FBXO21 mediated degradation of p85α regulates proliferation and survival of acute myeloid leukemia

Acute myeloid leukemia (AML) is a heterogeneous disease characterized by clonal expansion of myeloid blasts in the bone marrow (BM). Despite advances in therapy, the prognosis for AML patients remains poor, and there is a need to identify novel molecular pathways regulating tumor cell survival and proliferation. F-box ubiquitin E3 ligase, FBXO21, has low expression in AML, but expression correlates with survival in AML patients and patients with higher expression have poorer outcomes. Silencing FBXO21 in human-derived AML cell lines and primary patient samples leads to differentiation, inhibition of tumor progression, and sensitization to chemotherapy agents. Additionally, knockdown of FBXO21 leads to up-regulation of cytokine signaling pathways. Through a mass spectrometry-based proteomic analysis of FBXO21 in AML, we identified that FBXO21 ubiquitylates p85α, a regulatory subunit of the phosphoinositide 3-kinase (PI3K) pathway, for degradation resulting in decreased PI3K signaling, dimerization of free p85α and ERK activation. These findings reveal the ubiquitin E3 ligase, FBXO21, plays a critical role in regulating AML pathogenesis, specifically through alterations in PI3K via regulation of p85α protein stability.

Multi-omics reveals mitochondrial metabolism proteins susceptible for drug discovery in AML

Acute myeloid leukemia (AML) is a devastating cancer affecting the hematopoietic system. Previous research has relied on RNA sequencing and microarray techniques to study the downstream effects of genomic alterations. While these studies have proven efficacious, they fail to capture the changes that occur at the proteomic level. To interrogate the effect of protein expression alterations in AML, we performed a quantitative mass spectrometry in parallel with RNAseq analysis using AML mouse models. These combined results identified 34 proteins whose expression was upregulated in AML tumors, but strikingly, were unaltered at the transcriptional level. Here we focus on mitochondrial electron transfer proteins ETFA and ETFB. Silencing of ETFA and ETFB led to increased mitochondrial activity, mitochondrial stress, and apoptosis in AML cells, but had little to no effect on normal human CD34+ cells. These studies identify a set of proteins that have not previously been associated with leukemia and may ultimately serve as potential targets for therapeutic manipulation to hinder AML progression and help contribute to our understanding of the disease.

Effect of geriatric assessment (GA) and genetic profiling on overall survival (OS) of older adults with acute myeloid leukemia (AML)

7021

Background: GA can predict the risk of toxicities of chemotherapy in older adults. Genetic risk categories correlate with OS in AML. We previously reported a reduction in early mortality in a pre-planned interim analysis of a phase II trial with the use of GA and genetic profiling to personalize therapy selection (NCT03226418) (Blood 2019; 134(s1):120). Here, we present the results of a propensity score matched analysis demonstrating an improvement in OS over a historical control. Methods: Patients ≥60 years with a new diagnosis of AML underwent GA. Patients were considered fit for intensive chemotherapy if they had robust physical function [normal activities of daily living (ADL) and instrumental ADL, and short physical performance battery score of ≥10 out of 12], normal cognitive function (Montreal Cognitive Assessment score of ≥26 out of 30), and hematopoietic cell transplantation comorbidity index (HCT CI) of 0-2 (except for treatment related AML, where a score of 0-2 in addition to the prior history of malignancy was acceptable). Genetic profiling for therapy selection relied on karyotyping and followed the 2017 ELN criteria. Fit patients with good or intermediate-risk AML received intensive chemotherapy. Patients with high-risk AML received low-intensity chemotherapy, or CPX 351 if they were fit and met the FDA-approved indications. Pragmatic aspects of the trial included broad eligibility criteria (e.g. patients on treatment for other malignancy were enrolled) and co-management of patients with community oncologists. Mortality was compared with a historical control treated during the years 2004-2016 (after approval of HMA) and matched on gender, age, Karnofsky Performance Status (KPS), HCT CI and ELN risk category. Results: Treatment group (n = 27) vs. historical controls (n = 32) were matched in terms of age (median age, 70 vs. 68.5 years), ELN risk category (adverse risk 59% vs. 53%), HCT CI (median score of 2), KPS (median 80 vs. 85), and gender (male 44% vs. 50%). In the treatment group, 3 patients received intensive chemotherapy: CPX 351 (n = 2) or 7+3+ gemtuzumab (n = 1). Other patients received HMA alone (n = 16), decitabine and midostaurin (n = 3), or azacitidine and venetoclax after the approval of venetoclax (n = 5). Treatment in the historical control included intensive chemotherapy (n = 20) such as 7+3, or mostly HMA based low intensity chemotherapy (n = 12). OS was significantly higher in the treatment group over historical control with 1-year OS of 66% (95% CI 60-87%) vs. 16% (95% CI 7-35%). Conclusions: Our model to personalize AML therapy selection represents an innovative approach to precision medicine that incorporates both GA for patient profiling and genetic profiling of leukemia cells. Our results appear promising with superior OS (an absolute difference of 50% in 1-year OS) compared to a matched historical control. Clinical trial information: NCT03226418.

