Negation of the cancer-preventive actions of selenium by over-expression of protein kinase Cepsilon and selenoprotein thioredoxin reductase

Identification of PRDX5 as A Target for The Treatment of Castration-Resistant Prostate Cancer

Treatment of castration-resistant prostate cancer (CRPC) is a long-standing clinical challenge. Traditionally, CRPC drugs work by either reducing dihydrotestosterone biosynthesis or blocking androgen receptor (AR) signaling. Here it is demonstrated that AR inhibitor treatment gives rise to a drug-tolerant persister (DTP) state. The thioredoxin/peroxiredoxin pathway is up-regulated in DTP cells. Peroxiredoxin 5 (PRDX5) promotes AR inhibitor resistance and CRPC development. Inhibition of PRDX5 suppresses DTP cell proliferation in culture, dampens CRPC development in animal models, and stabilizes PSA progression and metastatic lesions in patients. Therefore, the study provides a novel mechanism and potential target for the management of castration-resistant prostate cancer.

Selenoprotein P as Biomarker of Selenium Status in Clinical Trials with Therapeutic Dosages of Selenite

Selenoprotein P (SELENOP) is an established biomarker of selenium (Se) status. Serum SELENOP becomes saturated with increasing Se intake, reaching maximal concentrations of 5–7 mg SELENOP/L at intakes of ca. 100–150 µg Se/d. A biomarker for higher Se intake is missing. We hypothesized that SELENOP may also reflect Se status in clinical applications of therapeutic dosages of selenite. To this end, blood samples from two supplementation studies employing intravenous application of selenite at dosages >1 mg/d were analyzed. Total Se was quantified by spectroscopy, and SELENOP by a validated ELISA. The high dosage selenite infusions increased SELENOP in parallel to elevated Se concentrations relatively fast to final values partly exceeding 10 mg SELENOP/L. Age or sex were not related to the SELENOP increase. Western blot analyses of SELENOP verified the results obtained by ELISA, and indicated an unchanged pattern of immunoreactive protein isoforms. We conclude that the saturation of SELENOP concentrations observed in prior studies with moderate Se dosages (<400 µg/d) may reflect an intermediate plateau of expression, rather than an absolute upper limit. Circulating SELENOP seems to be a suitable biomarker for therapeutic applications of selenite exceeding the recommended upper intake levels. Whether SELENOP is also capable of reflecting other supplemental selenocompounds in high dosage therapeutic applications remains to be investigated.

Redox regulation of protein kinase C by selenometabolites and selenoprotein thioredoxin reductase limits cancer prevention by selenium

The cancer-preventive mechanism of selenium should address the way low concentrations of selenometabolites react with cellular targets without being diffused from the sites of generation, the way selenium selectively kills tumor cells, and the intriguing U-shaped curve that is seen with dietary supplementation of selenium and cancer prevention. Protein kinase C (PKC), a receptor for tumor promoters, is well suited for this mechanism. Due to the catalytic redox cycle, low concentrations of methylselenol, a postulated active metabolite of selenium, react with the tumor-promoting lipid hydroperoxide bound to PKC to form methylseleninic acid (MSA), which selectively reacts with thiol residues present within the vicinity of the PKC catalytic domain to inactivate it. Given that lipid hydroperoxide levels are high in promoting cells, PKC inactivation selectively leads to death in these cells. A biphasic effect of MSA in inducing cell death was observed in certain prostate cancer cell lines; lower concentrations of MSA induced cell death, while higher concentrations failed to do so. Lower concentrations of selenium inactivate more sensitive antiapoptotic isoenzymes of PKC (ε and α), sparing less sensitive proapoptotic isoenzymes (PKCδ and PKCζ). Higher concentrations of selenium also inactivate proapoptotic isoenzymes and consequently make tumor cells resistant to apoptosis. Due to a high-affinity binding of thioredoxin to the PKC catalytic domain, this thiol oxidation is explicitly reversed by thioredoxin reductase (TXNRD), a selenoprotein. Therefore, overexpression of TXNRD in advanced tumor cells could make them resistant to selenium-induced death. Conceivably, this mechanism, at least in part, explains why selenium prevents cancer only in certain cases.

Cancer chemoprevention research with selenium in the post-SELECT era: Promises and challenges

The negative efficacy outcomes of double-blinded, randomized, placebo-controlled Phase III human clinical trials with selenomethionine (SeMet) and SeMet-rich selenized-yeast (Se-yeast) for prostate cancer prevention and Se-yeast for prevention of nonsmall cell lung cancer (NSCLC) in North America lead to rejection of SeMet/Se-yeast for cancer prevention in Se-adequate populations. We identify 2 major lessons from the outcomes of these trials: 1) the antioxidant hypothesis was tested in wrong subjects or patient populations, and 2) the selection of Se agents was not supported by cell culture and preclinical animal efficacy data as is common in drug development. We propose that next-generation forms of Se (next-gen Se), such as methylselenol precursors, offer biologically appropriate approaches for cancer chemoprevention but these are faced with formidable challenges. Solid mechanism-based preclinical efficacy assessments and comprehensive safety studies with next-gen Se will be essential to revitalize the idea of cancer chemoprevention with Se in the post-SELECT era. We advocate smaller mechanism-driven Phase I/II trials with these next-gen Se to guide and justify future decisions for definitive Phase III chemoprevention efficacy trials.

