mTOR and PI3 Kinase Inhibitor Synergy Figure 1. Temsirolimus differentially regulates cell viability and Akt phosphorylation in a dose-dependent manner. A, B, Endometrial cancer cells were treated with increasing doses of temsirolimus for 72 hrs. Results are separated by (A) sensitivity or (B) resistance as determined by cell viability. C, D, Phosphorylation of rS6 (P-rS6) and Akt (P-Akt T308 and P-Akt S473) after treatment with indicated doses of temsirolimus for 72 hrs in temsirolimus-sensitive (C) or temsirolimus-resistant (D) cells was determined by Western blotting. Expression of total rS6 and Akt protein serves as loading controls. doi:10.1371/journal.pone.0026343.g001 of rapalog-induced Akt phosphorylation in many cancer cell lines, human xenograft models, and patient tumors [16]. Compensatory hyper-Akt phosphorylation suggests one potential mechanism whereby cells that are initially sensitive escape the growth inhibitory effects of the drug and become secondarily resistant. From these data, we distinguish between primary resistance with a lack of baseline Akt dependence versus acquired resistance demonstrated by hyper-Akt activation in response to temsirolimus. PLoS ONE | www.plosone.org 3 October 2011 | Volume 6 | Issue 10 | e26343