Characterization of activating mutations of NOTCH3 in T-cell acute lymphoblastic leukemia and anti-leukemic activity of NOTCH3 inhibitory antibodies

Notch receptors have been implicated as oncogenic drivers in several cancers, the most notable example being NOTCH1 in T-cell acute lymphoblastic leukemia (T-ALL). To characterize the role of activated NOTCH3 in cancer, we generated an antibody that detects the neo-epitope created upon gamma-secretase cleavage of NOTCH3 to release its intracellular domain (ICD3), and sequenced the negative regulatory region (NRR) and PEST domain coding regions of NOTCH3 in a panel of cell lines. We also characterize NOTCH3 tumor-associated mutations that result in activation of signaling and report new inhibitory antibodies. We determined the structural basis for receptor inhibition by obtaining the first co-crystal structure of a NOTCH3 antibody with the NRR protein and defined two distinct epitopes for NRR antibodies. The antibodies exhibit potent anti-leukemic activity in cell lines and tumor xenografts harboring NOTCH3 activating mutations. Screening of primary T-ALL samples reveals that two of 40 tumors examined show active NOTCH3 signaling. We also identified evidence of NOTCH3 activation in 12 of 24 patient-derived orthotopic xenograft models, two of which exhibit activation of NOTCH3 without activation of NOTCH1. Our studies provide additional insights into NOTCH3 activation and offer a path forward for identification of cancers that are likely to respond to therapy with NOTCH3 selective inhibitory antibodies.

RARalpha antagonist Ro 41-5253 inhibits proliferation and induces apoptosis in breast-cancer cell lines

Ro 41-5253 is a RARalpha-selective antagonist that binds RARalpha but does not induce transcriptional activation and does not influence RAR/RXR heterodimerization and DNA binding. This retinoid inhibits proliferation and induces apoptosis in MCF-7 and ZR-75.1 estrogen-receptor-positive breast-carcinoma cells in a dose-dependent way. The anti-proliferative effect is more evident in ZR-75.1 cells than in MCF-7 cells and is probably mediated by anti-AP1 activity, a mechanism known to be implied in the action of several retinoids. In the induction of apoptosis also ZR-75.1 cells are more sensitive to treatment with Ro 41-5253 than MCF-7 cells. In ZR-75.1 cells an apoptotic/hypodiploid DNA peak is already evident after 2 days of incubation, whereas in MCF-7 cells it appears only after 4 days. The highest percentage of apoptotic cells, for both cell lines, is reached after 6 days of treatment. The apoptosis pathway is p53-independent and bcl-2 downregulation seems to be correlated with an increase in TGF-beta1 protein. The MDA-MB-231 estrogen-receptor-negative cell line is poorly responsive to Ro 41-5253 treatment, both in terms of proliferation inhibition and apoptosis induction. Ro 41-5253 has proliferation-inhibiting and apoptosis-inducing properties that are not mediated by transcriptional activation from retinoic-acid response elements. This retinoid antagonist seems to be a compound that exerts an anti-tumor activity but does not induce the toxic side effects of retinoids and might, therefore, be considered as a candidate for cancer therapy.

Differential expression of CRABP II, psoriasin and cytokeratin 1 mRNA in human skin diseases

In order to confirm and further explore the significance of the overexpression of the CRABP II (cellular retinoic acid binding protein type II) and psoriasin genes in psoriatic versus normal skin, we examined the mRNA expression levels of these two genes by in situ hybridization in skin samples from psoriatic plaques and in one case from the border between a psoriatic plaque and uninvolved skin. Both genes were markedly upregulated in lesional skin, with a shift from low to high expression in the transitional zone of the plaque. Expression of the cytokeratin 1 (K1) gene was, in contrast, high in normal skin and decreased in the transition from uninvolved skin to psoriatic plaque, Examination of mRNA levels of CRABP II and psoriasin in other hyperproliferative and inflammatory skin diseases showed high expression of psoriasin, and in some cases also of CRABP II, in atopic dermatitis, mycosis fungoides, Darier's disease and inflammatory lichen sclerosus et atrophicus. In atrophic lesions of lichen sclerosus et atrophicus that lacked an inflammatory infiltrate, these changes were only weakly expressed. These findings demonstrate that altered epidermal gene expression of K1, psoriasin and CRABP II is not disease-specific and may reflect instead an altered state of epidermal differentiation and/or may be linked to the inflammation and cellular infiltration common to all the conditions studied.

