**5. Concluding Remarks**

Ricin can be considered as a powerful tool to study intracellular pathways in general and cell death mechanisms that include apoptosis, inflammation or cell stress-induced signaling. On the other hand, exact knowledge about ricin action in the target cells is necessary to produce e ffectively working ricin-based immunotoxins or vaccines. One of the most interesting discoveries that has been made recently describes a vital sugar code for ricin toxicity, that is conserved from mouse to human [100]. This mechanism is based on defined glycosidic structures that determine cellular fate upon exposure to the toxin. The question of whether depurination of rRNA is necessary for ricin-induced cell death is still being discussed. It is believed that in addition to rRNA damage, ricin can induce apoptosis, inflammation and DNA damage. The correlation between these processes has been intensively studied. This knowledge is constantly being expanded as the huge contribution to this field has been made over the past years. An old dogma about ricin has currently been investigated. It was believed that a single A-chain molecule of ricin or other type-2 RIPs have the ability to kill one eukaryotic cell [20]. However, it was recently reported that one or a few molecules of ricin A-chain present in the cytosol is not su fficient to inhibit protein synthesis [302]. Moreover, cells with a partial inhibition of protein synthesis can, upon ricin removal, increase the level of protein production and survive the toxin challenge. Thus, in contrast to the previously accepted model, ongoing toxin delivery to the cytosol appears to be necessary for the death of cells exposed to sub-optimal ricin concentrations [302]. It was also suggested that ricin and other RIPs can be more toxic to cancer cells than to normal cells, due to the

higher rate of protein synthesis in malignant cells during proliferation or due to the changes in receptor concentration on their surfaces or altered intracellular transport of the toxin [30,42,303–305]. This is, however, not always the case (for review see e.g., Refs. [30,285]). Thus, for specific delivery of ricin to cancer cells, directing ricin to particular epitopes on tumor cells is necessary. On the other hand, some specific properties of ricin may enhance its effect on cancer cells. The ability of RTA to inhibit UPR may make it more potent in targeted therapy for cancer. It has been demonstrated that an increased level of spliced *XBP1* relatively to unspliced *XBP1* correlates with poor prognosis in breast cancer [306], and XBP1 has been proposed as a therapeutic target for solid tumors [307]. Thus, ricin treatment may be particularly useful in cancer cells where UPR is already accelerated by conditions such as hypoxia. These findings highlight the role of ricin as a valuable component of modern immunotoxins.

**Funding:** The work referred to as from the Słomi ´nska-Wojewódzka group was supported by the National Science Centre Poland gran<sup>t</sup> 2015/19/B/NZ3/03266. Work with toxins in the group of Sandvig is supported by the Norwegian Cancer Society and the Southern and Eastern Norway Regional Health Authority.

**Conflicts of Interest:** The authors declare no conflict of interest.