77Se and 125Te NMR spectroscopy on a selectivity study of organochalcogenanes with l-amino acids

Organochalcogenanes exhibited a remarkably high selectivity for l-cysteine which was monitored by ⁷⁷Se and ¹²⁵Te NMR spectroscopy.

Protein kinase C epsilon and genetic networks in osteosarcoma metastasis

Osteosarcoma (OS) is the most common primary malignant tumor of the bone, and pulmonary metastasis is the most frequent cause of OS mortality. The aim of this study was to discover and characterize genetic networks differentially expressed in metastatic OS. Expression profiling of OS tumors, and subsequent supervised network analysis, was performed to discover genetic networks differentially activated or organized in metastatic OS compared to localized OS. Broad trends among the profiles of metastatic tumors include aberrant activity of intracellular organization and translation networks, as well as disorganization of metabolic networks. The differentially activated PRKCε-RASGRP3-GNB2 network, which interacts with the disorganized DLG2 hub, was also found to be differentially expressed among OS cell lines with differing metastatic capacity in xenograft models. PRKCε transcript was more abundant in some metastatic OS tumors; however the difference was not significant overall. In functional studies, PRKCε was not found to be involved in migration of M132 OS cells, but its protein expression was induced in M112 OS cells following IGF-1 stimulation.

Susceptibility of the antioxidant selenoenyzmes thioredoxin reductase and glutathione peroxidase to alkylation-mediated inhibition by anticancer acylfulvenes

Selenium, in the form of selenocysteine, is a critical component of some major redox-regulating enzymes, including thioredoxin reductase (TrxR) and glutathione peroxidase (Gpx). TrxR has emerged as an anticancer target for drug development due to its elevated expression level in many aggressive human tumors. Acylfulvenes (AFs) are semisynthetic derivatives of the natural product illudin S and display improved cytotoxic selectivity profiles. AF and illudin S alkylate cellular macromolecules. Compared to AFs, illudin S more readily reacts with thiol-containing small molecules such as cysteine, glutathione, and cysteine-containing peptides. However, a previous study indicates that the reactivity of AFs and illudin S with glutathione reductase, a thiol-containing enzyme, is inversely correlated with the reactivity toward small molecule thiols. In this study, we investigate mechanistic aspects underlying the enzymatic and cellular effects of the AFs and illudin S on thioredoxin reductase. Both AF and HMAF were found to inhibit mammalian TrxR in the low- to submicromolar range, but illudin S was significantly less potent. TrxR inhibition by AFs was shown to be irreversible, concentration- and time-dependent, and mediated by alkylation of C-terminus active site Sec/Cys residues. In contrast, neither AFs nor illudin S inhibits Gpx, demonstrating that enzyme structure-specific small molecule interactions have a significant influence over the inherent reactivity of the Sec residue. In human cancer cells, TrxR activity can be inhibited by low micromolar concentrations of all three drugs. Finally, it was demonstrated that preconditioning cells by the addition of selenite to the cell culture media results in an enhancement in cell sensitivity toward AFs. These data suggest potential strategies for increasing drug activity by combination treatments that promote selenium enzyme activity.

Hypervalent organochalcogenanes as inhibitors of protein tyrosine phosphatases

A series of organochalcogenanes was synthesized and evaluated as protein tyrosine phosphatases (PTPs) inhibitors. The results indicate that organochalcogenanes inactivate the PTPs in a time- and concentration-dependent fashion, most likely through covalent modification of the active site sulfur-moiety by the chalcogen atom. Consequently, organochalcogenanes represent a new class of mechanism-based probes to modulate the PTP-mediated cellular processes.

Alteration of thioredoxin reductase 1 levels in elucidating cancer etiology

Thioredoxin reductase 1 (TR1) is a major antioxidant and redox regulator in mammalian cells and appears to function as a double-edged sword in that it has roles in preventing and promoting/sustaining cancer. TR1 is overexpressed in many cancer cells and targeting its removal often leads to a reversal in numerous malignant characteristics which has marked this selenoenzyme as a prime target for cancer therapy. Since alterations in TR1 activity may lead to a better understanding of the etiology of cancer and new avenues for providing better therapeutic procedures, we have described herein techniques for removing and reexpressing TR1 employing RNAi technology and for assessing the catalytic activity of this enzyme.