Piperlongumine (PPLGM) is a bioactive compound isolated from long peppers that shows selective toxicity towards a variety of cancer cell types including colon cancer. The signaling pathways that lead to cancer cell death in response to PPLGM exposure have not been previously identified. Our objective was to identify the intracellular signaling mechanisms by which PPLGM leads to enhanced colon cancer cell death. We found that PPLGM inhibited the growth of colon cancer cells in time- and concentration-dependent manners, but was not toxic toward normal colon mucosal cells at concentrations below 10 μM. Acute (0-60 minutes) and prolonged (24 hours) exposure of HT-29 cells to PPLGM resulted in phosphorylation of ERK. To investigate whether ERK signaling was involved in PPLGM-mediated cell death, we treated HT-29 cells with the MEK inhibitor U0126, prior to treating with PPLGM. We found that U0126 attenuated PPLGM-induced activation of ERK and partially protected against PPLGM-induced cell death. These results suggest that PPLGM works, at least in part, through the MEK/ERK pathway to result in colon cancer cell death. A more thorough understanding of the molecular mechanisms by which PPLGM induces colon cancer cell death will be useful in developing therapeutic strategies to treat colon cancer.

BackgroundHuman mitochondrial peptide deformylase (PDF) has been proposed as a novel cancer therapeutic target. However, very little is known about its expression and regulation in human tissues. The purpose of this study was to characterize the expression pattern of PDF in cancerous tissues and to identify mechanisms that regulate its expression.MethodsThe mRNA expression levels of PDF and methionine aminopeptidase 1D (MAP1D), an enzyme involved in a related pathway with PDF, were determined using tissue panels containing cDNA from patients with various types of cancer (breast, colon, kidney, liver, lung, ovarian, prostate, or thyroid) and human cell lines. Protein levels of PDF were also determined in 2 colon cancer patients via western blotting. Colon cancer cells were treated with inhibitors of ERK, Akt, and mTOR signaling pathways and the resulting effects on PDF and MAP1D mRNA levels were determined by qPCR for colon and lung cancer cell lines. Finally, the effects of a PDF inhibitor, actinonin, on the proliferation of breast, colon, and prostate cell lines were determined using the CyQUANT assay.ResultsPDF and MAP1D mRNA levels were elevated in cancer cell lines compared to non-cancer lines. PDF mRNA levels were significantly increased in breast, colon, and lung cancer samples while MAP1D mRNA levels were increased in just colon cancers. The expression of PDF and MAP1D varied with stage in these cancers. Further, PDF protein expression was elevated in colon cancer tissue samples. Inhibition of the MEK/ERK, but not PI3K or mTOR, pathway reduced the expression of PDF and MAP1D in both colon and lung cancer cell lines. Further, inhibition of PDF with actinonin resulted in greater reduction of breast, colon, and prostate cancer cell proliferation than non-cancer cell lines.ConclusionsThis is the first report showing that PDF is over-expressed in breast, colon, and lung cancers, and the first evidence that the MEK/ERK pathway plays a role in regulating the expression of PDF and MAP1D. The over-expression of PDF in several cancers and the inhibition of cancer cell growth by a PDF inhibitor suggest this enzyme may act as an oncogene to promote cancer cell proliferation.