CHD7 and Runx1 interaction provides a braking mechanism for hematopoietic differentiation

Hematopoietic stem and progenitor cell (HSPC) formation and lineage differentiation involve gene expression programs orchestrated by transcription factors and epigenetic regulators. Genetic disruption of the chromatin remodeler chromodomain-helicase-DNA-binding protein 7 (CHD7) expanded phenotypic HSPCs, erythroid, and myeloid lineages in zebrafish and mouse embryos. CHD7 acts to suppress hematopoietic differentiation. Binding motifs for RUNX and other hematopoietic transcription factors are enriched at sites occupied by CHD7, and decreased RUNX1 occupancy correlated with loss of CHD7 localization. CHD7 physically interacts with RUNX1 and suppresses RUNX1-induced expansion of HSPCs during development through modulation of RUNX1 activity. Consequently, the RUNX1:CHD7 axis provides proper timing and function of HSPCs as they emerge during hematopoietic development or mature in adults, representing a distinct and evolutionarily conserved control mechanism to ensure accurate hematopoietic lineage differentiation.

IL-33/IL1RL1 axis regulates cell survival through the p38 MAPK pathway in acute myeloid leukemia

Acute myeloid leukemia (AML) is often characterized by the presence of specific and recurrent chromosomal abnormalities. Current treatments have greatly increased remission rate, but relapse still occurs. Therefore, novel therapeutic approaches are required. Previously, using a conditional Cbfb-MYH11 knockin mouse model, we showed that Cbfb-MYH11 induces the expression of a cytokine receptor, IL1RL1. Treatment with IL-33, the only known ligand of IL1RL1, promotes leukemia cell survival in vitro. We further found that IL1RL1⁺ cells survive better with chemotherapy than IL1RL1^- population. However, the mechanism is not clear. Here, we show that IL-33 treatment decreased drug sensitivity in the human inv(16) AML cell line ME-1. By RT-PCR, we found that IL-33 increased the expression of IL-4 and IL-6 and led to the activation of both p38 MAPK and NF-κB. We also showed that IL-33 decreased apoptosis with increased phosphorylation of p38 MAPK. Moreover, pre-treatment with MAPK inhibitor attenuated the phosphorylation of p38 enhanced by IL-33 and reversed the anti-apoptotic effect by IL-33. Taken together, our findings give news insights into the potential mechanism of the anti-apoptotic effect by IL-33/IL1RL1 axis in AML which will help in future drug development.

