GROWTH OF METHIONINE-DEPENDENT HUMAN PROSTATE-CANCER (PC-3) IS INHIBITED BY ETHIONINE COMBINED WITH METHIONINE STARVATION

Methionine (MET) is required for cell metabolism. MET endogenously syn thesized from homocysteine (HCY) supports the proliferation of normal cells, but not that of numerous malignant cells, as shown previously. MET starvation should have an anti-tumour effect, and its deleterious effects on the hosts might be prevented by HCY. Anti-tumour effects of MET starvation must be reinforced by ethionine (ETH), a MET analogue. MET dependency of PC-3, a human prostate cancer cell line, was studie d in vitro. Proliferation of PC-3 cells, cultivated in MET-free medium , was 29% compared with growth in MET + HCY- medium. Addition of HCY t o MET-free medium increased the proliferation rate to 56%. The concent ration of ETH required to decrease the PC-3 cell proliferation rate to 50% (IC50) was 0.5 mg ml(-1) in MET-HCY- medium. ETH-induced inhibiti on was abolished by MET addition and was reinforced by HCY. PC-3 cell cycle was blocked in the S-G(2)-phase after 30 h culture in the absenc e of MET; this blockage was not reversed by addition of HCY. ETH at th e IC50 in MET-HCY+ medium blocked DNA replication. Apoptotic cells app eared after 30 h incubation in MET-HCY+ medium only when ETH was added . ATP pools were decreased after 15 h of culture in MET-free medium. I n vivo, MET starvation was obtained by feeding tumour-bearing mice a d iet containing a synthetic amino acid mixture as the protein supply, i n which HCY replaced MET. Given to nude mice bearing xenografted PC-3, from day 1 after grafting and for 3 weeks, this diet inhibited tumour growth (34% on day 20, P < 0.007); this effect was potentiated by ETH (200 mg kg(-1) day(-1) i.p.) (56% on day 20, P < 5 x 10(-5)). The dif ferences between the effects of these two treatments were significant (P < 0.017) and optimal on day 20. These data showed that combination of ETH and HCY slowed the proliferation of prostate cancer cells in vi tro and in vivo, decreased ATP synthesis and caused cell cycle arrest and apoptosis. Experimental therapy based on cancer cell MET metabolis m deficiency could be efficient for treating advanced prostate cancers refractory to current therapies.