Isosilybin B

Angiopreventive efficacy of pure flavonolignans from milk thistle extract against prostate cancer: targeting VEGF-VEGFR signaling.[Pubmed: 22514647 ]

PLoS One. 2012;7(4):e34630.

The role of neo-angiogenesis in prostate cancer (PCA) growth and metastasis is well established, but the development of effective and non-toxic pharmacological inhibitors of angiogenesis remains an unaccomplished goal. In this regard, targeting aberrant angiogenesis through non-toxic phytochemicals could be an attractive angiopreventive strategy against PCA.
METHODS AND RESULTS:
The rationale of the present study was to compare the anti-angiogenic potential of four pure diastereoisomeric flavonolignans, namely silybin A, silybin B, isosilybin A and Isosilybin B, which we established previously as biologically active constituents in Milk Thistle extract. Results showed that oral feeding of these flavonolignans (50 and 100 mg/kg body weight) effectively inhibit the growth of advanced human PCA DU145 xenografts. Immunohistochemical analyses revealed that these flavonolignans inhibit tumor angiogenesis biomarkers (CD31 and nestin) and signaling molecules regulating angiogenesis (VEGF, VEGFR1, VEGFR2, phospho-Akt and HIF-1α) without adversely affecting the vessel-count in normal tissues (liver, lung, and kidney) of tumor bearing mice. These flavonolignans also inhibited the microvessel sprouting from mouse dorsal aortas ex vivo, and the VEGF-induced cell proliferation, capillary-like tube formation and invasiveness of human umbilical vein endothelial cells (HUVEC) in vitro. Further studies in HUVEC showed that these diastereoisomers target cell cycle, apoptosis and VEGF-induced signaling cascade. Three dimensional growth assay as well as co-culture invasion and in vitro angiogenesis studies (with HUVEC and DU145 cells) suggested the differential effectiveness of the diastereoisomers toward PCA and endothelial cells.
CONCLUSIONS:
Overall, these studies elucidated the comparative anti-angiogenic efficacy of pure flavonolignans from Milk Thistle and suggest their usefulness in PCA angioprevention.

Isosilybin B causes androgen receptor degradation in human prostate carcinoma cells via PI3K-Akt-Mdm2-mediated pathway.[Pubmed: 18332867 ]

Oncogene. 2008 Jun 26;27(28):3986-98.

The identification and development of novel nontoxic phytochemicals that target androgen and androgen receptor (AR) signaling remains a priority for prostate cancer (PCA) control.
METHODS AND RESULTS:
In the present study, we assessed the antiandrogenic efficacy of Isosilybin B employing human PCA LNCaP (mutated AR), 22Rv1 (mutated AR) and LAPC4 (wild-type AR) cells. Isosilybin B (10-90 microM) treatment decreased the AR and prostate specific antigen (PSA) levels in LNCaP, 22Rv1 and LAPC4 cells, but not in non-neoplastic human prostate epithelial PWR-1E cells. Isosilybin B treatment also inhibited synthetic androgen R1881-induced nuclear localization of AR, PSA expression and cell growth, and caused G(1) arrest. In mechanistic studies identifying AR degradation, Isosilybin B caused increased phosphorylation of Akt (Ser-473 and Thr-308) and Mdm2 (Ser-166), which was linked with AR degradation as pretreatment with PI3K inhibitor (LY294002)-restored AR level. Further, overexpression of kinase-dead Akt largely reversed Isosilybin B mediated-AR degradation suggesting a critical role of Akt in AR degradation. Antibody pull-down results also indicated that Isosilybin B treatment enhances the formation of complex between Akt, Mdm2 and AR, which promotes phosphorylation-dependent AR ubiquitination and its degradation by proteasome.
CONCLUSIONS:
Together, present findings identify a novel mechanism for Isosilybin B-mediated anticancer effects in human PCA cells.

Isosilybin B and isosilybin A inhibit growth, induce G1 arrest and cause apoptosis in human prostate cancer LNCaP and 22Rv1 cells.[Pubmed: 17389612]

Carcinogenesis. 2007 Jul;28(7):1533-42.

