Targeting heat shock protein 27 (HspB1) interferes with bone metastasis and tumour formation in vivo

Heat shock protein 27 (Hsp27), also called HspB1, is a member of the human small heat shock protein family characterised by a highly conserved α-crystalline domain. This protein has a complex structural organisation that modulates its chaperone activity (Paul et al, 2010). Thus, like other small heat shock proteins, Hsp27 can sequester damaged proteins and prevent their aggregation (Wyttenbach et al, 2002; Arrigo, 2011). This property of molecular sponge leads to increased cell survival, and is correlated with complex patterns of Hsp27 oligomerisation and phosphorylation on three serine residues (Jaya et al, 2009; Paul et al, 2010). Hsp27 is also well characterised to counteract apoptotic cell death induced by different inducers (Mehlen et al, 1996; Paul et al, 2010). It modulates cell death negatively by blocking the apoptotic cascade at different levels. For example, Hsp27 binds and inactivates cytochrome-c released from mitochondria during apoptotic cell death (Bruey et al, 2000), as well as the pro-domain of procaspase-3, and inhibits the activation process of this caspase, two crucial events for apoptosis execution (Voss et al, 2007). Moreover, Hsp27 is able to directly stabilise actin filaments to protect cell integrity in response to heat, reactive oxygen species, cytotoxic agents (Venkatakrishnan et al, 2006; Pivovarova et al, 2007) and some apoptotic inducers, as for example staurosporine (Paul et al, 2002, 2010). Hsp27 is also implicated in radio- and chemo-resistance and can negatively modulate cell death induced by anticancerous cytotoxic agents like cisplatin, adriamycin, etoposide or γ-rays (Zhang and Shen, 2007; Aloy et al, 2008). γ-Ray resistance may be related to the ability of this protein to counteract oxidative stress (Arrigo, 1998). In murine vivo models, a direct demonstration of the tumourigenic and metastatic potential of this protein has been done (Garrido et al, 1998; Gibert et al, 2011). Of interest, Hsp27 is highly expressed in many types of human tumours, particularly in those of carcinoma origin, including ovarian, breast, head and neck cancer. Hsp27 overexpression is considered a bad prognostic for patient survival (Ciocca and Calderwood, 2005). This increase in Hsp27 level is probably due to the pro-survival effect of the protein allowing the proliferative potential of cancerous cells in adverse in vivo conditions. All these faculties implicate Hsp27 as a major therapeutic target in cancer (Arrigo et al, 2007; Gibert et al, 2011). The role and clinical outcome of Hsp27 in primary tumours has been well studied and documented (Ciocca and Calderwood, 2005; Calderwood et al, 2006; Tsuruta et al, 2008; Huang et al, 2010). However, its function in metastasis invasion has been less studied, even though Hsp27 gene has been shown by proteomic analysis to be overexpressed in tumour cells of patients that are able to metastasise (Song et al, 2006; Tian et al, 2007). It is also interesting to note that surface expression of murine Hsp27 stimulates tumour growth and metastasis of the highly metastatic murine 4T1 breast adenocarcinoma cells, whereas silencing of this protein eliminates migration capability (Bausero et al, 2004; Kaur et al, 2011). In the present study, we showed that Hsp27 is overexpressed in tumours that are able to metastasise, in 53 human breast cancer patients. We further investigated the role of human Hsp27 during bone metastasis formation and tumour growth in mice. Bone metastasis invasion is very often a dramatic stage of tumour progression, which frequently occurs and generally leads to patient death. Bone metastases in breast cancer patients are also associated with bone destruction that correlates with hypercalcaemia and fractures. We used the breast MDA-MB231/B02 metastatic cell line as a model to analyse the effect of Hsp27 silencing (Pecheur et al, 2002; Peyruchaud et al, 2003). This cell line displays a unique tropism for bone dissemination and constitutively expresses a high level of Hsp27. The aim of this work was to investigate the role of Hsp27 in bone colonisation by breast cancer cells. We provide here experimental evidence for a role of Hsp27 in migration and invasion of bone tissue by metastatic breast cancer cells.