Marked resistance of RAR gamma-deficient mice to the toxic effects of retinoic acid

Excessive intake of retinol or of retinoic acid causes a syndrome of characteristic toxic effects known as hypervitaminosis A. To test the role of the nuclear retinoic acid receptor (RAR gamma) in this process we produced mice with a targeted disruption of the RAR gamma gene and examined toxic effects of repeated doses of retinoic acid and two other synthetic retinoids, Ro 15-1570 and Ro 40-6055. Surprisingly, homozygous mutant mice were resistant to fourfold higher doses of retinoic acid than wild-type mice as well as to elevated doses of the synthetic retinoids, indicating that RAR gamma may have a major role in mediating retinoid toxicity, a finding that possibly has practical implications for reducing the toxicity of synthetic retinoids in clinical use.

Different agonist- and antagonist-induced conformational changes in retinoic acid receptors analyzed by protease mapping

The pleiotropic effects of retinoic acid on cell differentiation and proliferation are mediated by two subfamilies of nuclear receptors, the retinoic acid receptors (RARs) and the retinoid X receptors (RXRs). Recently the synthetic retinoid Ro 41-5253 was identified as a selective RAR alpha antagonist. As demonstrated by gel retardation assays, Ro 41-5253 and two related new RAR alpha antagonists do not influence RAR alpha/RXR alpha heterodimerization and DNA binding. In a limited trypsin digestion assay, complexation of RAR alpha with retinoic acid or several other agonistic retinoids altered the degradation of the receptor such that a 30-kDa proteolytic fragment became resistant to proteolysis. This suggests a ligand-induced conformational change, which may be necessary for the interaction of the DNA-bound RAR alpha/RXR alpha heterodimer with other transcription factors. Our results demonstrate that antagonists compete with agonists for binding to RAR alpha and may induce a different structural alteration, suggested by the tryptic resistance of a shorter 25-kDa protein fragment in the digestion assay. This RAR alpha conformation seems to allow RAR alpha/RXR alpha binding to DNA but not the subsequent transactivation of target genes. Protease mapping with C-terminally truncated receptors revealed that the proposed conformational changes mainly occur in the DE regions of RAR alpha. Complexation of RAR beta, RAR gamma, and RXR alpha, as well as the vitamin D3 receptor, with their natural ligands resulted in a similar resistance of fragments to proteolytic digestion. This could mean that ligand-induced conformational changes are a general feature in the hormonal activation of vitamin D3 and retinoid receptors.

RXR-dependent and RXR-independent transactivation by retinoic acid receptors

The binding affinity of retinoic acid receptors (RARs) to their response elements is strongly enhanced in vitro by the formation of heterodimers with retinoid X receptors (RXRs) suggesting that heterodimerization with RXR may be a prerequisite for a RAR-mediated transcriptional response. We found that in Drosophila SL-3 cells that are devoid of endogenous RARs and RXRs the presence of RAR is sufficient to confer a response to all-trans retinoic acid (RA). The transfection of both RAR and RXR and stimulation with their respective ligands all-trans and 9-cis RA leads to a synergistic response. On point mutations of the RAR beta 2 gene promoter RA response element (RARE) the stimulation by RARs showed distinct differences in the absence and presence of RXR. The same differences in transcriptional activity are observed, if mammalian cells containing endogenous RARs and RXRs are stimulated with all-trans RA only or additionally with 9-cis RA. This establishes an RXR-independent and an RXR-dependent pathway of all-trans RA action in Drosophila SL-3 cells as well as in mammalian cells. The presence or absence of 9-cis RA determines by which of the two pathways a response to all-trans RA is mediated.

A retinoic acid receptor alpha antagonist selectively counteracts retinoic acid effects

Retinoic acid (RA) exerts its pleiotropic effects on cell growth and differentiation through the activation of a family of transcription factors-the RA receptors (RARs). Three subtypes of these receptors exist, RAR alpha, RAR beta, and RAR gamma. The receptors are differentially expressed in different cell types and stages of development, suggesting that they may regulate different sets of genes. We have identified a synthetic retinoid with the characteristics of a selective RAR alpha antagonist. This antagonist counteracts RA effects on HL-60 cell differentiation and on B-lymphocyte polyclonal activation. Beyond its potential practical relevance, this and other specific antagonists will be useful to dissect the RAR system and to assign to one given receptor each of the many RA-regulated functions.

The specificities of Sex combs reduced and Antennapedia are defined by a distinct portion of each protein that includes the homeodomain

The sequence requirements for distinguishing the functional specificities of two homeodomain proteins, Antennapedia and Sex combs reduced, involved in the specification of segmental identities in Drosophila, have been determined. A series of deletions and hybrid proteins was generated and assayed for their function in vivo after heat shock-induced ectopic expression during development. A distinct portion of each protein, including the residues within and adjacent to both ends of the homeodomain, has been found to almost entirely determine its functional specificity as measured by diagnostic cuticular transformations of embryonic and adult head structures. The remaining sequences contribute to the potency with which the proteins act in different cells and are to a limited extent functionally transferable from one protein to the other.