Efficacy Of Combined Inhibition Of MTOR And ERK/MAPK Signaling Pathways In Treating A Tuberous Sclerosis Complex Cell Model

Tuberous Sclerosis Complex(TSC) is a systematic tumor syndrome with the characteristics of multiple tissues' abnormality especially hamartoma.Since there is no cure for TSC,patients may develop severe mental retardation and succumb to renal or respiratory failure.TSC is arisen by autosomal dominant inactive mutations of either TSC1 or TSC2 tumor suppressor gene.It is divided into TSC1 or TSC2 two types according to different gene location to TSC1 or TSC2,of which TSC2 is more common in clinic.Inactivation of TSC1/TSC2 protein complex causes hyperactivation of the mammalian target of rapamycin(mTOR) which leads to uncontrolled cell growth and proliferation.Rapamycin is the inhibitor of mTOR,and considered commonly as the potential drug to cure TSC.However, recent clinical trials of targeted suppression of mTOR have just yielded modest success in patients with TSC and also had the drug resistant problems.We proposed that abrogation of the newly identified mTOR-mediated negative feedback regulation on ERK mitogen-activated protein kinase(MAPK) signaling pathway,in addition to on the well-documented RTK/PI3K/AKT signaling cascade, could limit the efficacy of mTOR inhibitors in the treatment of TSC patients.That is to say,inhibiting mTOR will simultaneously recover the mTOR's feedback inhibition to ERK/MAPK pathway and cause the activation of the pathway unfortunately.Furthermore,dual inhibition of mTOR and ERK/MAPK pathways would overcome the disadvantage of single agent therapy and boost the efficacy of mTOR targeted therapy for TSC patients.We therefore tested our hypothesis in a TSC cell model(Tsc2-/- mouse embryonic fibroblast cell).Here we report that mTOR suppression with mTOR inhibitor,rapamycin(sirolimus),led to up-regulation of ERK/MAPK signaling in mouse Tsc2 knockout cells,which is coincidence with the former reports.This augmented activation of ERK/MAPK singling was attenuated by concurrent administration of MEK1/2 inhibitor, PD98059.The two drugs of the mTOR's inhibitor rapamycin and the MEK's inhibitor PD98059 act through additive rather than synergism effect.Furthermore, when compared with monotherapy,combinatorial application of rapamycin and PD98059 had greater inhibitory effect on the proliferation of Tsc2 deficient cells. We thus suggest that combined suppression of mTOR and MAPK signaling pathways may have the advantage over single mTOR inhibition in the treatment of TSC patients.This strategy may be a new method in the treatment of TSC patients.