As shown in Figures 2E and 2F, both STAT5 and STAT6 inhibition led to a significantly decreased survival after 4 Gy in all cell lines. For STAT6 inhibition this was only an additive effect, while STAT5 inhibition and 4 Gy had a supra additive ef fect on cell many survival in UT SCC40. Both pSTAT5 and pSTAT6 levels were low and difficult to detect on western blot. Reduction of pSTAT5 was observed in UT SCC40 and of pSTAT6 in UT SCC5 and UT SCC40. Discussion In this study, an antibody based array was used to de termine which activated kinases involved in growth fac tor signaling were correlated with radiosensitivity in HNSCC. This screen resulted in multiple kinases of dif ferent pathways, which could be potential targets to in crease radiosensitivity.

Pathways known to be associated with radiosensitivity were found, including the RAS/ RAF/ERK and the PI3 K/AKT pathways, valida ting our approach. In addition, kinases Inhibitors,Modulators,Libraries not known to be involved in radiosensitivity were Inhibitors,Modulators,Libraries identified, including STAT5 and STAT6. Moreover, inhibitors of these kinases were able to decrease survival after radiotherapy, par ticularly inhibitors against MEK1/2, STAT5 and STAT6. Hence, these kinases represent potential new targets Inhibitors,Modulators,Libraries to improve outcome after radiotherapy in HNSCC patients. The PI3 K/AKT pathway has been shown to regulate important cell survival mechanisms that Inhibitors,Modulators,Libraries induce radiore sistance, including DNA repair and proliferation. Hence, inhibition of this pathway has been shown to be a major mechanism for the radiosensitizing effect of EGFR inhibitors and this is strengthened by the observation that Inhibitors,Modulators,Libraries activation of AKT has been implicated in resistance to EGFR inhibition.

Here, we show that pAKT inhibition via MK 2206 can decrease survival after radiotherapy. This effect was supra additive in one cell line, indicating that pAKT inhibition specifically decreased survival after radiotherapy in this cell line. However, pAKT inhibition did not decrease survival in all cell lines we tested, despite consistently good inhib ition of pAKT levels. Lenalidomide 191732-72-6 Several mechanisms could explain this difference in radiosensitizing effect of MK 2206 between cell lines. Firstly, the importance of AKT activity for cell survival could differ between cell lines. for example also other kinases were highly ex pressed in resistant line UT SCC5, and, therefore, inhib ition of pAKT would not be deleterious for all cell lines. Moreover, numerous feedback systems are present be tween growth factor receptors and their downstream pathways, whereby inhibition of one kinase can lead to activation of receptors and consequently activation of other downstream pathways. These feedback me chanisms can greatly impact the sensitivity of cells to kinase inhibitors.