SCD1 Inhibition Causes Cancer Cell Death by Depleting Mono-Unsaturated Fatty Acids

Increased metabolism is a requirement for tumor cell proliferation. To understand the dependence of tumor cells on fatty acid metabolism, we evaluated various nodes of the fatty acid synthesis pathway. Using RNAi we have demonstrated that depletion of fatty-acid synthesis pathway enzymes SCD1, FASN, or ACC1 in HCT116 colon cancer cells results in cytotoxicity that is reversible by addition of exogenous fatty acids. This conditional phenotype is most pronounced when SCD1 is depleted. We used this fatty-acid rescue strategy to characterize several small-molecule inhibitors of fatty acid synthesis, including identification of TOFA as a potent SCD1 inhibitor, representing a previously undescribed activity for this compound. Reference FASN and ACC inhibitors show cytotoxicity that is less pronounced than that of TOFA, and fatty-acid rescue profiles consistent with their proposed enzyme targets. Two reference SCD1 inhibitors show low-nanomolar cytotoxicity that is offset by at least two orders of magnitude by exogenous oleate. One of these inhibitors slows growth of HCT116 xenograft tumors. Our data outline an effective strategy for interrogation of on-mechanism potency and pathway-node-specificity of fatty acid synthesis inhibitors, establish an unambiguous link between fatty acid synthesis and cancer cell survival, and point toward SCD1 as a key target in this pathway

SCD1 inhibitors are potent, specific, and kill cancer cells exclusively by depleting mono-unsaturated fatty acids.

<p><b>A</b> HCT116 cells were treated and analyzed for cell viability or cellular SCD1 inhibition (LC/MS/MS) as described above. <b>B</b> HCT116 were treated with DMSO or SCD1 inhibitor #28c in the presence of various fatty acids (25 uM) (Biomol, #2803) for 72 hours, and analyzed for cell viability. Data are displayed as a heat map continuum from green (live cells) to red (dead cells). <b>C</b> HCT116 cells were treated for 36 hours with various doses of SCD1 inhibitors as indicated. Cells were lysed in LDS loading dye (Invitrogen) and analyzed by western blotting for PARP cleavage (Cell Signaling). Staurosporine, a broad-spectrum kinase inhibitor, was included as a positive control for PARP cleavage. <b>D</b> HCT116 cells were treated as in <b>C</b>, in the presence or absence of exogenous oleate, followed by analysis of PARP cleavage.</p

SCD1 is a fatty acid synthesis pathway node critical for cancer cell survival.

<p><b>A </b><i>de novo</i> synthesis of mono-unsaturated fatty acids. <b>B</b> HCT116 colon cancer cells (ATCC) cultured in RPMI-1640 (Cambrex) containing 2%FBS plated at a density of 4000 cells per well in 100 ul media in 96-well plates were transfected with siRNA pools (Dharmacon, 50 nM) targeting three fatty-acid-synthesis pathway nodes,or two unrelated survival genes, using Lipofectamine 2000 (Invitrogen). 16 hours after transfection, cells were treated with 25 uM fatty acids (Sigma, 100× stocks dissolved in 10%MeOH/0.9%BSA/PBS) as indicated, and viability was determined 72 hours after transfection (Cell Titer Glo, Promega). <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0033823#s2" target="_blank">Results</a> are expressed as percent viability versus cells transfected with a non-targeting control siRNA (designated 100% viability) treated with the same fatty acid. <b>C</b> DU145 prostate cancer cells, HCT116 colon cancer cells, and MIA PaCa2 pancreatic cancer cells (ATCC) cultured in RPMI-1640 containing 2% FBS were treated with single siRNAs targeting SCD1 or PSMD14 (Dharmacon, 25 nM), followed 16 hours later by treatment with oleate as indicated. Viability was determined 72 hours after transfection. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0033823#s2" target="_blank">Results</a> are expressed as percent viability versus cells transfected with a non-targeting control siRNA (designated 100% viability) treated with the same fatty acid. <b>D</b> HCT116 colon cancer cells plated at a density of 1000 cells per well in 25 ul media in 384-well plates were treated with small-molecule inhibitors of ACC1 (CP640186, Pfizer), FASN (#10v, Merck), or SCD1 (#7n, Abbott), in media containing fatty acids as indicated, 72 hours prior to viability determination. Inibitors were synthesized at Genzyme (Waltham, MA).</p

SCD1 inhibition is not universally toxic, and slows growth of HCT116 xenograft tumors.

<p><b>A</b> HCT116 or SKOV3 cells were treated and analyzed for cell viability or cellular SCD1 inhibition (LC/MS/MS) as described above. <b>B</b> HCT116 or SKOV3 cells were treated and analyzed for cell viability. Table expresses the ratio of SKOV3 EC50 versus HCT116 EC50. <b>C</b>, <b>D</b> Nude mice harboring passage five 200 mm3 HCT116 tumors (passaged as trocar fragments) (n = 10 per group) were dosed by oral gavage twice daily with 160 mg/kg #28c for 20 days or with intravenous CPT11 on three consecutive days starting when tumors reached 200 mm3. Tumor growth (<b>C</b>) and body weight (<b>D</b>) were monitored and plotted as mean +/− standard deviation.</p