NAC, a common dietary supplement and an antioxidant membrane-permeable metal-binding compound, has been shown to inhibit inflammatory responses, tumor growth including lung cancer [13, 14]. However, the mechanisms by which this reagent in control of NSCLC cell growth has not been well elucidated. We have found that NAC inhibited NSCLC cell proliferation through reduction of PDK1, a kinase and master regulator of a number of downstream signal cascades that are involved in suppression of apoptosis and promotion of tumor growth including lung cancer [4, 15]. High expression of PDK1 has been detected in invasive cancers including lung  and inhibition of PDK1 in several cancer cells results in significant cell growth inhibition . These observations suggest that PDK1 can be considered as a target for therapies. This result, together with the finding that exogenous PDK1 diminishes the inhibitory effect of NAC on cell growth, indicates an important role of targeting PDK1 in mediating the inhibitory effect of NAC on growth of NSCLC cells.
PPARα, a ligand-inducible nuclear transcription factor that has been implicated in the pathogenesis and treatment of tumor including lung cancer both in vitro and in vivo[7, 16, 17]. The exact role that PPARα signaling plays in NSCLC and the mechanisms by which PPARα ligands suppress tumor cell growth have not been fully elucidated. A report showed that NAC could increase PPARα activity . Because of this, we will further test the role of PPARα and the effect of PPARα ligands on PDK1 expression. Our results showed that NAC increased protein expression of PPARα and the synergism of NAC and PPARα ligands on cell growth inhibition demonstrated an important role of this nuclear transcription factor in mediating the inhibitory effect of NAC on PDK1 expression and on NSCLC cell proliferation. Our result suggested that PPARα agonist could sensitize the effect of NAC on cell growth inhibition and also implied that NAC may act as a potential PPARα ligand. Consistent with this, one report demonstrated a synergistic effect of PPARα agonist and NAC in control of brain tumor cells .
Note that no report showed a link between PPARα ligand and PDK1 although PDK1 was reported to be a target gene of PPARσ/β , another isoforms of PPAR family, which strongly expressed in the majority of lung cancers, and activation of this isoform induced proliferation of lung cancer through pathways including activation of Akt phosphorylation correlated with up-regulation of PDK1 . Note that the PDK1 promoter contains peroxisome proliferator responsive element (PPRE) , our data showed that PPARα ligand inhibited PDK1 promoter activity suggesting a distinct function of PPARα activation as compared to that of PPARσ/β. More studies are required to elucidate this.
Furthermore, our results indicated that NAC–mediated downregulation of PDK1 reflected inhibition of transactivation of the PDK1 gene and also demonstrated that NAC, through activation of PPARα, increased tumor suppressor, p53 and reduced p65, a subunit of NF-κB, which played important roles in mediating the effect of NAC on inhibition of PDK1 expression. This again suggested the characteristic of NAC acted as PPARα ligand. Silencing of p53 and overexprerssion of p65 blocked the effects of NAC on PDK1 expression further confirm the key roles of p53 and p65 in this process. P53 plays a critical role in tumor suppression mainly by inducing growth arrest, blocking angiogenesis and conferring the cancer cell sensitivity to chemoradiation . Transcription factor NF-κB has been shown to regulate the expression of a number of genes that involve in many cellular processes such as inflammation and tumor growth . Interestingly, the link of p53 in the regulation of glycolysis-dependent activation of NF-κB signaling in cancer has been reported . However, the role of p53 and NF-κB in the direct regulation of PDK1 expression remains unknown. On the contrary, one study showed that overexpression of PDK1 resisted the apoptotic cell death caused by hypoxic injury and increased the expression of survival proteins, such as p53, in cultured rat cardiomyocytes . Also, reports found that PDK1 plays a critical role by nucleating the T cell receptor-induced NF-κB activation pathway, which is important for T cell proliferation and activation during the adaptive immune response . Together, these findings indicated that PDK1 was a critical regulator of tumor cell survival by modulating the p53 and NF-κB signaling pathways. NAC also had a direct or indirect effect on the regulation of p53 and NF-κB [26, 27]. The activation of p53 has been shown to mediate the effects of NAC on prostate cancer cell growth . One study reported that NAC inhibited growth of liver cancer cells through reducing the expression of NF-κB subunit p65 suggesting that NAC may be used for the treatment of liver tumor . Thus, knockdown of p53 and overexpression of p65 abrogated the effect of NAC on PDK1 expression and cell proliferation highlighted the critical role of p53 and p65 in this process.