BMP2 sensitizes glioblastoma stem-like cells to Temozolomide by affecting HIF-1α stability and MGMT expression

Glioblastoma multiforme (GBM) is the most common brain tumour, characterized by a central and partially necrotic (i.e., hypoxic) core enriched in cancer stem cells (CSCs). We previously showed that the most hypoxic and immature (i.e., CSCs) GBM cells were resistant to Temozolomide (TMZ) in vitro, owing to a particularly high expression of O6-methylguanine-DNA-methyltransferase (MGMT), the most important factor associated to therapy resistance in GBM. Bone morphogenetic proteins (BMPs), and in particular BMP2, are known to promote differentiation and growth inhibition in GBM cells. For this reason, we investigated whether a BMP2-based treatment would increase TMZ response in hypoxic drug-resistant GBM-derived cells. Here we show that BMP2 induced strong differentiation of GBM stem-like cells and subsequent addition of TMZ caused dramatic increase of apoptosis. Importantly, we correlated these effects to a BMP2-induced downregulation of both hypoxia-inducible factor-1 alpha (HIF-1 alpha) and MGMT. We report here a novel mechanism involving the HIF-1 alpha-dependent regulation of MGMT, highlighting the existence of a HIF-1 alpha/MGMT axis supporting GBM resistance to therapy. As confirmed from this evidence, over-stabilization of HIF-1 alpha in TMZ-sensitive GBM cells abolished their responsiveness to it. In conclusion, we describe a HIF-1 alpha-dependent regulation of MGMT and suggest that BMP2, by down-modulating the HIF-1 alpha/MGMT axis, should increase GBM responsiveness to chemotherapy, thus opening the way to the development of future strategies for GBM treatment. Cell Death and Disease (2012) 3, e412; doi:10.1038/cddis.2012.153; published online 18 October 2012