Verticillin C

The HL-60 cell line, derived from a patient with acute promyelocytic leukemia, is a widely used model system to study the cellular and molecular events involved in differentiation of leukemic cells. In a screening for inducers of differentiation of HL-60 cells, cultures of Gliocladium strain 4-93 were found to produce Sch 52900, a previously isolated diketopiperazine (Chu et al,. J Antibiotics 48, 1440-1445). Sch 52900 induced the differentation of 50-69% of HL-60 cells at concentrations of 6.8-13.6 nM as measured by nitro-blue tetrazolium chloride (NBT) reduction which was followed by apoptosis as shown by DNA fragmentation. Our results demonstrate that growth arrest and the induction of differentiation by Sch 52900 is due to the induction of the cell cycle inhibitor p21WAF and an inhibition of the extracellular signal-regulated kinase (ERK) signaling pathway which leads to the activation of the transcription factor AP-1.

Sch 52900 (1) and Sch 52901 (2), two new inhibitors of c-fos proto-oncogene induction, have been isolated from the fermentation of broth of the fungal culture (SCF-1168), Gliocladium sp. Along with compounds 1 and 2, a known compound verticillin A (3) was also obtained from the culture. Structure elucidation of 1 and 2, accomplished by analysis of spectral data in comparison with the data of 3, revealed both 1 and 2 were found to be closely related to the verticillin family of diketopiperazines. All three compounds prevented serum-stimulated transcription of the human c-fos promoter, using a fos/lac Z reporter gene assay, with IC50 values of 1.5, 18 and 0.5 microM of 1, 2 and 3, respectively. Northern analysis revealed the exposure of cells to compound 3 causes inhibition of both phorbol ester-induced c-fos induction of serum-induced JE induction in the absence of inhibiting RNA synthesis, as measured by [3H]uridine incorporation. There results suggest that this class of compounds exerts antitumor activity by blocking a signal transduction pathway that is common to and necessary for the induction of at least a subset of immediate early genes involved in cell proliferation.

Three new epidithiodioxopiperazine compounds, bionectins A (1), B (2), and C (3), along with a known compound, verticillin D (4), have been isolated from the mycelium of liquid fermentation cultures of the fungus Bionectra byssicola F120. The structures of compounds 1-3 were assigned on the basis of MS and NMR data. Compounds 1 and 2 incorporate a dioxopiperazine moiety with a disulfide bridge in their molecules, while 3 contains a dioxopiperazine ring with two methylsulfanyl groups. Compounds 1 and 2 exhibited antibacterial activity against S. aureus including methicillin-resistant S. aureus (MRSA) and quinolone-resistant S. aureus (QRSA), with MIC values of 10-30 microg/mL, while 3 showed no antibacterial activity even at 100 microg/mL.