HDAC1 Is a Required Cofactor of CBFβ-SMMHC and a Potential Therapeutic Target in Inversion 16 Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is a neoplastic disease characterized by the uncontrolled proliferation and accumulation of immature myeloid cells. A common mutation in AML is the inversion of chromosome 16 [inv (16)], which generates a fusion between the genes for core binding factor beta (CBFB) and smooth muscle myosin heavy chain gene (MYH11), forming the oncogene CBFB-MYH11. The expressed protein, CBFβ-SMMHC, forms a heterodimer with the key hematopoietic transcription factor RUNX1. Although CBFβ-SMMHC was previously thought to dominantly repress RUNX1, recent work suggests that CBFβ-SMMHC functions together with RUNX1 to activate transcription of specific target genes. However, the mechanism of this activity or a requirement for additional cofactors is not known. Here, we show that the epigenetic regulator histone deacetylase 1 (HDAC1) forms a complex with CBFβ-SMMHC, colocalizes with RUNX1 and CBFβ-SMMHC on the promoters of known fusion protein target genes, and that Hdac1 is required for expression of these genes. These results imply that HDAC1 is an important component of the CBFβ-SMMHC transcriptional complex, and that leukemia cells expressing the fusion protein may be sensitive to treatment with HDAC1 inhibitors. Using a knock-in mouse model expressing CBFβ-SMMHC, we found that in vivo treatment with the HDAC1 inhibitor entinostat decreased leukemic burden, and induced differentiation and apoptosis of leukemia cells. Together, these results demonstrate that HDAC1 is an important cofactor of CBFβ-SMMHC and a potential therapeutic target in inv (16) AML. IMPLICATIONS: This report describes a novel role for HDAC1 as a cofactor for the leukemogenic fusion protein CBFβ-SMMHC and shows that inhibitors of HDAC1 effectively target leukemia cells expressing the fusion protein in vivo.

IL1RL1 is dynamically expressed on Cbfb-MYH11+ leukemia stem cells and promotes cell survival

Acute myeloid leukemia (AML) is often characterized by the presence of specific, recurrent chromosomal abnormalities. One of the most common aberrations, inversion of chromosome 16 [inv(16)], generates the fusion oncogene CBFB-MYH11. Previously, we used a mouse knock-in model to show that Cbfb-MYH11 induces changes in gene expression and results in the accumulation of abnormal myeloid cells, a subset of which are enriched for leukemia stem cell (LSC) activity. One gene upregulated by Cbfb-MYH11 encodes the cytokine receptor IL1RL1 (ST2). IL1RL1 and its ligand IL-33 are known regulators of mature myeloid cells, but their roles in AML are not known. Here, we use Cbfb-MYH11 knock-in mice to show that IL1RL1 is expressed by cell populations with high LSC activity, and that the cell surface expression of IL1RL1 is dynamic, implying that the expression of IL1RL1 is not restricted to a specific stage of differentiation. We also show that treatment with IL-33 increased serial replating ability and expression of pro-survival proteins in vitro. Finally, we show that IL1RL1⁺ cells can survive chemotherapy better than IL1RL1⁻ cells in vivo. Collectively, our results indicate that IL1RL1 is dynamically expressed in Cbfb-MYH11⁺ leukemia cells and promotes their survival.

Abstract 5419: HDAC1 regulates RUNX1 activity in inv(16) acute myeloid leukemia

RUNX1 and CBFβ form a transcription factor dimer that regulates normal hematopoiesis and leukemogenesis. Inversion of chromosome 16 (inv(16)) is one of the most common mutations in acute myeloid leukemia (AML), fusing CBFβ with the gene encoding smooth muscle myosin heavy chain (MYH11). The fusion protein encoded by CBFB-MYH11 (CM), retains the ability to bind to RUNX1, and together they cause changes in gene expression leading to leukemogenesis. Recently, we found that Histone Deacetylase 1 (HDAC1) is part of the RUNX1:CM complex, that all three proteins co-localize on the promoters of target genes, and that HDAC1 is required for target gene expression. By deacetylating histones, HDAC1 can act as a transcriptional repressor. However, HDAC1 can also deacetylate non-histone proteins, which may explain its unexpected role in gene activation. We hypothesized that HDAC1 acts by deacetylating proteins in the RUNX1:CM complex.

