The programmed cell death 4 (PDCD4) gene was originally identified as a tumor-related gene in humans and acts as a tumor-suppressor in mouse epidermal carcinoma cells. However, its function and regulatory mechanisms of expression in human cancer remain to be elucidated. We therefore investigated the expression of PDCD4 in human hepatocellular carcinoma (HCC) and the role of PDCD4 in human HCC cells. Downregulation of PDCD4 protein was observed in all HCC tissues tested compared with corresponding noncancerous liver, as revealed by Western blotting or immunohistochemical staining. Human HCC cell line, Huh7, transfected with PDCD4 cDNA showed nuclear fragmentation and DNA laddering characteristic of apoptotic cells associated with mitochondrial changes and caspase activation. Transforming growth factor-b1 (TGF-b1) treatment of Huh7 cells resulted in increased PDCD4 expression and occurrence of apoptosis, also concomitant with mitochondrial events and caspase activation. Transfection of Smad7, a known antagonist to TGF-b1 signaling, protected cells from TGF-b1-mediated apoptosis and suppressed TGF-b1-induced PDCD4 expression. Moreover, antisense PDCD4 transfectants were resistant to apoptosis induced by TGF-b1. In conclusion, these data suggest that PDCD4 is a proapoptotic molecule involved in TGF-b1-induced apoptosis in human HCC cells, and a possible tumor suppressor in hepatocarcinogenesis.

PDCD4 is a novel tumor suppressor to show multi-functions inhibiting cell growth, tumor invasion, metastasis, and inducing apoptosis. PDCD4 protein binds to the translation initiation factor eIF4A, some transcription factors, and many other factors and modulates the function of the binding partners. PDCD4 downregulation stimulates and PDCD4 upregulation inhibits the TPA-induced transformation of cells. However, PDCD4 gene mutations have not been found in tumor cells but gene expression was post transcriptionally downregulated by micro environmental factors such as growth factors and interleukins. In this review, we focus on the suppression mechanisms of PDCD4 protein that is induced by the tumor promotors EGF and TPA, and in the inflammatory conditions. PDCD4-protein is phosphorylated at 2 serines in the SCFβTRCP ubiquitin ligase binding sequences via EGF and/or TPA induced signaling pathway, ubiquitinated, by the ubiquitin ligase and degraded in the proteasome system. The PDCD4 protein synthesis is inhibited by microRNAs including miR21.

One-carbon metabolism, in which folate plays an essential role, is involved in DNA methylation and synthesis, and is suspected of impacting on colorectal carcinogenesis. Alcohol is well recognized as a risk factor for colorectal cancer (CRC) and interactions with one-carbon metabolism have also been suggested. Therefore, functional polymorphisms in genes encoding members of this pathway, MTHFR C677T and A1298C (genes for methylenetetrahydrofolate reductase), MTR A2756G (gene for methionine synthase) and TS (gene for thymidylate synthase) tandem repeats polymorphisms, have attracted attention. We conducted a matched case-control study with 257 incident CRC cases and 771 non-cancer controls at the Aichi Cancer Center to clarify associations among folate intake and four polymorphisms with reference to CRC risk. Gene-environment interaction between polymorphisms, drinking and folate consumption was also evaluated. None of the polymorphisms showed any significant impact on CRC risk by genotype alone, but when combined with alcohol consumption the MTHFR 677CC type showed a significantly reduced risk (odds ratio (OR) = 0.45, 95% confidence interval (CI): 0.23-0.86) (P = 0.01). MTR GG showed increased risk only among drinkers (OR = 3.35, 1.40-8.05) (P = 0.047). TS polymorphism did not show statistical significance by genotype alone, while interaction with drinking was significant (P = 0.028). The association was not changed even after stratification by daily folate consumption and drinking habit. In conclusion, we found consistently significant interactions between one-carbon metabolism-related polymorphisms and alcohol drinking.

Alcohol consumption is recognized as a potential risk factor for colorectal cancer (CRC). Genetic polymorphisms, aldehyde dehydrogenase (ALDH2) Glu487Lys and alcohol dehydrogenase 2 (ADH2) His47Arg, which have a strong impact on alcohol metabolism, are common in Japanese population but their significance for CRC carcinogenesis remains to be clarified in detail. We, therefore, conducted a matched case-control study with 257 incident CRC cases and 771 non-cancer controls at Aichi Cancer Center, including analysis of interactions between polymorphisms, drinking and folate consumption. The ADH2 Arg allele was found to be associated with increased risk, the odds ratios (ORs) being 1.35 (95% confidence interval: 1.00-1.84) and 1.93 (1.06-3.53) for the His/Arg and Arg/Arg genotypes, respectively. In contrast, no apparent links were observed with the ALDH2 genotypes. Individuals having ALDH2 Glu/Glu with ADH2 Arg+, ALDH2 Lys+ with ADH2 His/His and ALDH2 Lys+ with ADH2 Arg+ showed ORs of 0.10(0.04-0.21), 0.10 (0.06-0.19) and 1.36 (0.94-1.97), respectively, compared with ALDH2 Glu/Glu with ADH2 His/His. Statistical gene-gene interaction was significant between the two polymorphisms for the risk of CRC (P< 0.001). The impact of ALDH2 Lys+ with ADH2 Arg+ was more evident in low folate consumer (OR = 2.32, 1.19-4.55) than high folate consumer (OR 1.38, 0.80-2.38). In conclusion, while we failed to find any significant association with the ALDH2 polymorphism itself, significant interaction between ALDH2 and ADH2 polymorphism was observed. Replication in the future study is warranted.

Hepatocellular carcinoma (HCC) often arises from chronically diseased livers. Persistent liver inflammation causes the accumulation of excessive extracellular matrix (ECM) proteins and impairs the liver function, finally leading to the development of HCC. A pleiotropic cytokine, transforming growth factor (TGF)-β1, plays critical roles throughout the process of fibrogenesis and hepatocarcinogenesis. In the liver, TGF-β1 inhibits the proliferation of hepatocytes and stimulates the production of ECM from hepatic stellate cells (HSCs) to maintain tissue homeostasis. During disease progression, both growth factors/cytokines and the ECM alter the TGF-β1 signals by modifying the phosphorylation of Smad proteins at their C-terminal and linker regions. TGF-β1 stimulates the expression of integrins, cellular receptors for ECM, along with an increase in ECM accumulation. The activation of integrins by the ECM modulates the response to TGF-β1 in hepatic cells, resulting in their resistance to TGF-β1-induced growth suppression in hepatocytes and the sustained production of ECM proteins in activated HSCs/myofibroblasts. Both growth factor receptors and integrins modify the expression and/or functions of the downstream effectors of TGF-β1, resulting in the escape of hepatocytes from TGF-β1-induced apoptosis. Recent studies have revealed that the alterations of Smad phosphorylation that occur as the results of the crosstalk between TGF-β1, growth factors and integrins could change the nature of TGF-β1 signals from tumor suppression to promotion. Therefore, the modification of Smad phosphorylation could be an attractive target for the prevention and/or treatment of HCC.