Edible mushrooms consumption has increased worldwide in the last decades mainly because, together with the Asiatic traditional large acceptance, Western consumers are now also integrating them in their usual menu. The extensive understanding that the scientific community is reaching about their composition and their potential benefits on human health is probably behind this trend (El-Ramady et al., 2022; Roncero-Ramos & Delgado-Andrade, 2017).
Among the large number of species, shiitake mushroom (Lentinula edodes) deserves a special mention because of its significant levels of bioactive molecules such as ergosterol, β-glucans, chitins, peptides or phenolics and also specific compounds such as eritadenine or lenthionine (Fukushima-Sakuno, 2020; Morales, Piris, et al., 2018; Morales, Tabernero, et al., 2018). These bioactives were related to important biological activities such as hypocholesterolemic, immune-modulatory, antihypertensive, antioxidant, antiproliferative properties etc. tested not only in vitro but also in animal and clinical studies (Dai et al., 2015; Morales, Smiderle, Villalva, et al., 2019; Morales, Tejedor-Calvo, Jurado-Chivato, et al., 2019; Wong et al., 2020).
Moreover, vitamin D2 (ergocalciferol) can be found in shiitake and other species although in highly variable contents in wild mushrooms and extremely low or undetectable levels in cultivated mushrooms (Mattila et al., 2002; Teichmann et al., 2007). However, these levels can be enhanced by UV-light action since fungal ergosterol, the main sterol of hyphal membranes, is photoconverted into vitamin D2. This transformation can be stimulated by direct exposure of fruiting bodies but also by irradiating specific extracts or fractions (Morales et al., 2017; Taofiq et al., 2017) and this procedure might be crucial to offer mushrooms or fungal products as an alternative vitamin D2 source considering the deficient levels noticed nowadays in Western population. Vitamin D plays a key role in many metabolic pathways, being involved not only in calcium homeostasis and bone system regulation, but also in immune-system modulation, neuronal and cardiovascular health, besides other biological functions (Janoušek et al., 2022). Its deficiency, among other disorders, is linked to infectious and autoimmune diseases since it is strongly correlated with immune cells, being expressed in monocytes/macrophages, T, B and dentritic cells together with 1α-hydroxylase, an enzyme that activates vitamin D (Ao et al., 2021). Furthermore, low levels of this molecule reduce vitamin D receptor activity, triggering an increase in the sterol regulatory element-binding protein 2 (SREBP2) activity and the 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMHCR) expression, with the consequent rise in cholesterol biosynthesis rate (Li et al., 2016).
Furthermore, chitins are present in fungal cell walls (approximately 2–10% in edible mushrooms) and these polysaccharides, considered as dietary fibres, can be subjected to chemical deacetylation (usually applying hot alkaline procedures) to obtain chitosan, another interesting polymer because of its hypocholesterolemic activities particularly when a high deacetylation degree is achieved. The positive charges of chitosan promote bile acids scavenging stimulating cholesterol transformation to restore their levels. Moreover, chitosan and other dietary fibres can also increase intestinal viscosity impairing cholesterol absorption (Silva et al., 2021). Regarding chitosan immune-modulating ability, it has shown potent immune-stimulatory effects by activating antigen-presenting cells and inducing cytokines secretion (Li et al., 2021) Although chitosan presence has been described in mushrooms, its content was significantly lower than chitin concentrations, particularly in shiitake mushrooms, being even undetectable in many cases (Chien et al., 2016; John Kasongo et al., 2020; Morales et al., 2019).
In a previous work, a dietary fibre extract containing shiitake β-glucans, chitins and ergosterol reduced the cholesterol levels in mice serum after 9 weeks of oral administration (Morales, Tejedor-Calvo, et al., 2019). However, when a clinical study was conducted on hypercholesterolemic subjects, no significant reduction of total or LDL-cholesterol levels was observed when compared to a placebo group after 8 weeks intaking. Moreover, no changes were noticed on the secretion of specific cytokines (IL1-β, IL-6, TNF-α). The extract only modulated the gut microbiota promoting the growth of beneficial microorganisms (Morales et al., 2021). Therefore, in this work, a procedure to convert part of the ergosterol into vitamin D2 and chitins into chitosan in shiitake fractions was explored using UV-irradiation and alkaline deacetylation as an attempt to increase the effectivity of the extract as hypocholesterolemic and immune-modulatory functional ingredients.