Ricin can be isolated from the seeds of the castor bean plant (Ricinus communis). It belongs to the ribosome-inactivating protein (RIP) family of toxins classified as a bio-threat agent due to its high toxicity, stability and availability. Ricin is a typical A-B toxin consisting of a single enzymatic A subunit (RTA) and a binding B subunit (RTB) joined by a single disulfide bond. RTA possesses an RNA N-glycosidase activity; it cleaves ribosomal RNA leading to the inhibition of protein synthesis. However, the mechanism of ricin-mediated cell death is quite complex, as a growing number of studies demonstrate that the inhibition of protein synthesis is not always correlated with long term ricin toxicity. To exert its cytotoxic effect, ricin A-chain has to be transported to the cytosol of the host cell. This translocation is preceded by endocytic uptake of the toxin and retrograde traffic through the trans-Golgi network (TGN) and the endoplasmic reticulum (ER). In this article, we describe intracellular trafficking of ricin with particular emphasis on host cell factors that facilitate this transport and contribute to ricin cytotoxicity in mammalian and yeast cells. The current understanding of the mechanisms of ricin-mediated cell death is discussed as well. We also comment on recent reports presenting medical applications for ricin and progress associated with the development of vaccines against this toxin.

The plant toxin ricin is transported retrogradely from the cell surface to the endoplasmic reticulum (ER) from where the enzymatically active part is retrotranslocated to the cytosol, presumably by the same mechanism as used by misfolded proteins. The ER degradation enhancing α-mannosidase I-like protein, EDEM, is responsible for directing aberrant proteins for ER-associated protein degradation. In this study, we have investigated whether EDEM is involved in ricin retrotranslocation. Overexpression of EDEM strongly protects against ricin. However, when the interaction between EDEM and misfolded proteins is inhibited by kifunensin, EDEM promotes retrotranslocation of ricin from the ER to the cytosol. Furthermore, puromycin, which inhibits synthesis and thereby transport of proteins into the ER, counteracted the protection seen in EDEM-transfected cells. Coimmunoprecipitation studies revealed that ricin can interact with EDEM and with Sec61α, and both kifunensin and puromycin increase these interactions. Importantly, vector-based RNA interference against EDEM, which leads to reduction of the cellular level of EDEM, decreased retrotranslocation of ricin A-chain to the cytosol. In conclusion, our results indicate that EDEM is involved in retrotranslocation of ricin from the ER to the cytosol.

Purpose Tamoxifen has been used for the treatment of estrogen receptor (ER)-positive breast cancers and in women who are at an increased risk of breast cancer. Acquired resistance to this drug and its toxicity still pose a clinically significant problem, especially in the prevention setting. Isothiocyanates present in cruciferous plants, such as sulforaphane or erucin, have been shown to reduce growth of breast cancer cells in vivo and in vitro. In this study, we explored their ability to sensitize cancer cells to 4-hydroxytamoxifen. Methods We used three ER-positive breast cancer cell lines, T47D, MCF-7 and BT-474, as well as the drug-resistant T47D and MCF-7 derivatives. We examined the effect of 4-hydroxytamoxifen, isothiocyanates and their combinations on cell viability by MTT and clonogenic assays. Impact of treatments on the levels of proteins engaged in apoptosis and autophagy was determined by Western blotting. Results Isothiocyanates act in a synergistic way with 4-hydroxytamoxifen, and co-treatment reduces breast cancer cell viability and clonogenic potential more effectively than treatment with any single agent. This is connected with a drop in the Bcl-2/Bax ratio and the level of survivin as well as increased PARP cleavage, and elevation in ADRP, the mitochondrial stress marker. Moreover, isothiocyanates sensitize 4-hydroxytamoxifen-resistant T47D and MCF-7 cells to the drug. Conclusion Isothiocyanates enhance response to 4-hydroxytamoxifen, which allows for reduction of the effective drug concentration. Combinatorial strategy may hold promise in development of therapies and chemoprevention strategies against ER-positive breast tumors, even those with acquired resistance to the drug.

Mucopolysaccharidoses (MPS) are inherited metabolic disorders caused by deficiencies in enzymes involved in degradation of glycosaminoglycans. MPS type III (Sanfilippo disease) is clinically characterized mainly by progressive and severe behavioral disturbances and cognitive dysfunction. Recent 1-year experimental treatment of 10 patients with a genistein (4′, 5, 7-trihydroxyisoflavone)-rich extract resulted in improvement of tested parameters, including cognitive and behavioral functions. Eight pediatric patients with Sanfilippo disease were enrolled into the study. The modified version of the Brief Assessment Examination was used to assess cognitive functions. Moreover, 18 different parameters concerning changes in conditions of patients were assessed by their parents. During the first year of the treatment, an improvement of cognitive functions in 7 patients and stabilization in 1 patient were assessed, while after the third year (2-year follow-up) further improvement was observed in 2 patients, stabilization in 3 patients and some deterioration in 3 patients. Monitoring of general and behavioral symptoms revealed improvement in all patients after the first year of the treatment, further improvement in 5 patients, and deterioration in 3 patients during the next 2 years. We conclude that the treatment of Sanfilippo patients with a genistein-rich soy isoflavone extract (called gene expression-targeted isoflavone therapy [GET IT]) may be effective in either inhibition (in some patients) or slowing down (in other patients) of behavioral and cognitive problems over a longer period. An increased dose of genistein may improve the efficacy of the treatment.

Several bacterial and plant AB-toxins are delivered by retrograde vesicular transport to the endoplasmic reticulum (ER), where the enzymatically active A subunit is disassembled from the holotoxin and transported to the cytosol. In this process, toxins subvert the ER-associated degradation (ERAD) pathway. ERAD is an important part of cellular regulatory mechanism that targets misfolded proteins to the ER channels, prior to their retrotranslocation to the cytosol, ubiquitination and subsequent degradation by a protein-degrading complex, the proteasome. In this article, we present an overview of current understanding of the ERAD-dependent transport of AB-toxins to the cytosol. We describe important components of ERAD and discuss their significance for toxin transport. Toxin recognition and disassembly in the ER, transport through ER translocons and finally cytosolic events that instead of overall proteasomal degradation provide proper folding and cytotoxic activity of AB-toxins are discussed as well. We also comment on recent reports presenting medical applications for toxin transport through the ER channels.