Statins hydroxy methylglutaryl coenzyme A HMG CoA reductase

Statins, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors, include lovastatin, simvastatin, mevastatin, fluvastatin, pravastatin, atorvastatin, rosuvastatin and cerivastatin. Statins have been widely used for lowering cholesterol, but recent research reveal other roles, such as anti-inflammation, immunomodulation, neuroprotection and antitumor effects (Zhang et al., 2013). During the last decade, the role of statins in cancer prevention or therapy has been controversial, but accumulating evidence based on retrospective analyses support that statins could be beneficial to patients with certain types of tumors (Nielsen et al., 2012; Singh et al., 2013; Hamilton et al., 2014). In addition, numerous experimental studies have revealed that statins have a role in inhibiting tumor growth and metastasis (Campbell et al., 2006; Clendening and Penn, 2012; Hindler et al., 2006). However, the underlying mechanisms are complex, and the working hypothesis can be divided into Rho GTPase dependent and independent mechanisms. Rho GTPases are considered to be important signaling molecules that drive tumor growth and metastasis. Statins may inhibit Rho GTPase activation by interfering with mevalonate pathway and Rho geranylgeranylation, a key posttranslational modification for Rho GTPase biological activities (Collisson et al., 2003; Kusama et al., 2001; Tsubaki et al., 2015). Furthermore, statins involve the innate immune response against human metastatic melanoma ABT-263 by inducing MHC class I Chain-related protein A (MICA) membrane expression. As a result, independent of Ras and Rho GTPase signaling pathways, melanoma cells are more sensitive to lysis by NK cells (Pich et al., 2013). In addition, simvastatin may prevent triple-negative breast cancer metastasis via regulation of FOXO3a (Wolfe et al., 2015). Although statins may trigger autophagy in a few cancer cells through inhibition of geranylgeranylation (Araki et al., 2012; Yang et al., 2010; Parikh et al., 2010), whether statin-induced autophagy plays a role in promotion or suppression of cancer metastasis is largely unknown. Only few in vitro studies showed that inhibition of autophagy may enhance statin-induced apoptosis or cytotoxicity in some cancer cell lines (Yang et al., 2010; Misirkic et al., 2012; Kang et al., 2014). The role of autophagy in tumorigenesis and progression is quite complicated, because induction of autophagy could either accelerate the tumor progression or suppression (Zhang et al., 2013; Su et al., 2015a). Many studies emphasize cancer stages, cancer types and tumor microenvironments as the determining factors. In our current study, we validate that statins prevent lung adenocarcinoma bone metastasis though triggering autophagy. Moreover, we propose a mechanism by which autophagy serves a positive role in suppression of cancer bone metastasis. p53, the most well-known tumor suppressor, serves a critical role in safeguarding the integrity of the genome. After activation by intracellular stresses, such as DNA damage, hypoxia and oncogene activation, p53 initiates a series of cell events including apoptosis, cell cycle arrest and DNA repair to suppress tumorigenesis (Jin, 2005). In recent years, p53 has been revealed to play an important role in either inducing or suppressing autophagy depending on its subcellular localization. Specifically, nuclear p53 acts as a transcription factor to activate a series of pro-autophagic genes; cytoplasmic p53 serves a role in repressing autophagy via unclear mechanisms (Maiuri et al., 2010; Tasdemir et al., 2008). In this study, we investigate the involvement of p53 in both fluvastatin-induced autophagy and suppression of lung adenocarcinoma bone metastasis and determine the signaling pathway bridging p53 to autophagy.
Materials & Methods
Results First, we examined the role of fluvastatin in the regulation of lung adenocarcinoma cell migration. As shown in Fig. 1A, fluvastatin significantly delayed the wound-healing time in A549 and SPC-A-1 cells compared with the control. Consistent with the wound-healing result, Matrigel invasive assay revealed that fluvastatin strongly prevented the invasion of adenocarcinoma cells (Fig. 1B). To further investigate the anti-metastatic role of fluvastatin in vivo, we established a lung adenocarcinoma bone metastatic model in nude mice by injecting SPC-A-1 cells into the heart\'s left ventricle. Bioluminescence imaging showed that tumor dissemination was effectively blocked by fluvastatin treatment, and overall, this treatment had greater efficacy than the positive anti-bone metastatic drug denosumab (Fig. 1C and D). Furthermore, X-ray images also showed reduced lesions and damage on bones after the fluvastatin treatment (Fig. 1C and E). H&E staining in Fig. 1C illustrated deep purple tumor cells with irregularly shaped nucleus occupying the bone marrow cavity and replacing normal bone marrow cells in sections from untreated mice. Moreover, metastatic lung adenocarcinoma cells caused abrupt lysis of long bones, reduction in trabecular bone density and gradual destruction of cortical bone. Bone sections from denosumab-treated mice showed a mixture of tumor and bone marrow cells in tibia but the tumor cells were restricted to the cortical bone. Fluvastatin treatment resulted in a complete reduction in the number of tumor cells in tibia and lesions of bone structure. These results suggest an inhibitory role of fluvastatin in lung adenocarcinoma metastasis, specifically in bone metastasis.