Strophasterols A to D with an Unprecedented Steroid Skeleton: From the Mushroom Stropharia rugosoannulata

Abstract

The mushroom Stropharia rugosoannulata is called saketsubatake in Japanese, and wine-cap stropharia in English. It belongs to the family Strophariaceae, which is widespread in northern temperate zones throughout the world. It is edible and is cultivated for food. During screening for anti-endoplasmic-reticulum (ER) stress and anti-methicillin-resistant Staphylococcus aureus (MRSA) effects of the extracts of various mushrooms, we found activity in the extract of this mushroom. ER stress induces apoptotic pathways with signaling between the ER and mitochondria. By triggering apoptosis on neural cells, the stress is a major cause of degenerative diseases such as Alzheimers disease. [1,2] MRSA has developed resistance to most antibiotics and is one of the most prevalent pathogens in nosocomial infections. Therefore, anti-ER-stress and anti-MRSA substances are urgently required. Recently we reported that several active steroids were isolated from this mushroom. [3] In further search for bioactive compounds from the mushroom, we discovered four novel steroids having a very unique and unprecedented carbon skeleton. Herein, we describe the isolation, structure determination, and biological activity of the compounds from the mushroom.Fresh fruiting bodies of S. rugosoannulata were extracted with EtOH and then with acetone. After the solutions were combined and concentrated, they were partitioned between n-hexane and H 2 O, CHCl 3 and H 2 O, and then EtOAc and H 2 O. The hexane-soluble residue was fractionated by repeated chromatography. As a result, four novel compounds (1-4), which were named strophasterols A, B, C, and D, were isolated (Scheme 1).Strophasterol A (1) was obtained as white crystals. Its molecular formula was determined to be C 28 H 44 O 4 by HRESIMS with m/z 467.3100 [M+Na] + (calcd for C 28 H 44 NaO 4 467.3137), thus indicating the presence of seven degrees of unsaturation in the molecule. The planar structure of 1 was elucidated by interpretation of the NMR spectra, including DEPT, COSY, HMBC, and HMQC data. The DEPT experiment indicated the presence of six methyl, eight methylene, and eight methane groups, as well as six quaternary carbon atoms. In the NMR spectra of 1, typical signals of a sterol corresponding to two hydroxymethines [C3: d H = 3.94 ppm (m), d C = 68.2 ppm; C6: d H = 4.84 ppm (m), d C = 63.0 ppm], four doublet methyls [C2'CH 3 : d H = 0.97 ppm (d, J = 6.7 Hz), d C = 20.8 ppm; C4''CH 3 : d H = 0.72 ppm (d, J = 6.7 Hz), d C = 16.4 ppm; C2''CH 3 : d H = 0.74 ppm (d, J = 6.7 Hz), d C = 15.6 ppm; C3''CH 3 : d H = 0.83 ppm (d, J = Scheme 1. Structures of strophastrerols A-

Synthesis of Strophasterol A Guided by a Proposed Biosynthesis and Innate Reactivity

“…[3] Thep resenilins (6-8 transmembrane proteins) are mainly localized to the endoplasmic reticulum, where they are part of the g-secretase protein complex that cleaves APP to generate Ab peptides.T he resulting amyloidogenic peptides can damage neurons by inducing membrane-associated oxidative stress,t hereby rendering neurons vulnerable to Ca 2+ overload, particularly under conditions of impaired mitochondrial energy production. [6] While the structures of the former two (strophasterols Aand B, 1A and 1B)h ave been unequivocally determined by singlecrystal X-ray diffraction, [7] thes tructures of strophasterols C and D( 1C and 1D)r emain uncertain with regard to their relative and absolute configuration. [5] Strophasterols A-D (1A-D;F igure 1), four secosterols with an unprecedented carbon skeleton, have recently been isolated from the mushroom Stropharia rugosoannulata, which is abundant in the northern temperate zone.…”

An unusual sterol from the mushroom Stropharia rugosoannulata

“…2 In further research, we discovered two novel sterols. Herein, we describe the isolation and structure determination of the compounds from the mushroom.…”

Terpenoids and Sterols from Mushrooms

“…Wu et al [61,62] reported the isolation of four new sterols having a very unique and unprecedented carbon skeleton, strophasterols A-D (95-98), from the fruiting bodies of Stropharia rugosoannulata (Fig. 1.15).…”

“…1.15). In a protective activity assay against ER-stress-dependent cell death caused by tunicamycin or thapsigargin, none of the compounds showed any inhibitory effect on tunicamycin toxicity, but compound 95 showed a dose-dependent inhibitory effect on thapsigargin toxicity (increase in cell viability compared with control, 10.3%) [61]. Compounds 95-98 were evaluated by endoplasmic reticulum (ER)-stress suppression and an anti-methicillinresistant Staphylococcus aureus (MRSA) assay.…”

“…The stereochemistry of 97 and 98 remains undetermined. In addition, compound 95 showed weak anti-MRSA activity [61]. In a protective activity assay against ER-stress-dependent cell death caused by tunicamycin or thapsigargin, none of the compounds showed any inhibitory effect on tunicamycin toxicity, but compound 95 showed a dose-dependent inhibitory effect on thapsigargin toxicity (increase in cell viability compared with control, 10.3%) [61].…”