To test this, we immunoprecipitated the RUNX1:CM complex and probed for acetyl-lysine. The only visible band was at the expected size for RUNX1, suggesting that RUNX1 is the target of HDAC1 activity. Indeed, cells transfected with RUNX1 and HDAC1 showed significantly less acetylation than cells with RUNX1 alone. To test if HDAC1 deacetylates endogenous RUNX1, we treated leukemia cells from CM-expressing knockin mice with the HDAC1 selective inhibitor entinostat. The level of RUNX1 acetylation was significantly increased in treated cells compared to control, providing further support that HDAC1 deacetylates RUNX1.

Acetylation at lysines 24 and 43 of RUNX1 was previously shown to increase its activity. To determine the effect of deacetylation on RUNX1 activity, we used the M-CSFR promoter fused to luciferase. Cells transfected with RUNX1, CBFβ, and HDAC1 showed significantly decreased promoter activity compared to cells with RUNX1 and CBFβ. Importantly, in cells expressing RUNX1 acetylation and deacetylation mimetics, promoter activity was unchanged with the addition of HDAC1, indicating that lysines 24 and 43 are the relevant HDAC1 targets. To test if RUNX1 acetylation status affects the activity of the CM complex, we transfected a CM+ cell line with WT or mutated RUNX1 and examined target gene expression. In CM+ cells over-expressing WT RUNX1, we found similar target gene expression as untransfected CM+ cells. However, expression of either of RUNX1 mutant blocked the CM-induced changes in gene expression. This implies that regulation of RUNX1 acetylation is important for the activity of the RUNX1:CM complex. The observation that both RUNX1 mimetics had the same effect on CM activity may imply that alternate rounds of RUNX1 acetylation and deacetylation is required for continuous RUNX1:CM transcriptional activity. This work provides a possible explanation for HDAC1's role in gene activation in inv(16) AML, and may have implications for other RUNX1 dependent leukemias as well.

Citation Format: Lisa Richter, Yiqian Wang, Michelle Becker, Jake Williams, R. Katherine Hyde. HDAC1 regulates RUNX1 activity in inv(16) acute myeloid leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5419.

Targeting binding partners of the CBFβ-SMMHC fusion protein for the treatment of inversion 16 acute myeloid leukemia

Inversion of chromosome 16 (inv(16)) generates the CBFβ-SMMHC fusion protein and is found in nearly all patients with acute myeloid leukemia subtype M4 with Eosinophilia (M4Eo). Expression of CBFβ-SMMHC is causative for leukemia development, but the molecular mechanisms underlying its activity are unclear. Recently, there have been important advances in defining the role of CBFβ-SMMHC and its binding partners, the transcription factor RUNX1 and the histone deacetylase HDAC8. Importantly, initial trials demonstrate that small molecules targeting these binding partners are effective against CBFβ-SMMHC induced leukemia. This review will discuss recent advances in defining the mechanism of CBFβ-SMMHC activity, as well as efforts to develop new therapies for inv(16) AML.

Runx1 is required for hematopoietic defects and leukemogenesis in Cbfb-MYH11 knock-in mice

CBFβ-SMMHC, the fusion protein generated by the chromosome 16 inversion fusion gene, CBFB-MYH11, is known to initiate leukemogenesis. However, the mechanism through which CBFβ-SMMHC contributes to leukemia development is not well understood. Previously it was proposed that CBFβ-SMMHC acts by dominantly repressing the transcription factor RUNX1, but we recently showed that CBFβ-SMMHC has activities that are independent of RUNX1 repression. In addition, we showed that a modified CBFβ-SMMHC with decreased RUNX1 binding activity accelerates leukemogenesis. These results raise questions about the importance of RUNX1 in leukemogenesis by CBFβ-SMMHC. To test this, we generated mice expressing Cbfb-MYH11 in a Runx1 deficient background, resulting from either homozygous Runx1 null alleles (Runx1^−/−) or a single dominant negative Runx1 allele (Runx1^+/lz). We found that loss of Runx1 activity rescued the differentiation defects induced by Cbfb-MYH11 during primitive hematopoiesis. During definitive hematopoiesis, RUNX1 loss also significantly reduced the proliferation and differentiation defects induced by Cbfb-MYH11. Importantly, Cbfb-MYH11 induced leukemia had much longer latency in Runx1^+/lz mice than in Runx1 sufficient mice. These data indicate that Runx1 activity is critical for Cbfb-MYH11 induced hematopoietic defects and leukemogenesis.

