Rapamycin inhibition of the Akt/mTOR pathway blocks select stages of VEGF-A driven angiogenesis in part through the blockade of S6Kinase

289 Objective_We evaluated the stages of VEGF-A driven angiogenesis inhibited by therapeutic doses of rapamycin and the potential role of S6K in the earliest stage of the VEGF-response.
 Methods and Results_Adenovirus expressing VEGF-A was introduced to the ear of adult mice and a series of 4 key developmental stages of vascular and lymphatic morphogenesis were followed. Rapamycin was used to inhibit each stage independently to assess its impact on the vascular and lymphatic response. Rapamycin (0.5mg/kg) effectively inhibited mTOR signaling to S6K and partially inhibited pSer473Akt signaling likely through feedback from TORC2. The effects on the vasculature were observed at the earliest stages through the inhibition of mother vessels and vascular permeability, and at the mid-stages through the near-complete elimination of glomeruloid vessels. However, vascular malformations and lymphatics remained unaffected. In this study we used retroviral delivery to alter gene expression in the dividing cells responding to VEGF-A. We demonstrated that S6K signaling has an important role to play in the VEGF-A response at these early stages using S6K siRNA and dominant active S6K gene expression in vivo.
 Conclusions_Rapamycin has potent effects on VEGF-A driven angiogenesis, but notably not on VEGF-A driven lymphangiogenesis. Moreover, in addition to the role that inhibition of Akt signaling plays in these effects, rapamycin inhibition of S6K signaling is also likely to be a major component of rapamycin’s inhibition of VEGF-A driven angiogenesis.