Citromycin

Chemical analysis of an Australian marine-derived isolate of Penicillium bilaii, collected from the Huon estuary, Port Huon, Tasmania, yielded the known fungal aromatic polyketides citromycetin (1) and citromycin (2) together with two dihydro analogues, (-)-2,3-dihydrocitromycetin (3) and (-)-2,3-dihydrocitromycin (4). An Australian terrestrial isolate of Penicillium striatisporum collected near Shalvey, New South Wales, also yielded citromycetin (1), citromycin (2), and the new dihydro analogue (-)-2,3-dihydrocitromycetin (3), together with fulvic acid (5), anhydrofulvic acid (6), and a selection of new methoxylated analogues, 12-methoxycitromycetin (7), 12-methoxycitromycin (8), (-)-12-methoxy-2,3-dihydrocitromycetin (9), and 12-methoxyanhydrofulvic acid (10). P. bilaii also yielded the rare siderophore pistillarin (11), the known diketopiperazines cyclo-(L-Phe -L-Pro) (12), cyclo-(L-Pro-L-Tyr) (13), cyclo-(L-Pro-L-Val) (14), and cis-bis(methylthio)silvatin (15), and three new diketopiperazines, bilains A-C (16-18). The structures for the Penicillium metabolites 1- 18 were assigned by a combination of detailed spectroscopic analysis, including correlation with relevant literature data, chemical derivatization, degradation, and biosynthetic considerations. The citromycin polyketides 2 and 4 and the diketopiperazine 15 were weakly cytotoxic.

A previously unreported chromene derivative, 1-hydroxy-12-methoxycitromycin (1c), and four previously undescribed chromone derivatives, including pyanochromone (3b), spirofuranochromone (4), 7-hydroxy-6-methoxy-4-oxo-3-[(1E)-3-oxobut-1-en-1-yl]-4H-chromene-5-carboxylic acid (5), a pyranochromone dimer (6) were isolated, together with thirteen known compounds: β-sitostenone, ergosterol 5,8-endoperoxide, citromycin (1a), 12-methoxycitromycin (1b), myxotrichin D (1d), 12-methoxycitromycetin (1e), anhydrofulvic acid (2a), myxotrichin C (2b), penialidin D (2c), penialidin F (3a), SPF-3059-30 (7), GKK1032B (8) and secalonic acid A (9), from cultures of the marine sponge- associated fungus Penicillium erubescens KUFA0220. Compounds 1a⁻e, 2a, 3a, 4, 7⁻9, were tested for their antibacterial activity against Gram-positive and Gram-negative reference and multidrug-resistant strains isolated from the environment. Only 8 exhibited an in vitro growth inhibition of all Gram-positive bacteria whereas 9 showed growth inhibition of methicillin-resistant Staphyllococus aureus (MRSA). None of the compounds were active against Gram-negative bacteria tested.

Recently, microorganisms and their metabolites in the Antarctic marine environment have attracted attention as useful sources for novel therapeutics, including anticancer drugs. Here, we investigated the effects of citromycin, isolated from the Antarctic marine-derived fungus, Sporothrix sp., on human ovarian cancer cells. Citromycin inhibited the migration and invasion of human ovarian cancer SKOV3 and A2780 cells, but had no cytotoxic activity against them. Additionally, it inhibited the expression of epithelial-mesenchymal transition (EMT) markers and the activation of matrix metalloproteinase (MMP)-2 and MMP9. Moreover, extracellular signal-regulated kinase (ERK)-1/2 signaling was inhibited after citromycin treatment, and the ectopic expression of ERK negated the anti-invasive activity of citromycin. Our findings suggest that citromycin inhibits the migration and invasion of human ovarian cancer cells by downregulating the expression levels of EMT markers and MMP-2/9 via inhibition of the ERK1/2 pathway.