Germline PAX5 mutations and B cell leukemia

The transcription factor PAX5 is required for normal B cell development and is frequently mutated or deleted in B cell precursor acute lymphoblastic leukemia (B-ALL). A new study demonstrates that germline hypomorphic mutations of PAX5 are associated with susceptibility to B-ALL, implicating PAX5 in a growing list of hematopoietic transcription factors mutated in familial leukemia predisposition syndromes.

Abstract 3850: The interaction of RUNX1 with CBFβ-SMMHC during leukemogenesis

Chromosome 16 inversion is associated with acute myeloid leukemia subtype M4Eo and produces a fusion gene CBFB-MYH11 that contains part of the core binding factor (CBF) β gene CBFB, and part of the smooth muscle myosin heavy chain (SMMHC) gene MYH11. This fusion gene encodes a fusion protein CBFβ-SMMHC, which is oncogenic and binds to the runt domain (RD) of RUNX1, another member of the CBF transcription factor family, resulting in repression of RUNX1 transactivation. We have generated mouse models by conventional and conditional knock-in of the Cbfb-MYH11 fusion gene and demonstrated that Cbfb-MYH11 represses Runx1 function in hematopoiesis and predisposes mice to myeloid leukemia (Castilla et. al., Cell 1996; Nat Genet, 1999).

RUNX1 binding and repression was previously considered a key step in leukemogenesis by CBFβ-SMMHC. In order to dissect the molecular mechanism of RUNX1 and CBFβ-SMMHC interaction during leukemogenesis, we generated a knock-in mouse model with deleted high affinity binding site of Cbfb-MYH11. We found accelerated leukemia development in these mice (Kamikubo et.al., Cancer cell, 2010) suggesting that Cbfb-MYH11 play an independent role apart from Runx1 binding and repression. To test if Runx1 is involved in the leukemia development and progression, we crossed Cbfb-MYH11 knock-in mice with mice harboring one of the two mutant alleles of Runx1 - Runx1+/- and Runx1+/Lzd. Runx1+/- contains a null allele while Runx1+/Lzd contains a knocked-in fusion between the RD of Runx1 and the LacZ gene, which is partially dominant-negative in reporter assays. We have determined the rate and percentage of leukemia development in these mice. We found that the Cbfb-MYH11 mice that were Runx1+/- had a similar rate of leukemogenesis when compared with Cbfb-MYH11 mice that were Runx1+/+. However, the Cbfb-MYH11 mice that were Runx1+/Lzd had significantly delayed leukemogenesis as compared to Cbfb-MYH11 mice that were Runx1+/+. Moreover, some of the Cbfb-MYH11; Runx1+/Lzd mice did not develop leukemia at the end of the one-year observation. We detected a decrease of BrdU incorporation in the bone marrow cells in mice with the Runx1+/Lzd allele, suggesting that the delayed leukemia development resulted, at least in part, from decreased proliferation. These data demonstrated that Runx1 is likely required for leukemogenesis by CBFβ-SMMHC.

Citation Format: Ling Zhao, R Katherine Hyde, Lemlem Alemu, P Paul Liu. The interaction of RUNX1 with CBFβ-SMMHC during leukemogenesis. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3850. doi:10.1158/1538-7445.AM2013-3850

The role of microRNAs in acute myeloid leukemia

MicroRNAs (miRs) are short (18-22 nucleotides) non-coding RNAs that are important in regulating gene expression. MiR expression is deregulated in many types of cancers, including leukemias. In acute myeloid leukemia (AML), the expression of specific miRs has been linked with both prognostically and cytogenetically defined subgroups. Recent studies have shown that deregulation of miR expression is not simply a consequence of AML but a potential contributer to leukemogenesis. This commentary will focus on select findings that describe the different mechanistic roles for miRs in the development of leukemia.

