy approach for this subgroup of AML patients. We could show that AML blasts from GFI1-KD mice and from AML patients with low GFI1 levels were more sensitive to treatment with histone acetyltransferase inhibitors than cells with normal GFI1 expression levels. We suggest therefore that GFI1 has a dose-dependent role in AML progression and development. GFI1 levels are involved in epigenetic regulation, which could open new therapeutic approaches for AML patients.

Human insulin receptor substrate-2: gene organization and promoter characterization.

Insulin receptor substrate-2 (IRS-2) belongs to a family of cytoplasmic adaptor proteins, which link insulin, IGF-1, and cytokine receptor tyrosine kinases to signaling pathways regulating metabolism, growth, and differentiation (1-3). IRS-2-deficient mice display all characteristics of type 2 diabetes, suggesting that dysfunction of the IRS-2 gene may contribute to the pathogenesis of human type 2 diabetes (4). Based on its progesterone inducibility, we have recently cloned and sequenced a full-length human IRS-2 cDNA containing an open reading frame (ORF) of 4,014 bp and 5'- and 3'-untranslated regions (UTRs) of 516 and 2,466 bp (5). Although the IRS-2 gene has previously been thought to lack introns within the coding region (6,7), the amino acid sequence predicted from our cDNA sequence differed at its very COOH-terminal end from an IRS-2 protein sequence derived from genomic IRS-2 sequences. Therefore, we carefully analyzed the genomic structure of the IRS-2 gene and found that the IRS-2 gene contains an intron that disrupts the ORF. Characterization of promoter and 5'-flanking regions of IRS-2 by sequencing, reporter gene assays, and chromatin structure analysis suggests that elements conferring progesterone inducibility are not located immediately upstream of the gene promoter.

Human insulin receptor substrate-2 (IRS-2) is a primary progesterone response gene.

Elevated cAMP has been shown to unmask agonist activity of antiprogestin/antiglucocorticoid RU486. In our search for cellular target genes induced through this cross-talk mechanism, we identified human insulin receptor substrate-2 (IRS-2), a cytoplasmic signaling molecule that mediates effects of insulin, insulin-like growth factor-1 (IGF-I), and other cytokines by acting as a molecular adaptor between diverse receptor tyrosine kinases and downstream effectors. Our analysis of the regulation of IRS-2 in HeLa cell models shows that synergistic induction of IRS-2 by cAMP and RU486 can be mediated by progesterone receptors (PR) and glucocorticoid receptors (GR) and occurs through a relative slow mechanism that requires ongoing protein synthesis. Importantly, we demonstrate that IRS-2 mRNA is also inducible by progesterone, while glucocorticoid effects are only observed in the presence of cAMP. Up-regulation of IRS-2 by progesterone depends strictly on the presence of PR and occurs through a rapid mechanism, suggesting that it represents a primary transcriptional response. Furthermore, we show that expression of IRS-1, which also binds to receptors of insulin, IGF-I, and cytokines, is unaffected by progesterone. Thus, our results demonstrate that progesterone alters the ratio of IRS-1 and IRS-2 in PR-positive cells and implicate a mechanism through which progesterone can modulate the effects of insulin, IGF-I, and cytokines on cell proliferation, differentiation, and homeostasis.

Targets of activated steroid hormone receptors: basal transcription factors and receptor interacting proteins.

Steroid hormone receptors constitute a family of inducible transcription factors that mediate the multi-fold effects of steroids on development, reproduction, proliferation, and cellular homeostasis. Activation through the binding of the cognate hormone enables the receptors to bind with high affinity to specific response elements in the promoters of target genes, resulting in stimulation or repression of transcription. While protein-protein interactions were early postulated to play an important role in the mechanism through which steroid hormone receptors exert their effects on transcription initiation, recent research has revealed a number of potential targets within the basal transcription machinery. Moreover, aided by the development of protein-protein interaction screening techniques, a rapidly increasing number of factors has been identified which associate with hormonally activated receptors and may be involved in the transactivation process. This review summarizes the basal transcription factors and cofactors which are targeted by steroid hormone receptors, describes their structure and properties, and discusses possible mechanisms.

Synergistic enhancement of PRB-mediated RU486 and R5020 agonist activities through cyclic adenosine 3',5'-monophosphate represents a delayed primary response.

Activators of protein kinase A have been shown to affect the transactivation potential of progestins and antiprogestins. To analyze the mechanisms and factors involved, we have created HeLa and CV1 cell clones stably expressing isoform B of progesterone receptor. In the HeLa cell background, the progesterone antagonist RU486 significantly induces progesterone-regulatable reporter genes, and this agonistic effect is synergistically enhanced by elevating cAMP or through overexpression of protein kinase A catalytic subunit. In contrast, in CV1 cells containing functional progesterone receptors no agonist activity of RU486 could be detected, suggesting the involvement of cell specifically expressed factors. In a PR(B)-positive HeLa cell clone containing stably integrated copies of a thymidine kinase-luciferase reporter gene with two progesterone response elements, we observed a complete loss of RU486 antagonist potential upon cotreatment with cAMP for 25 h while partial antagonist potential was maintained in a 5-h experiment. This result shows that, particularly in the presence of protein kinase A activators, the duration of hormone treatment is a crucial parameter in the evaluation of antagonist properties of antiprogestins. A detailed analysis of the kinetics of the hormone effects on transcription revealed that the onset of cAMP/RU486 synergism is delayed relative to the responses induced by RU486 or R5020 alone. Moreover, partial inhibition of protein synthesis by cycloheximide completely abolished cAMP/RU486 synergism while R5020 and RU486 responses were not inhibited. Together, these data indicate that cAMP/RU486 synergism is a delayed primary response requiring the intermediate induction of an essential factor.