title: Rapamycin induces Bad phosphorylation in association with its resistance to human lung cancer cells.
abstract: Inhibition of mTOR signaling by rapamycin has been shown to activate extracellular signal-regulated kinase 1 or 2 (ERK1/2) and Akt in various types of cancer cells, which contributes to rapamycin resistance. However, the downstream effect of rapamycin-activated ERKs and Akt on survival or death substrate(s) remains unclear. We discovered that treatment of human lung cancer cells with rapamycin results in enhanced phosphorylation of Bad at serine (S) 112 and S136 but not S155 in association with activation of ERK1/2 and Akt. A higher level of Bad phosphorylation was observed in rapamycin-resistant cells compared with parental rapamycin-sensitive cells. Thus, Bad phosphorylation may contribute to rapamycin resistance. Mechanistically, rapamycin promotes Bad accumulation in the cytosol, enhances Bad/14-3-3 interaction, and reduces Bad/Bcl-XL binding. Rapamycin-induced Bad phosphorylation promotes its ubiquitination and degradation, with a significant reduction of its half-life (i.e., from 53.3-37.5 hours). Inhibition of MEK/ERK by PD98059 or depletion of Akt by RNA interference blocks rapamycin-induced Bad phosphorylation at S112 or S136, respectively. Simultaneous blockage of S112 and S136 phosphorylation of Bad by PD98059 and silencing of Akt significantly enhances rapamycin-induced growth inhibition in vitro and synergistically increases the antitumor efficacy of rapamycin in lung cancer xenografts. Intriguingly, either suppression of Bad phosphorylation at S112 and S136 sites or expression of the nonphosphorylatable Bad mutant (S112A/S136A) can reverse rapamycin resistance. These findings uncover a novel mechanism of rapamycin resistance, which may promote the development of new strategies for overcoming rapamycin resistance by manipulating Bad phosphorylation at S112 and S136 in human lung cancer.
authors: State Key Laboratory of Cancer Biology, Department of Biopharmaceutics, School of Pharmacy, Fourth Military Medical University, Xi'an, People's Republic of China.
authors: Yan
authors: Y
authors: Liu
authors: Shi-Yong
authors: SY
authors: Sun
authors: Taofeek K
authors: TK
authors: Owonikoko
authors: Gabriel L
authors: GL
authors: Sica
authors: Walter J
authors: WJ
authors: Curran
authors: Fadlo R
authors: FR
authors: Khuri
authors: Xingming
authors: X
authors: Deng
mesh_terms: D017209:Apoptosis / Q000502:physiology; D045744:Cell Line, Tumor; D019008:Drug Resistance, Neoplasm; D048049:Extracellular Signal-Regulated MAP Kinases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D005419:Flavonoids / Q000494:pharmacology; D006801:Humans; D008175:Lung Neoplasms / Q000378:metabolism* / Q000473:pathology; D010766:Phosphorylation; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism*; D034622:RNA Interference; D034741:RNA, Small Interfering; D015398:Signal Transduction; D020123:Sirolimus / Q000494:pharmacology*; D058570:TOR Serine-Threonine Kinases / Q000037:antagonists & inhibitors / Q000187:drug effects; D054875:Ubiquitination; D051025:bcl-Associated Death Protein / Q000378:metabolism*
chemical_list: C494819:BAD protein, human; D005419:Flavonoids; D034741:RNA, Small Interfering; D051025:bcl-Associated Death Protein; C546842:MTOR protein, human; D051057:Proto-Oncogene Proteins c-akt; D058570:TOR Serine-Threonine Kinases; D048049:Extracellular Signal-Regulated MAP Kinases; C093973:2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one; D020123:Sirolimus