Silymarin and, one of its constituents, silibinin exert strong efficacy against prostate cancer (PCA); however, anticancer efficacy and associated mechanisms of other components of silymarin, which is a mixture of flavonolignans, are largely unknown.
METHODS AND RESULTS:
Here we have assessed the anticancer efficacy of two pure compounds Isosilybin B and isosilybin A, isolated from silymarin, in human prostate carcinoma LNCaP and 22Rv1 cells. Isosilybin B and isosilybin A treatment resulted in growth inhibition and cell death together with a strong G(1) arrest and apoptosis in both the cell lines. In the studies examining changes in cell cycle and apoptosis regulators, Isosilybin B and isosilybin A resulted in a decrease in the levels of both cyclins (D1, D3, E and A) and cyclin-dependent kinases (Cdk2, Cdk4 and cell division cycle 25A), but caused an increase in p21, p27 and p53 levels, except in 22Rv1 cells where Isosilybin B caused a decrease in p21 protein level. Isosilybin B- and isosilybin A-induced apoptosis was accompanied with an increase in the cleavage of poly (ADP-ribose) polymerase, caspase-9 and caspase-3 and a decrease in survivin levels. Compared with LNCaP and 22Rv1 cells, the antiproliferative and cytotoxic potentials of Isosilybin B and isosilybin A were of much lesser magnitude in non-neoplastic human prostate epithelial PWR-1E cells suggesting the transformation-selective effect of these compounds.
CONCLUSIONS:
Together, this study for the first time identified that Isosilybin B and isosilybin A, two diastereoisomers isolated from silymarin, have anti-PCA activity that is mediated via cell cycle arrest and apoptosis induction.

The effect of milk thistle (Silybum marianum) and its main flavonolignans on CYP2C8 enzyme activity in human liver microsomes.[Pubmed: 28457856 ]

Tyrosinase inhibitory study of flavonolignans from the seeds of Silybum marianum (Milk thistle).[Pubmed: 30871862 ]

Bioorg Med Chem. 2019 Jun 15;27(12):2499-2507.

Anti-melanogenesis effects of silymarin from milk thistle have been reported recently, but detailed tyrosinase inhibition properties of individual components have not been investigated. This study purported to substantiate tyrosinase inhibition and its mechanism based on a single metabolite.
METHODS AND RESULTS:
The responsible components for tyrosinase inhibition of target source were found out as flavonolignans which consist of isosilybin A (1), Isosilybin B (2), silydianin (3), 2,3-dihydrosilychristin (4), silychristin A (5), silychristin B (6) and silybin (7), respectively. The isolated flavonolignans (1-7) inhibited both monophenolase (IC50?=?1.7-7.6?µM) and diphenolase (IC50?=?12.1-44.9?µM) of tyrosinase significantly. Their inhibitions were 10-fold effective in comparison with their mother skeletons (8-10). Inhibitory functions were also proved by HPLC analysis using N-acetyl-l-tyrosine as substrate. The predominant formation of Emet·I was confirmed from a long prolongation of lag time and a decrease of the static state activity of the enzyme.
CONCLUSIONS:
All tested compounds had a significant binding affinity to tyrosinase with KSV values of 0.06-0.27?×?104?L·mol-1, which are well correlated with IC50s. In kinetic study, all flavonolignan (1-7) were mixed type I (KI?<?KIS) inhibitors, whereas their mother skeletons (8-10) were competitive ones. The UPLC-ESI-TOF/MS analysis showed that the isolated inhibitors are the most abundant metabolites in the target plant.

Chem Biol Interact. 2017 Jun 1;271:24-29.

Milk thistle is a widely-consumed botanical used for an array of purported health benefits. The primary extract of milk thistle is termed silymarin, a complex mixture that contains a number of structurally-related flavonolignans, the flavonoid, taxifolin, and a number of other constituents. The major flavonolignans present in most extracts are silybin A, silybin B, isosilybin A and Isosilybin B, silydianin, silychristin and isosilychristin. Silymarin itself has been reported to inhibit CYP2C8 activity in vitro, but the effect of the individual flavonolignans on this enzyme has not been studied.
METHODS AND RESULTS:
To investigate the effects of milk thistle extract and its main flavonolignans (silybin A, silybin B, isosilybin A and Isosilybin B) on CYP2C8 activity at relevant concentrations, the effect of milk thistle extract and the flavonolignans on CYP2C8 enzyme activity was studied in vitro using human liver microsomes (HLM) incorporating an enzyme-selective substrate for CYP2C8, amodiaquine. Metabolite formation was analyzed using liquid chromatography-tandem mass spectrometry (LC/MS-MS). The concentration causing 50% inhibition of enzyme activity (IC50) was used to express the degree of inhibition. Isosilibinin, a mixture of the diastereoisomers isosilybin A and Isosilybin B, was found to be the most potent inhibitor, followed by Isosilybin B with IC50 values (mean ± SE) of 1.64 ± 0.66 μg/mL and 2.67 ± 1.18 μg/mL, respectively. The rank order of observed inhibitory potency after isosilibinin was silibinin > isosilybin A > silybin A > milk thistle extract > and silybin B.
CONCLUSIONS:
These in vitro results suggest a potentially significant inhibitory effect of isosilibinin and Isosilybin B on CYP2C8 activity. However, the observed IC50 values are unlikely to be achieved in humans supplemented with orally administered milk thistle extracts due to the poor bioavailability of flavonolignans documented with most commercially available formulations.