IGF-1 receptor is down-regulated by sunitinib induces MDM2-dependent ubiquitination

The insulin like growth factor receptor subtype 1(IGF-1R) plays an important role in cancers transformation and progression. The aim is to investigate the effects of sunitinib on IGF-1R cell signaling transduction, especially on receptor phosphorylation and ubiquitination. In HEK293 cells, IGF-1R signaling pathways are activated in response to IGF-1, which induces obvious phosphorylations of receptor tyrosine and Akt, ERK. However, the phosphorylations of receptor tyrosine, Akt and ERK were significant inhibited by sunitinib. We found that both IGF-1 and sunitinib obviously down regulated the IGF-1R expression. For analysis the ubiquitination, HEK293 cells were simulated with 100 ng/ml IGF-1 or 10 nM sunitinib for 10 min after serum starvation for 24 h. Both IGF-1 and sunitinib could obviously induce the IGF-1R ubiquitination at 10 min compared with control (only serum free, no stimulation), indicating IGF-1 and sunitinib down-regulate the IGF-1R by increasing the receptor degradation through ubiquitination dependent proteasome pathway. We also found that MDM2 combined to IGF-1R in response to sunitinib stimulation. To confirm it, HEK293 cells were transfected with human HA-MDM2 (+MDM2) or siRNA to MDM2 (-MDM2). Following 24 h serum starvation, cells were stimulated with 10 nM sunitinib for 10 min. In over-expressed MDM2 cells, IGF-1R was more ubiquitinated than that in mock-transfected cells (control), and no ubiquitination in -MDM2 cells. These results mean that sunitinib mediates ubiquitination of IGF-1R dependent on MDM2. In summary, sunitinib could block signaling transduction and mediate degradation of IGF-1R.