Field Survey of Glycyrrhiza Plants in Central Asia (5). Chemical Characterization of G. bucharica Collected in Tajikistan.

One triterpene and five triterpene glycosides, including four new compounds, have been identified in the underground parts of Glycyrrhiza bucharica, which was shown to be closely related to Glycyrrhizin-producing Glycyrrhiza species, G. uralensis, G. glabra and G. inflata, based on their chloroplast rbcL sequences. Two known compounds were identified squasapogenol and macedonoside C. The structures of four new compounds, bucharosides A, B, C, and D, were determined to be 3-O-α-L-rhamnopyranosyl-(1→2)-ß-D-glucuronopyranosyl-(1→2)-ß-D-glucuronopyranosyl-22-O-α-L-rhamnopyranosyl squasapogenol, 3-O-α-L-rhamnopyranosyl-(1→2)-ß-D-glucuronopyranosyl-(1→2)-ß-D-glucuronopyranosyl-macedonic acid, 3-O-α-L-rhamnopyranosyl-(1→2)-ß-D-glucuronopyranosyl-(1→2)-ß-D-glucuronopyranosyl-squasapogenol, and 22-O-α-L-rhamnopyranosyl squasapogenol, respectively. Contents of these triterpene glycosides were less than 0.5% of dry weight, and no main saponin, like glycyrrhizin or macedonoside C found in other Glycyrrhiza species, was found in the underground parts of G. bucharica.

Field Survey of Glycyrrhiza Plants in Central Asia (4). Characterization of G. glabra and G. bucharica Collected in Tajikistan.

The characteristics of 2 Glycyrrhiza plants, G. glabra and G. bucharica (=Meristotropis bucharica), were investigated in Tajikistan. The glycyrrhizin content in the underground parts of G. glabra varied from 2.56 to 9.29% of the dry weight, and the content of glabridin, a species-specific flavonoid of G. glabra, varied from 0.09 to 0.92% of the dry weight. Seeds of G. glabra plants from Tajikistan were cultivated for 3 years in Japan, and the glycyrrhizin content of the harvested roots ranged from 0.75 to 1.82% of the dry weight. In addition, HPLC analysis of leaf extracts indicated that the G. glabra plants collected in Tajikistan could be divided into various types, according to the flavonoid contents of the leaves. The endemic G. bucharica was also collected. A phylogenetic tree of rbcL nucleotide sequences from various Glycyrrhiza plants indicated that G. bucharica was closely related to the three glycyrrhizin-producing Glycyrrhiza spp. (G. uralensis, G. inflata, and G. glabra), even though G. bucharica does not produce glycyrrhizin.

α-Tocopherol suppresses 24(S)-hydroxycholesterol-induced cell death via inhibition of endoplasmic reticulum membrane disruption.

The brain-specific cholesterol metabolite 24(S)-hydroxycholesterol (24S-OHC) has been shown to cause neuronal cell death when subjected to esterification by acyl-CoA:cholesterol acyltransferase 1 (ACAT1). Accumulating 24S-OHC esters in the endoplasmic reticulum (ER) provoked ER membrane disruption and an integrated stress response (ISR), a signaling pathway that regulates adaptation to various stresses. We have previously reported that α-tocopherol (α-Toc) but not α-tocotrienol (α-Toc3), among vitamin E homologs, suppressed 24S-OHC-induced cell death without affecting ACAT1 activity in human neuroblastoma SH-SY5Y cells. However, the precise mechanisms underlying the inhibitory activity of α-Toc have yet to be elucidated. In the present study, we aimed to investigate the effects of α-Toc on the 24S-OHC-induced cell death machinery. We showed that α-Toc, but not α Toc3, suppressed 24S-OHC-induced ISR and downstream eukaryotic translation initiator factor 2α (eIF2α) phosphorylation. We also found that α-Toc inhibited stress granule formation and robust downregulation of nascent protein synthesis, which were induced by 24S-OHC treatment. Furthermore, disruption of ER membrane integrity was suppressed by α-Toc, but not by α-Toc3. Our findings suggest that the inhibitory effects of α-Toc on 24S-OHC-induced cell death may be attributed to its protective function against ER membrane disruption.

Integrated stress response is involved in the 24(S)-hydroxycholesterol-induced unconventional cell death mechanism.

Perturbation of proteostasis triggers the adaptive responses that contribute to the homeostatic pro-survival response, whereas disruption of proteostasis can ultimately lead to cell death. Brain-specific oxysterol-i.e., 24(S)-hydroxycholesterol (24S-OHC)-has been shown to cause cytotoxicity when esterified by acyl-CoA:cholesterol acyltransferase 1 (ACAT1) in the endoplasmic reticulum (ER). Here, we show that the accumulation of 24S-OHC esters caused phosphorylation of eukaryotic translation initiator factor 2α (eIF2α), dissociation of polysomes, and formation of stress granules (SG), resulting in robust downregulation of global protein de novo synthesis in human neuroblastoma SH-SY5Y cells. We also found that integrated stress response (ISR) activation through PERK and GCN2 activation induced by 24S-OHC treatment caused eIF2α phosphorylation. 24S-OHC-inducible SG formation and cell death were suppressed by inhibition of ISR. These results show that ACAT1-mediated 24S-OHC esterification induced ISR and formation of SG, which play crucial roles in 24S-OHC-inducible protein synthesis inhibition and unconventional cell death.