RUNX1 repression-independent mechanisms of leukemogenesis by fusion genes CBFB-MYH11 and AML1-ETO (RUNX1-RUNX1T1)

The core binding factor (CBF) acute myeloid leukemias (AMLs) are a prognostically distinct subgroup that includes patients with the inv(16) and t(8:21) chromosomal rearrangements. Both of these rearrangements result in the formation of fusion proteins, CBFB-MYH11 and AML1-ETO, respectively, that involve members of the CBF family of transcription factors. It has been proposed that both of these fusion proteins function primarily by dominantly repressing normal CBF transcription. However, recent reports have indicted that additional, CBF-repression independent activities may be equally important during leukemogenesis. This article will focus on these recent advances.

Accelerated leukemogenesis by truncated CBF beta-SMMHC defective in high-affinity binding with RUNX1

Dominant RUNX1 inhibition has been proposed as a common pathway for CBF leukemia. CBF beta-SMMHC, a fusion protein in human acute myeloid leukemia (AML), dominantly inhibits RUNX1 largely through its RUNX1 high-affinity binding domain (HABD). However, the type I CBF beta-SMMHC fusion in AML patients lacks HABD. Here, we report that the type I CBF beta-SMMHC protein binds RUNX1 inefficiently. Knockin mice expressing CBF beta-SMMHC with a HABD deletion developed leukemia quickly, even though hematopoietic defects associated with Runx1-inhibition were partially rescued. A larger pool of leukemia-initiating cells, increased MN1 expression, and retention of RUNX1 phosphorylation are potential mechanisms for accelerated leukemia development in these mice. Our data suggest that RUNX1 dominant inhibition may not be a critical step for leukemogenesis by CBF beta-SMMHC.

Core binding factor beta (CBFB) haploinsufficiency due to an interstitial deletion at 16q21q22 resulting in delayed cranial ossification, cleft palate, congenital heart anomalies, and feeding difficulties but favorable outcome

The core binding factor beta gene (CBFB), essential to bone morphogenesis, is located at 16q22.1. Homozygous deficiency of CBFB leads to ossification defects in mice. CBFB forms a heterodimer with RUNX2 (CBFA1) during embryonic bone development. RUNX2 mutations lead to cleidocranial dysplasia in humans. We describe an infant boy with an interstitial deletion of 16q21q22, delayed skull ossification, cleft palate, and heart anomalies who had a difficult course in infancy but eventually improved and is healthy. He was found to have CBFB haploinsufficiency, but did not have mutations in RUNX2. We suggest that 16q21q22 deletion be considered when there are antenatal or postnatal findings of enlarged cranial sutures with or without cleft palate. The finding of CBFB haploinsufficiency in our case and the similarity of cranial ossification defects with a mouse model of CBFB deletion suggest a role for CBFB in cranial bone development in humans.

Examination of the pRb-dependent and pRb-independent functions of E7 in vivo

High-risk human papillomaviruses encode two oncogenes, E6 and E7, expressed in nearly all cervical cancers. Although E7 protein is best known for its ability to inactivate the retinoblastoma tumor suppressor protein, pRb, many other activities for E7 have been proposed in in vitro studies. Herein, we describe studies that allowed us to define unambiguously the pRb-dependent and -independent activities of E7 for the first time in vivo. In these studies, we crossed mice transgenic for human papillomavirus 16 E7 to knock-in mice genetically engineered to express a mutant form of pRb (pRb(DeltaLXCXE)) that is selectively defective for binding E7. pRb inactivation was necessary for E7 to induce DNA synthesis and to overcome differentiation-dependent cell cycle withdrawal and DNA damage-induced cell cycle arrest. While most of E7's effects on epidermal differentiation were found to require pRb inactivation, a modest delay in terminal differentiation with resulting hyperplasia was observed in E7 mice on the Rb(DeltaLXCXE) mutant background. E7-induced p21 upregulation was also pRb dependent, and genetic Rb inactivation was sufficient to reproduce this effect. While E7-mediated p21 induction was partially p53 dependent, neither p53 nor p21 induction by E7 required p19(ARF). These data show that E7 upregulates the expression of p53 and p21 via pRb-dependent mechanisms distinct from the proposed p19-Mdm2 pathway. These results extend our appreciation of the importance of pRb as a relevant target for high-risk E7 oncoproteins.

