Insulin-like growth factor-binding protein-1 (IGFBP-1) regulates human schwannoma proliferation, adhesion and survival

Abstract

Merlin is a tumour suppressor involved in the development of a variety of tumours including mesotheliomas. Neurofibromatosis type 2 (NF2), a dominantly inherited tumour disease, is also caused by loss of merlin. NF2 patients suffer from multiple genetically well-defined tumours, schwannomas are most frequent among those. Using our in vitro model for human schwannoma, we found that schwannoma cells display enhanced proliferation because of the overexpression/activation of platelet-derived growth factor receptor and ErbB2/3, increased cell-matrix adhesion because of the overexpression of integrins, and decreased apoptosis. Mechanisms underlying schwannomas basal proliferation and cell-matrix adhesion are not understood. Here, we investigated insulin-like growth factor-binding protein-1 (IGFBP-1), which is expressed and released from central nervous system tumours and strongly overexpressed in schwannoma at the mRNA level. IGFBP-1 acts via b1-integrin and focal-adhesion-kinase (FAK), which are strongly overexpressed and basally activated in schwannoma. Using short hairpin RNA knockdown, small inhibitors and recombinant IGFBP-1, we demonstrate that schwannoma cells, in contrast to Schwann cells, release IGFBP-1 that activates the Src/FAK pathway, via integrin b1, potentiating schwannoma's proliferation and cell-matrix adhesion. We show that FAK localizes to the nucleus and Src triggers IGFBP-1 production. Further, we observed downregulation of the tumour-suppressor phosphatase and tensin homolog in schwannoma cells leading to increased activity of antiapoptotic AKT. Thus, IGFBP-1/integrin b1/Src/FAK pathway has a crucial role in merlin-related tumourigenesis and therefore represents an important therapeutic target in the treatment of merlin-deficient tumours.

Merlin, a multi‐suppressor from cell membrane to the nucleus

“…Thus FAK/Src are activated downstream of integrins after merlin loss. RNA interference experiments have confirmed that integrin b1 directly functions upstream of FAK, as knockdown of integrin b1 completely abolished IGFBP1 (acting via integrin alpha 5) mediated FAK phosphorylation/activity at Y397 in human schwannoma cells [14]. Furthermore, the nuclear localisation of FAK in schwannoma cells suggests its regulation of proliferation in the cells without merlin [14].…”

“…It has been described that activated Src recruits FAK and Paxillin in NF2 À/À mouse astrocyte cells [35]. In addition, both Src activity (Y416) and FAK phosphorylation at Y397 and Y925 (Src phosphorylation site) are significantly increased in schwannoma cells compared with Schwann cells [14,33]. Thus FAK/Src are activated downstream of integrins after merlin loss.…”

“…The upregulation of integrins a6, b1 and b4 was confirmed in an array analysis when mRNA from schwannomas was compared to Schwann cells [13]. Further activation of integrin has been linked to merlin deficiency [14,15]. Indeed, knockdown of integrin b1 with lentiviral shRNA in human schwannoma cells decreased the proliferation and adhesion upon IGFBP stimulation [14].…”

“…Further activation of integrin has been linked to merlin deficiency [14,15]. Indeed, knockdown of integrin b1 with lentiviral shRNA in human schwannoma cells decreased the proliferation and adhesion upon IGFBP stimulation [14].…”

“…Due to their activation in many tumours FAK/Src are considered as promising therapeutic targets [32]. In NF2 À/À (merlin null) tumours cells it has also been shown that FAK functions upstream of PI3K/AKT and Raf/MEK/ERK pathways to potentiate schwannoma proliferation, migration and cell survival [14,33,34]. In mouse embryonic fibroblasts (MEFs), it has been shown that FAK phosphorylation at Tyr397, which is an auto-phosphorylation site triggered by integrin, and FAK's connections with Src and PI3K are tightly regulated by merlin's expression [34].…”