A Molecular Cascade Modulates MAP1B and Confers Resistance to mTOR Inhibition in Human Glioblastoma

Abstract

Background

Clinical trials of therapies directed against nodes of the PI3K/AKT/mTOR signaling axis in Glioblastoma (GBM) have had disappointing results. Resistance to mTOR inhibitors limits their efficacy.

Methods

To determine mechanisms of resistance to chronic mTOR inhibition, we performed tandem screens on patient-derived GBM cultures.

Results

An unbiased phosphoproteomic screen quantified phosphorylation changes associated with chronic exposure to the mTOR inhibitor rapamycin and our analysis implicated a role for GSK3B attenuation in mediating resistance that was confirmed by functional studies. A targeted shRNA screen and further functional studies both in vitro and in vivo demonstrated that MAP1B, a protein previously associated predominantly with neurons, is a downstream effector of GSK3B mediated resistance. Furthermore, we provide evidence that chronic rapamycin induces microtubule stability in a MAP1B-dependent manner in GBM cells. Additional experiments explicate a signaling pathway wherein combinatorial ERK/mTOR targeting abrogates inhibitory phosphorylation of GSK3B, leads to phosphorylation of MAP1B, and confers sensitization.

Conclusions

These data portray a compensatory molecular signaling network that imparts resistance to chronic mTOR inhibition in primary, human GBM cell cultures and points towards new therapeutic strategies.