Introduction: Despite improved understanding of the pathogenesis of glioblastoma (GBM), outcomes have changed little in recent decades. The prognosis of GBM remains poor with a 14-month median survival. Therefore, investigation of novel anti-tumor therapies is imperative. Propranolol is a beta-1/beta-2 adrenergic receptor (BAR1, BAR2) antagonist used for the treatment of hypertension. Propranolol was fortuitously discovered to harbor anti-tumor effects in infantile hemangiomas, thought to be due to inverse agonism of BARs leading to decreased intracellular cAMP levels. Preclinical studies suggest broad anti-tumor activity against other solid tumors. We therefore investigated whether propranolol may have a role in the treatment of GBM.
Methods: Four GBM cell lines (U251, S635, GL261 and 9L) were cultured and treated with propranolol or metoprolol (another BAR antagonist). Cell viability, quantitative RT-PCR, flow-cytometry and migration assays were performed using clinically relevant concentrations of propranolol. RNA interference (RNAi) with siRNA was used to determine the dependency of propranolol anti-tumor activity on BAR1/BAR2 expression.
Results: Propranolol treatment reduced GBM viability with an IC50 of 100uM 24-hours post-exposure. Metoprolol did not affect cell survival up to 200uM. Treated cells demonstrated a 6-fold reduction in migratory capacity and 400% increase in apoptosis. Propranolol led to a 10-fold upregulation of VEGF expression (p=0.02) with no effect on other HIF inducible genes. An increase in Bax:Bcl-2 expression was consistent with observed pro-apoptotic effects. A receptor independent mechanism of propranolol was suggested by failure of 5mM 8-Br-cAMP to rescue propranolol treated cells. Similarly, siRNA against BAR1/BAR2 did not affect treatment response to propranolol.
Conclusions: Propranolol induces apoptosis and decreases glioma viability in-vitro. This result appears specific to propranolol and is not observed with other beta antagonists such as metoprolol. Consistent with previous findings in infantile hemangioma, VEGF transcription is paradoxically increased. Preliminary data suggests propranolol may exert its anti-tumor effects independent of BAR expression.
Patient Care: Glioblastoma (GBM) remains a uniformly fatal disease. Despite improvements in image-guided surgery, chemotherapy and radiation, overall survival remains poor. Furthermore, treatment strategies targeting pathways central to GBM, such as bevacizumab, have not led reliably to improved overall survival rates. Propranolol is an FDA approved, orally administered, well tolerated beta antagonist that readily crosses the blood brain barrier. Serendipitous discoveries demonstrated that propranolol has anti-tumor effects in infantile hemangiomas by augmenting the pseudohypoxia pathway. Furthermore, propranolol shows promising results in other malignancies such as melanoma, breast and prostate cancer. The work reported herein lays the framework for pre-clinical animal studies and perhaps eventual clinical trials. Strengthened by the well-documented safety profile of propranolol and its low cost of administration, these findings encourage further investigation into the potential use of propranolol for treatment of GBM.
Learning Objectives: 1) Describe the current treatment strategies and prognosis for GBM
2) Describe the anti-tumor effects of propranolol for GBM in-vitro
3) Detail potential mechanisms by which propranolol exerts anti-proliferative effects on GBM and other solid tumors
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