Unique roles for E2F1 in the mouse lens in the absence of functional pRB proteins

PURPOSE

Normal lens fiber cell differentiation requires functional retinoblastoma protein (pRB), because inactivation of this protein results in proliferation and apoptosis in normally postmitotic, differentiating fiber cells. Loss of either E2F1 or -3 can partially rescue the lens phenotype in Rb-deficient mice, implying that these E2Fs may have specific targets in this system. The purpose of this study was to determine what unique role E2F1 may play.

METHODS

Expression of E2F family members and target genes was analyzed in the lenses of nontransgenic, E2F1-null, alphaAE7;E2F1-sufficient; and alphaAE7;E2F1-null mice by in situ hybridization, Northern blot analysis, and RT-PCR.

RESULTS

In lenses of E2F1-null mice, there was no change in the expression of E2F-2 to -5 or their target genes, compared with E2F1-sufficient mice. However, in the lens of alphaAE7 mice where pRB proteins are inactivated, expression of E2F2 and -3a was increased. The E2F3a increase, but not that of E2F2, was dependent on E2F1. Expression of E2F target genes was increased with expression of E7 and expression of one of these, p19ARF, was E2F1 dependent.

CONCLUSIONS

Although in the normal lens there do not appear to be unique roles for E2F1 that cannot be fulfilled by other E2F family members, in the absence of functional pRB proteins, E2F1 is specifically responsible for the increased expression of E2F3a and p19ARF. These findings suggest that E2F1 may be the preferred E2F regulating these target genes in the normal lens.

The position of the alpha and beta subunits in a single chain variant of human chorionic gonadotropin affects the heterodimeric interaction of the subunits and receptor-binding epitopes

The glycoprotein hormone family represents a class of heterodimers, which include the placental hormone human chorionic gonadotropin (CG) and the anterior pituitary hormones follitropin, lutropin, and thyrotropin. They are composed of common alpha subunit and a hormone-specific beta subunit. Based on the CG crystal structure, it was suggested that the quaternary subunit interactions are crucial for biological activity. However, recent observations using single chain glycoprotein hormone analogs, where the beta and alpha subunits are linked (NH(2)-CGbeta-alpha; CGbetaalpha orientation), implied that the heterodimeric-like quaternary configuration is not a prerequisite for receptor binding/signal transduction. To study the heterodimeric alignment of the two subunit domains in a single chain and its role in the intracellular behavior and biological action of the hormone, a single chain CG variant was constructed in which the carboxyl terminus of alpha was fused to the CGbeta amino terminus (NH(2)-alpha-CGbeta; alphaCGbeta orientation). The secretion rate of alphaCGbeta from transfected Chinese hamster ovary cells was less than that seen for CGbetaalpha. The alphaCGbeta tether was not recognized by dimer-specific monoclonal antibodies and did not bind to lutropin/CG receptor. To define if one or both subunit domains were modified in alphaCGbeta, it was co-transfected with a monomeric alpha or CGbeta gene. In each case, alphaCGbeta/alpha and alphaCGbeta/CGbeta complexes were formed indicating that CG dimer-specific epitopes were established. The alphaCGbeta/alpha complex bound to receptor indicating that the beta domain in the alphaCGbeta tether was still functional. In contrast, no significant receptor binding of alphaCGbeta/CGbeta was observed indicating a major perturbation in the alpha domain. These results suggest that although dimeric-like determinants are present in both alphaCGbeta/alpha and alphaCGbeta/CGbeta complexes, the receptor binding determinants in the alpha domain of the tether are absent. These results show that generating heterodimeric determinants do not necessarily result in a bioactive molecule. Our data also indicate that the determinants for biological activity are distinct from those associated with intracellular behavior.