Abstract LB-009: Mnk targeting enhances vulnerability of medulloblastoma stem-like cancer cells to PI3K-p110alpha inhibition
Authors: Bell Jonathan B. , Eckerdt Frank , Ma Quanhong , Clymer Jessica R. , Goldman Stewart , Hashizume Rintaro , Platanias Leonidas C. 
Published: July 2016

Our recent work suggested that inhibition of mTORC1 activates Mnk in a PI3K-dependent manner, thereby providing a survival mechanism for medulloblastoma cells. The PI3K-Akt axis represents an important survival pathway that also confers therapy resistance to medulloblastoma stem cells, resulting in tumor recurrence. Here, we investigated a role for p110 isoforms of PI3K in medulloblastoma stem-like cancer cell biology and studied the potential of Mnk inhibition for sensitizing medulloblastoma stem-like cancer cells and orthotopic xenograft tumors to PI3K inhibition.

We used medulloblastoma cell lines Daoy and D556 grown as conventional 2-D cultures or under stem-cell conditions as 3-D neurospheres to elucidate the roles of PI3K-Akt signaling in medulloblastoma proliferation, colony formation and stem cell function. We employed extreme limiting dilution analysis (ELDA) to ask whether concomitant Mnk inhibition enhances antineoplastic effects of PI3K inhibition on cancer stem cell growth. Additionally, in a preliminary intracereballar xenograft mouse study, we investigated the effects of pharmacologic PI3K and Mnk inhibition.

We found that Akt activity greatly increased when 2-D cultures were converted into 3-D neurospheres. This Akt activation coincided with increased expression of CD133 and nestin, suggesting an important role for PI3K-Akt signaling in medulloblastoma stem cells. The p110a specific inhibitor BYL-719 blocked this Akt activation in neurospheres indicating this Akt activation is mediated by p110a. Consistently, of all class I PI3K catalytic isoforms (p110a, p110b, p110d and p110g) only knockdown of p110a disrupted stem cell frequencies in ELDA. We previously reported that mTORC1 inhibition engages Mnk signaling in a negative feedback manner to promote survival. Here we show that Mnk inhibition by CGP57380 sensitized medulloblastoma cells for pharmacologic inhibition and siRNA-mediated knockdown of p110a both in 2-D cancer cells and 3-D stem-like cancer cell cultures. After intracerebellar injection of medulloblastoma cells in nude mice, we found that combined targeting of PI3K-p110a and Mnk results in inhibition of tumor growth and increased survival.

In summary, pharmacologic inhibition of PI3K-p110a by BYL-719 showed potent activity against medulloblastoma cells and stem-like cancer cells. Knockdown of p110a disrupted cancer stem cell frequency in ELDA and this effect was greatly enhanced by pharmacologic inhibition of Mnk. Finally, in an orthotopic mouse model we found that concomitant inhibition of p110a and Mnk prolonged survival and reduced tumor size. The striking effects of concomitant Mnk inhibition on stem-like cancer cells, neurospheres and tumors is particularly interesting as it suggests enhanced vulnerability of the therapy-resistant, tumor-initiating cancer stem cell population to p110a inhibition in medulloblastoma.

Citation Format: Jonathan B. Bell, Frank Eckerdt, Quanhong Ma, Jessica R. Clymer, Stewart Goldman, Rintaro Hashizume, Leonidas C. Platanias. Mnk targeting enhances vulnerability of medulloblastoma stem-like cancer cells to PI3K-p110alpha inhibition. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr LB-009.