Targeting mTOR downstream of LIN28 in Atypical Teratoid Rhabdoid Tumors promotes apoptosis and suppresses tumorigenicity

Jeffrey Rubens, Antoinette Price, Charles Eberhart, Eric Raabe
Johns Hopkins Hospital, Baltimore, MD, USA

LIN28A and LIN28B are stem cell factors that regulate thousands of RNAs and are expressed in aggressive cancers. One of the canonical downstream targets of LIN28 is the mTOR pathway. We have previously shown that overexpression of LIN28 in Atypical Teratoid Rhabdoid Tumors (AT/RT) contributes to the aggressive nature of the tumor. We hypothesized that AT/RT would have increased expression of mTOR signaling due to high-level expression of LIN28. We found that primary human AT/RT samples do have high levels of P-S6 and P-AKT expression by immunohistochemistry staining (mean H-score for P-S6 36.3; p-AKT 150.4). The dual TORC1/2 inhibitor MLN0128 has good brain penetration and is currently being tested in phase I clinical trials in children. Treatment of AT/RT cell lines with MLN0128 inhibits TORC1/2 targets in vitro and suppresses cell proliferation at 10nM concentration (MTS assay for BT12 p=0.0034 vs DMSO control; BT37 p=0.0064 vs DMSO control; and CHLA-06 p=0.002 vs DMSO control by t-test). MLN0128 also slows cell proliferation via BrdU assay (BT-37 p=0.0241 10nM vs DMSO control; CHLA-06 p=0.045 10nM vs DMSO control by t-test) and induces apoptosis measured by Western blot for PARP and cleaved caspase 3 assay (BT-37 p=0.001 100nM vs DMSO; CHLA-06 p=0.007 10nM vs DMSO by t-test). MLN0128 treatment of AT/RT xenograft mouse models leads to rapid reduction in tumor volume compared to controls treated with vector alone (61.9% average reduction in tumor volume with MLN0128 treatment vs 272.1% average increase in tumor volume in vector control; p=0.006 by t-test). Targeting the overexpressed mTOR pathway with the TORC1/TORC2 inhibitor MLN0128 leads to potent in vitro and in vivo activity against AT/RT. MLN0128 may be a candidate for future clinical trials to treat this aggressive tumor.