The entire plant of Morus alba L. (Family Moraceae), or mulberry, possesses medical benefits, including anticancer properties. In this study, we investigated the effect of mulberry leaf extracts on the human hepatoma HepG2 cell line, which is related to hepatocellular carcinoma. Mulberry leaf extracts were prepared using four solvents, each with different polarities: 100% methanol (MeOH), 50% aqueous MeOH, 1-butanol (BuOH), and hot water (W). The phenolic profile, total polyphenol content, antioxidant capacity, and effect on human hepatoma HepG2 cells of the leaf extracts were analyzed by examining cytotoxicity, cell cycle progression, apoptosis, expression of topoisomerase IIα, and proteins involved in cell cycle progression. High-performance liquid chromatography-mass spectrometry analysis revealed that 100% MeOH, 50% MeOH, and BuOH extracts contained rutin, isoquercetin, and various derivatives of kaempferol and quercetin glycosides as their major constituents; the W extract contained primarily chlorogenic acid and caffeoylquinic acid derivatives. Total phenolic content based on rutin equivalents was 17.1%, 9.6%, 8.3%, and 6.5% of dry 100% MeOH, 50% MeOH, BuOH, and W extracts, respectively. 2,2-Diphenyl-1-picrylhydrazyl radical scavenging activities were 70.0%, 45.8%, 41.0%, and 33.6%, and 50% inhibitory concentration values were 33.1, 79.4, 35.6, and 204.2 μg/mL for HepG2 cell proliferation inhibition for 100% MeOH, 50% MeOH, BuOH, and W extracts, respectively. MeOH extracts caused cell cycle G2/M arrest and induced the caspase cascade and apoptosis, but the W extract had very little effect on cell cycle progression. MeOH extracts reduced the level of topoisomerase IIα but increased the level of p27^Kip1, with no significant effect on p21^Cip1/waf1. Therefore, we concluded that phenolic-containing organic extracts of mulberry leaves inhibit the growth of HepG2 hepatoma cells through coordinated actions of inducing cell cycle arrest in the G2/M phase (with p27^Kip1 protein expression), inhibiting topoisomerase IIα activity, and inducing cell apoptosis by activation of caspases.

Two-pore channel 2 (TPC2) resides in endolysosomal membranes but also in lysosome-related organelles such as the melanin producing melanosomes. Gain-of-function polymorphisms in hTPC2 are associated with decreased melanin production and blond hair color. Vice versa genetic ablation of TPC2 increases melanin production. We show here an inverse correlation between melanin production and melanoma proliferation, migration, and invasion due to the dual activity of TPC2 in endolysosomes and melanosomes. Our results are supported by both genetic ablation and pharmacological inhibition of TPC2. Mechanistically, our data show that loss/block of TPC2 results in reduced protein levels of MITF, a major regulator of melanoma progression, but an increased activity of the melanin-generating enzyme tyrosinase. TPC2 inhibition thus provides a twofold benefit in melanoma prevention and treatment by increasing, through interference with tyrosinase activity, the synthesis of UV blocking melanin in melanosomes and by decreasing MITF-driven melanoma progression by increased GSK3β-mediated MITF degradation.

Twenty-seven flavonoids isolated from Dalbergia parviflora with vast structural diversity were screened for inhibitory activity against mushroom and murine tyrosinases using l-DOPA as the substrate. Among the flavonoids tested, only four—khrinone (5), cajanin (9), (3RS)-3′-hydroxy-8-methoxy vestitol (21), and (6aR,11aR)-3,8-dihydroxy-9-methoxy pterocarpan (27)—reacted with mushroom tyrosinase, with IC₅₀ values of 54.0, 67.9, 67.8, and 16.7 μM, respectively, and only compound 27 showed inhibitory activity against murine tyrosinase. With cell-based assays, only compounds 9 and 27 effectively inhibited melanogenesis in B16-F10 melanoma cells (by 34% and 59%, respectively), at a concentration of 15 μM, without being significantly toxic to the cells. However, the crude extract of D. parviflora and some of the flavonoid constituents appeared to increase melanin production in B16-F10 cells, suggesting that there are flavonoids with both inhibitory and stimulatory melanogenesis in the crude extract. Studies on the correlation between the enzyme-based and cell-based assays showed that only the flavonoids with IC₅₀ values below 50 μM against mushroom tyrosinase could inhibit the mammalian tyrosinase, and thus, reduce melanogenesis in B16-F10. Flavonoids with the IC₅₀ values greater than 50 μM, on the other hand, could not inhibit the mammalian tyrosinase, and had either no effect or enhancement of melanogenesis. In conclusion, the tyrosinase enzyme from mushroom is not as selective as the one from mammalian source for the enzyme-based melanogenesis inhibitory screening, and the mammalian cell-based assay appears to be a more reliable model for screening than the enzyme-based one.

The antioxidant activities of 24 isoflavonoids that were previously isolated as pure compounds from Dalbergia parviflora were evaluated using three different in vitro antioxidant-based assay systems: xanthine/xanthine oxidase (X/XO), ORAC, and DPPH. The isolates consisted of three subgroups, namely isoflavones, isoflavanones, and isoflavans, each of which appeared to have diversified substituents, and were thus ideal for the study of their structure-activity relationships (SARs). The SAR analysis was performed using the results obtained from both the inter-subgroup isoflavonoids with the same substitution pattern and the intra-subgroup compounds with different substitution patterns. The inter-subgroup comparison showed that the isoflavones exhibited the highest antioxidant activities based on all three assays. The intra-subgroup analysis showed that the additional presence of an OH group in Ring B at either R3′ or R5′ from the basic common structure of the R7-OH of Ring A and the R4′-OH (or -OMe) of Ring B greatly increased the antioxidant activities of all of the isoflavonoid subgroups and that other positions of OH and OMe substitutions exerted different effects on the activities depending on the subgroup and assay type. Therefore, based on the structural diversity of the isoflavonoids in D. parviflora, the present study provides the first clarification of the detailed antioxidant SARs of isoflavonoids.