Abstract
Polyphenols, a class of natural products, have been shown to exhibit cancer protective properties. Proprotein convertases form a family of mammalian subtilisin-like serine endoproteases. Increased expression of these enzymes has been associated with numerous pathologies including cancer. It has been suggested that the cancer protective effect of polyphenols might be related to their proprotein convertase inhibitory effects. Furin, the most studied proprotein convertase, was shown to be inhibited by polyphenols in an in vitro fluorescence peptide-based assay. Protein substrates or the presence of protein prevented this inhibition by prototype members of various classes of polyphenolic compounds. Inhibition appeared to be related to the reactivity of polyphenol auto-oxidation products to proteins. While direct inhibition by polyphenols of furin has, therefore, not been observed in cells, the existence of indirect mechanisms cannot be excluded. In the present investigation, 26 polyphenols and 5 control compounds were screened for indirect inhibition of furin in a cellular environment. Five polyphenols showed moderate inhibitory activity and three of these: octyl gallate, dodecyl gallate and nordihydroguariaretic acid were further studied. The processing in cells of several genuine furin substrates, including pro-IGF-1R, appeared to be inhibited by these polyphenols. The inhibition was not specific for furin but also affected other proprotein convertases. The three polyphenols inhibited the maturation of the furin zymogen, thereby limiting the formation of the active enzyme. The three polyphenols inhibited focus formation of HepG2 liver carcinoma cells suggesting reversal of the malignant phenotype. Anchorage-independent growth of these cells, a hallmark feature of tumor cells, was also inhibited. Since, dependent of the molecular subclass of hepatocellular carcinoma, overexpression of furin can have either favourable or detrimental effects, it seems advisable to take indirect proprotein convertase inhibitory activity into account when polyphenols are considered for therapy of hepatocellular carcinoma.