Inhibitory effect of silibinin on ligand binding to erbB1 and associated mitogenic signaling, growth, and DNA synthesis in advanced human prostate carcinoma cells

We recently showed the inhibitory effect of a flavonoid antioxidant silymar in, on erbB1-Shc activation in prostate cancer (PCA) DU145 cells. In the pr esent study, we performed more detailed mechanistic and molecular modeling studies with pure silibinin to assess and define its effect on membrane sig naling related to erbB1 activation in human PCA LNCaP and DU145 cells. Stud ies also were performed to establish the biologic responses toward extracel lular signal-regulated protein kinase 1/2 (ERK1/2) activation, cell growth, and DNA synthesis. Treatment of serum-starved cells with various doses of silibinin for 2 h followed by I-125-epidermal growth factor (EGF) showed 30 -75% inhibition in ligand binding and 55-95% inhibition in its internalizat ion in LNCaP cells and 20-64% and 12-27% inhibition in these two events in DU145 cells. Time-response studies showed similar effects. In further studi es, treatment of serum-starved cultures with silibinin followed by EGF show ed strong inhibitory effects on membrane and cytoplasmic signaling molecule s. In the case of erbB1 activation, silibinin showed a 58-75% decrease in L NCaP and a 40-100% decrease in DU145 cells at 50, 75, and 100-mug/mL doses. Inhibitory effects of silibinin also were evident on ERK1/Z activation (20 -80% inhibition) in both cell lines. Treatment of serum-starved cultures wi th silibinin resulted in 20-40% and 30-55% inhibition of LNCaP and DU145 ce ll growth, respectively, at similar doses after 1-3 d of treatment, and 10- 50% cell death in both cell lines. Under 10% serum conditions, identical si libinin treatments resulted in 20-65% inhibition of cell growth in LNCaP an d DU145 cells but did not cause any cell death. Similar doses of silibinin treatments for 24 h also resulted in 25-60%, 35-40%, and 36-50% inhibition of DNA synthesis when cells were cultured in 10% serum, totally serum starv ed, and serum starved plus stimulated with EGF, respectively. Molecular mod eling of silibinin showed that it is a highly lipophilic compound, suggesti ng that it interacts with lipid-rich plasma membrane, including binding wit h erbB1, thereby competing with the EGF-erbB1 interaction. Because the liga nd-erbB1 autocrine-loop is causally involved in advanced and androgen-indep endent PCA, the observed effects of silibinin and its strong lipophilic nat ure could be useful in developing this agent for the prevention and therapy of PCA. (C) 2001 Wiley-Liss, Inc.