Combination of Solamargine and Metformin Strengthens IGFBP1 Gene Expression Through Inactivation of Stat3 and Reciprocal Interaction Between FOXO3a and SP1.

BACKGROUND/AIMS: Solamargine, one natural photochemical component from traditional plants, has been shown to have anti-cancers properties. We previously showed that solamargine inhibited the growth of non-small-cell lung cancer (NSCLC) cells through suppression of prostaglandin E2 (PGE2) receptor EP4 gene and regulation of downstream signaling pathways. However, the detailed mechanism underlying this, especially in combination of metformin, a known AMPK activator, still remained to be determined.

METHODS: Cell viability was measured using a 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) and colorimetric 5-bromo-2-deoxyuridine (BrdU) ELISA methods, respectively. Western blot analysis and immunohistochemistry were performed to examine the phosphorylation and protein expressions of signal transducer and activator of transcription 3 (Stat3), SP1, forkhead box O3a (FOXO3a), and insulin-like growth factor (IGF)-IGF binding protein 1 (IGFBP1). The expression of IGFBP1 mRNA was measured by quantitative real time PCR (qRT-PCR). Silencing of FOXO3a and IGFBP1 were examined by siRNA procedures. Exogenously expression of SP1, FOXO3a, and IGFBP1 were carried out by transient transfection assays. The promoter activity of IGFBP1 was tested using Secrete-PairTM Dual Luminescence Assay Kit. A xenografted tumor model was used to further test the effect of solamargine in combining with metformin in vivo.

RESULTS: We further demonstrated that solamargine inhibited growth and induced cell cycle arrest in other NSCLC cell lines. Through mechanism-based approaches, we showed that solamargine decreased the phosphorylation of Stat3; In addition, solamargine induced FOXO3a, whereas reduced SP1 protein levels; all of which were abrogated in cells with overexpressed Stat3 gene. Interestingly, there is interaction between FOXO3a and SP1. Moreover, solamargine increased mRNA, protein expression and promoter activity of IGFBP1, which was not observed in cells with overexpressed SP1 or with silenced FOXO3a genes. Finally, ablation of IGFBP1 expression by siRNA blocked the effect of solamargine on cell growth inhibition. More importantly, there was a synergy of combination of solamargine and metformin. Similar findings were also observed in vivo.

CONCLUSION: Our results show that solamargine increases IGFBP1 gene expression through inactivation of Stat3, followed by regulation and reciprocal interaction of FOXO3a and SP1 in vitro and in vivo. This ultimately leads to suppression of human lung cancer cell growth. Moreover, this is a synergy of solamargine in combination with metformin in this process. This study unravels a novel mechanism underlying the anti-lung cancer effects of solamargine in combination of metformin, and suggests a potential new lung cancer associated therapy.