Silybins as multi-target proteostasis rescuers in Alzheimer’s Disease and Type II Diabetes Mellitus

A defective protein folding and its abnormal aggregation and accumulation is the central pathogenic event in Protein Conformational diseases (PCDs), such as Alzheimer`s Disease (AD) and Type II Diabetes Mellitus (T2DM), which are characterized by the accumulation of the amyloid-β (Aβ) peptide in neuronal tissues and islet amyloid polypeptide (IAPP) in pancreatic β-cells, respectively. So far, all the designed anti-aggregating drugs (amyloid hypothesis) have failed in clinical trials and the latest studies suggest that pathological conditions occur when the equilibrium between production and clearance of the involved protein results unbalanced. For this reason, currently, new drugs are designed to restore the proteostasis network (PN) targeting any of its components and not just simple anti-aggregating compounds. However, due to the multifactorial nature of protein diseases, it may be difficult to find effective drugs by screening molecules on a single target, and very few molecules, if any, selected by using this strategy, have shown to have the potential to be pipelined to clinical trials. Due to the significant interconnection between AD and T2DM which share common pathogenic mechanisms, we have proposed here a new, multi-target strategy against both diseases, to profile small molecules for their potential to be used as Aβ/hIAPP proteostasis rescuers. In particular, we have investigated the cytoprotective effect of two natural compounds (Silybin A and Silybin B) extracted from the plant Silybum marianum. Both compounds have shown to halt and rescue the cytotoxicity of Aβ oligomers in differentiated SH-SY5Y neuroblastoma cells, a neuron-like cell system. The use of several spectroscopic, biophysical, biochemical, and computational techniques has allowed us to establish with high-level of accuracy the molecular reasons that entail their cytoprotective effect and the important role of the stereochemistry in this processes; in particular, silybins may i) inhibit Aβ/hIAPP aggregation; ii) counteract amyloid-mediated membrane disruption, iii) enhance 20S proteasome activity, iv) foster ubiquitin signaling and v) protect from oxidative stress. However, despite the high potential of silybins as anti-AD and -T2DM drugs, their use is limited due to their scarce solubility and bioavailability. We have overcome these limitations by conjugating silybins with a trehalose moiety through a phosphate linker, following a pro-drug approach. This approach allows the administration of higher amounts of the pro-drug and the progressively release of the parental compound on a time scale of days. In addition, the synthesized pro-drugs are not devoiced of activity but, rather, partially maintain or, in some cases, even increase the ability of silybins against the studied targets. The activity of the pro-drugs represents an additional advantage, addressing not only the solubility related problem and permitting the delivery of the drug, but furthermore, being a part of the whole drug system where both drug and pro-drug present desirable activities in restoring proteostasis.