Abstract
Background: Emodin has anti-neoplastic activities on multiple tumors. However, the molecular mechanisms underlying this effect still remain to be fully understood. Methods: Cell viability and cell cycle distribution were measured using 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) assays and flow cytometry, respectively. Cell invasion and migration were examined by transwell invasion and wound healing assays. Western blot analysis was performed to examine the phosphorylation and protein expression of AMP-activated protein kinase alpha (AMPKα), extracellular signaling-regulated kinase 1/2 (ERK1/2), peroxisome proliferators-activated receptor gamma (PPARγ), insulin-like growth factor (IGF) binding protein 1 (IGFBP1) and the transcription factor Sp1. QRT-PCR was used to examine the mRNA levels of the IGFBP1 gene. Small interfering RNAs (siRNAs) were used to knockdown PPARγ and IGFBP1 genes. Exogenously expression of IGFBP1 and Sp1 was determined by transient transfection assays. IGFBP1 promoter activity was measured by Secrete-Pair Dual Luminescence Assay Kit. In vivo nude mice xenograft model and bioluminescent imaging system were used to confirm the findings. Results: We showed that emodin induced cell cycle arrest of NSCLC cells. Emodin increased PPARγ protein and luciferase reporter activity, which were abolished by inhibitors of MAPK extracellular signaling-regulated kinase (ERK) kinase (MEK)/ERK and AMPK. Silencing of PPARγ abrogated emodin-inhibited cell growth and cell cycle arrest. Furthermore, emodin elevated IGFBP1 mRNA, protein, and promoter activity through activation of PPARγ. Intriguingly, overexpressed Sp1 attenuated emodin-induced IGFBP1 expression, which was not observed in cells with silenced PPARγ gene. Moreover, silencing of IGFBP1 gene blunted emodin-induced inhibition of cell growth and cell cycle arrest. On the contrary, overexpressed IGFBP1 enhanced emodin-induced phosphorylation of AMPKα and ERK1/2, and restored emodin-inhibited growth in cells with silenced endogenous IGFBP1 gene. Emodin also inhibited growth of lung xenograft tumors and Sp1, and increased IGFBP1 and PPARγ protein expressions In vivo. Conclusion: Collectively, our results show that emodin inhibits growth of non-small-cell lung cancer (NSCLC) cells through ERK and AMPKα-mediated induction of PPARγ, followed by reduction of Sp1. This in turn induces IGFBP1 gene expression. Thus, the signaling cascades, positive feedback loop and cooperative interplay between transcription factors-induced the expression of IGFBP1 gene contribute to the overall responses of emodin. This study provides a novel mechanism by which emodin inhibits growth of human lung cancer cells.
