Retinoblastoma vulnerability to combined de novo and salvage pyrimidine ribonucleotide synthesis pharmacologic blockage.

Retinoblastoma is an eye cancer that commonly affects young children. Despite significant advances, current treatments cause side effects even when administered locally, and patients may still have to undergo enucleation. This is particularly disheartening in cases of bilateral retinoblastoma. Hence, there is an urgent need for novel therapeutic strategies. Inhibitors of the enzyme dihydroorotate dehydrogenase (DHODH), which is involved in the de novo pyrimidine ribonucleotide synthesis pathway, have proven to be effective in preclinical trials against several cancers including pediatric cancers. Here we tested whether blocking pyrimidine ribonucleotide synthesis promotes retinoblastoma cell death. Cultured retinoblastoma cell lines were treated with small molecule inhibitors of DHODH alone or in combination with inhibitors of nucleoside uptake to also block the salvage pathway for pyrimidine ribonucleotide formation. On their own, DHODH inhibitors had a moderate killing effect. However, the combination with nucleoside uptake inhibitors greatly enhanced the effect of DHODH inhibition. In addition, we observed that pyrimidine ribonucleotide synthesis blockage can cause cell death in a p53 mutant retinoblastoma cell line derived from a patient with metastasis. Explaining these results, the analysis of a published patient cohort revealed that loss of chr16q22.2 (containing the DHODH gene) is amongst the most frequent alterations in retinoblastoma and that these tumors often show gains in chromosome regions expressing pyrimidine ribonucleotide salvage factors. Furthermore, these genome alterations associate with malignancy. These results indicate that targeting pyrimidine ribonucleotide synthesis may be an effective therapeutic strategy to consider as a treatment for retinoblastoma.

Modulating pyrimidine ribonucleotide levels for the treatment of cancer.

By providing the necessary building blocks for nucleic acids and precursors for cell membrane synthesis, pyrimidine ribonucleotides are essential for cell growth and proliferation. Therefore, depleting pyrimidine ribonucleotide pools has long been considered as a strategy to reduce cancer cell growth. Here, we review the pharmacological approaches that have been employed to modulate pyrimidine ribonucleotide synthesis and degradation routes and discuss their potential use in cancer therapy. New developments in the treatment of myeloid malignancies with inhibitors of pyrimidine ribonucleotide synthesis justify revisiting the literature as well as discussing whether targeting this metabolic pathway can be effective and sufficiently selective for cancer cells to warrant an acceptable therapeutic index in patients.

Publisher Correction: A DHODH inhibitor increases p53 synthesis and enhances tumor cell killing by p53 degradation blockage.

The original PDF version of this Article listed the authors as "Marcus J.G.W. Ladds," where it should have read "Marcus J. G. W. Ladds, Ingeborg M. M. van Leeuwen, Catherine J. Drummond et al.#".Also in the PDF version, it was incorrectly stated that "Correspondence and requests for materials should be addressed to S. Lín.", instead of the correct "Correspondence and requests for materials should be addressed to S. Laín."This has been corrected in the PDF version of the Article. The HTML version was correct from the time of publication.

A DHODH inhibitor increases p53 synthesis and enhances tumor cell killing by p53 degradation blockage.

The development of non-genotoxic therapies that activate wild-type p53 in tumors is of great interest since the discovery of p53 as a tumor suppressor. Here we report the identification of over 100 small-molecules activating p53 in cells. We elucidate the mechanism of action of a chiral tetrahydroindazole (HZ00), and through target deconvolution, we deduce that its active enantiomer (R)-HZ00, inhibits dihydroorotate dehydrogenase (DHODH). The chiral specificity of HZ05, a more potent analog, is revealed by the crystal structure of the (R)-HZ05/DHODH complex. Twelve other DHODH inhibitor chemotypes are detailed among the p53 activators, which identifies DHODH as a frequent target for structurally diverse compounds. We observe that HZ compounds accumulate cancer cells in S-phase, increase p53 synthesis, and synergize with an inhibitor of p53 degradation to reduce tumor growth in vivo. We, therefore, propose a strategy to promote cancer cell killing by p53 instead of its reversible cell cycle arresting effect.

Human neural progenitor cells decrease photoreceptor degeneration, normalize opsin distribution and support synapse structure in cultured porcine retina.

Retinal neurodegenerative disorders like retinitis pigmentosa, age-related macular degeneration, diabetic retinopathy and retinal detachment decrease retinal functionality leading to visual impairment. The pathological events are characterized by photoreceptor degeneration, synaptic disassembly, remodeling of postsynaptic neurons and activation of glial cells. Despite intense research, no effective treatment has been found for these disorders. The current study explores the potential of human neural progenitor cell (hNPC) derived factors to slow the degenerative processes in adult porcine retinal explants. Retinas were cultured for 3 days with or without hNPCs as a feeder layer and investigated by terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL), immunohistochemical, western blot and quantitative real time-polymerase chain reaction (qRT-PCR) techniques. TUNEL showed that hNPCs had the capacity to limit photoreceptor cell death. Among cone photoreceptors, hNPC coculture resulted in better maintenance of cone outer segments and reduced opsin mislocalization. Additionally, maintained synaptic structural integrity and preservation of second order calbindin positive horizontal cells was also observed. However, Müller cell gliosis only seemed to be alleviated in terms of reduced Müller cell density. Our observations indicate that at 3 days of coculture, hNPC derived factors had the capacity to protect photoreceptors, maintain synaptic integrity and support horizontal cell survival. Human neural progenitor cell applied treatment modalities may be an effective strategy to help maintain retinal functionality in neurodegenerative pathologies. Whether hNPCs can independently hinder Müller cell gliosis by utilizing higher concentrations or by combination with other pharmacological agents still needs to be determined.