Polyisoprenylation is a set of secondary modifications involving proteins whose aberrant activities are implicated in cancers and degenerative disorders. The last step of the pathway involves an ester-forming polyisoprenylated protein methyl transferase- and hydrolytic polyisoprenylated methylated protein methyl esterase (PMPMEase)-catalyzed reactions. Omega-3 and omega-6 polyunsaturated fatty acids (PUFAs) have been linked with antitumorigeneis and tumorigenesis, respectively. PUFAs are structurally similar to the polyisoprenyl groups and may interfere with polyisoprenylated protein metabolism. It was hypothesized that PUFAs may be more potent inhibitors of PMPMEase than their more polar oxidative metabolites, the prostaglandins. As such, the relative effects of PUFAs and prostaglandins on PMPMEase could explain the association between cyclooxygenase-2 (COX-2) expression in tumors, the chemopreventive effects of the non-steroidal anti-inflammatory (NSAIDs) COX-2 inhibitors and PUFAs. PUFAs such as arachidonic (AA), eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids inhibited PMPMEase activity with Ki values of 0.12 to 3.7 μM. The most potent prostaglandin was 63-fold less potent than AA. The PUFAs were also more effective at inducing neuroblastoma cell death at physiologically equivalent concentrations. The lost PMPMEase activity in AA-treated degenerating cells was restored by incubating the lysates with COX-1 or COX-2. PUFAs may thus be physiological regulators of cell growth and could owe these effects to PMPMEase inhibition.

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Protein polyisoprenylation is essential for the activities of oncogenic proteins such as Ras. The final, reversible step of polyisoprenylation pathway is catalyzed by an ester forming polyisoprenylated protein methyl transferase and polyisoprenylated methylated protein methyl esterase (PMPMEase). Various free fatty acids (FFAs) were analyzed to test whether their beneficial health effects are associated with PMPMEase inhibition. The potent inhibitory FFAs were further analyzed for their effects on cell viability. Arachidonic acid (AA) and lauric acid (LA) inhibited PMPMEase with 0.19 and 6.5 ìM, respectively. AA was over 63‐fold more inhibitory than Prostaglandin A2 (Ki of 12 ìM) while prostaglandin E2 and long chain saturated fatty acids were not inhibitory even at 1 mM. AA and LA inhibited the viability of human neuroblastoma cells with EC50 values of 11 and 25 ìM respectively, after 72 h exposure. Therefore AA and other PUFAs may be involved in the regulation of cell proliferation and that the elevated levels of COX‐2 in tumors convert PUFAs which inhibit PMPMEase at physiological concentrations, to prostanoids that are ineffective at inhibiting both PMPMEase and thus cell proliferation. NSAIDs may thus owe their anticancer effects through their ability to protect the PMPMEase‐ and cell proliferation‐inhibiting PUFAs from COX‐2 metabolism.Grant Funding Source: NIH/NIGMS/SCORE (GM 08111‐35) & NIH/NCRR (G12 RR0 3020)

Polyisoprenylation is a protein modification with 15 or 20 carbon isoprenes. Polyisoprenylated proteins (PP) undergo S‐adenosyl‐L‐methionine‐dependent methylation of the terminal ‐COOH. PP mediate various intracellular processes such as signaling, cell proliferation, differentiation and apoptosis and mutations of PP are encountered in about 30% of cancers. The methylation by PP methyl transferase (PPMTase) is counteracted by PMPMEase and constitute the only reversible step of the pathway. The relative amounts of the acid and ester forms of PP are determined by the two enzymes. Since PMPMEase and PPMTase may influence functional conformations of PP, a thorough knowledge of these enzymes is essential to the understanding of the significance of PP. Unlike the well studied PPMTase, PMPMEase is less understood. PMPMEase's ability to hydrolyze food‐derived substances has led us to hypothesize a regulatory role of PUFAs on the enzyme activity. A series of saturated and PUFAs were tested for PMPMEase inhibition using an HPLC‐based assay. Shorter chain, more unsaturated, cis fatty acids were the most inhibitory, with an IC50 of 11.6 μM for eicosapentaenoic acid. The anticancer and other benefits of short chain fatty acids and PUFAs may be related to PMPMEase inhibition given the many putative PP substrates that impact various biochemical events. Supported by NIH/NIGMS/SCORE (GM 08111‐35) & NIH/NCRR (G12 RR0 3020).