Polyisoprenylated Methylated Protein Methyl Esterase Is Both Sensitive to Curcumin and Overexpressed in Colorectal Cancer: Implications for Chemoprevention and Treatment

Inhibition of PMPMEase, a key enzyme in the polyisoprenylation pathway, induces cancer cell death. In this study, purified PMPMEase was inhibited by the chemopreventive agent, curcumin, with a K-i of 0.3 mu M (IC50 = 12.4 mu M). Preincubation of PMPMEase with 1mM curcumin followed by gel-filtration chromatography resulted in recovery of the enzyme activity, indicative of reversible inhibition. Kinetics analysis with N-para-nitrobenzoyl-S-trans, trans-farnesylcysteine methyl ester substrate yielded K-M values of 23.6 +/- 2.7 and 85.3 +/- 15.3 mu M in the absence or presence of 20 mu M curcumin, respectively. Treatment of colorectal cancer (Caco2) cells with curcumin resulted in concentration-dependent cell death with an EC50 of 22.0 mu g/mL. PMPMEase activity in the curcumin-treated cell lysate followed a similar concentration-dependent profile with IC50 of 22.6 mu g/mL. In colorectal cancer tissue microarray studies, PMPMEase immunoreactivity was significantly higher in 88.6% of cases compared to normal colon tissues (P < 0.0001). The mean scores +/- SEM were 91.7 +/- 11.4 (normal), 75.0 +/- 14.4 (normal adjacent), 294.8 +/- 7.8 (adenocarcinoma), and 310.0 +/- 22.6 (mucinous adenocarcinoma), respectively. PMPMEase overexpression in colorectal cancer and cancer cell death stemming from its inhibition is an indication of its possible role in cancer progression and a target for chemopreventive agents.