Cinnabaramides A−G:  Analogues of Lactacystin and Salinosporamide from a Terrestrial Streptomycete

Abstract

The cinnabaramides A-G (1-7) were isolated from a terrestrial strain of Streptomyces as potent and selective inhibitors of the human 20S proteasome. Their chemical and biological properties resemble those of salinosporamide A, a recently identified lead compound from an obligate marine actinomycete, which is currently under development as an anticancer agent. Cinnabaramides F and G (6, 7) combine essential structural features of salinosporamide A and lactacystin and show about equal potency in vitro, with IC50 values in the 1 nM range. The properties and phylogenetic position of the producer organism, the production and isolation of compounds 1-7, their structure elucidation by MS and NMR, and their biological activities are reported. Additionally, an X-ray crystal structure was obtained from cinnabaramide A (1).

Mining the Cinnabaramide Biosynthetic Pathway to Generate Novel Proteasome Inhibitors

“…As shown in Table 2, salinosporamide A has a higher inhibitory potential towards all three proteolytic subunits than the tested cinnabaramides. The IC 50 values determined for the CT-L activity were all in the low nanomolar range, as reported previously, [9,[32][33][34] although the chloro derivatives are more potent than the parent compound. With regard to inhibition at the T-L active site, 15-chlorocinnabaramide A showed the highest activity amongst the derivatives.…”

“…[9] Cells of Escherichia coli were grown in Luria-Bertani (LB) medium at 37 8C. dNTPs, restriction enzymes, T4 DNA ligase, isopropyl b-d-thiogalactopyranoside (IPTG), and X-gal were purchased from Fermentas.…”

“…JS360 is known to produce cinnabaramides A-G, each exhibiting a g-lactam-b-lactone bicyclic ring structure attached to a cyclohexenyl unit, together with a hexyl side chain. [9] With the exception of the hexyl side chain, significant structure similarity was found between these compounds and the salinosporamides (Scheme 1), a class of PKS/NRPS antitumor metabolites isolated from the marine actinomycetes Salinispora tropica CNB-440 [10][11][12] and Salinospora…”

“…[13] Both cinnabaramides and salinosporamides exhibit potent cancer cell cytotoxicities and exert their effects through binding to the 20S proteasome core particle, thereby inducing cell cycle arrest and programmed cell death (apoptosis). [9,[14][15][16] The key to the superior activity of salinosporamide A versus the cinnabaramides is assumed to be the presence of the chlorine atom attached to the side chain, which is required for the drug's irreversible binding to its biological target and is thus a major reason for the compound's effectiveness against cancer. [17] We anticipated that the factor behind the varying side chains in the two compound classes should be a biosynthetic enzyme responsible for forming a hypothetical and unusual hexylmalonyl-CoA extender unit.…”

Carboxylases in Natural and Synthetic Microbial Pathways

“…A homolog of the latter enzyme was also shown to be responsible for the synthesis of the unusual extender units chloroethylmalonyl-CoA (from chlorocrotonyl-CoA) and propylmalonyl-CoA (from 2-pentenylCoA) (46,80). Moreover, in the biosynthesis of the proteasome inhibitor cinnabaramide A and the macrolide antibiotic filipin, CCR homologs supply hexylmalonyl-CoA via reductive carboxylation of 2-octenoyl-CoA (97,112,130).…”