MDM2-mediated degradation of SIRT6 phosphorylated by AKT1 promotes tumorigenesis and trastuzumab resistance in breast cancer

Sirtuin 6 (SIRT6) is geassocieerd met een lange levensduur en is ook een tumor suppressor. Identificatie van moleculaire regulatoren van SIRT6 zou de activering therapeutisch staat bij kankerpatiënten. In verscheidene borstkankercellijnen, vonden we dat SIRT6 op Ser338 werd gefosforyleerd door de kinase AKT1, die de interactie en ubiquitinatie van SIRT6 geïnduceerd door MDM2, gericht SIRT6 protease-afhankelijke degradatie

De overleving van patiënten met borstkanker positief gecorreleerd met de overvloed aan SIRT6 en omgekeerd gecorreleerd met de fosforylering van SIRT6 op Ser338. In een panel van borsttumor biopten, SIRT6 overvloed omgekeerd gecorreleerd met de overvloed van gefosforyleerd AKT. Remmende AKT of voorkomen SIRT6 fosforylering door mutatie Ser338 voorkomen de afbraak van SIRT6 gemedieerd door MDM2, onderdrukt de proliferatie van borstkankercellen in kweek, en remde de groei van borstkanker xenotransplantaten in muizen. Overexpressie MDM2 verminderde de overvloed aan SIRT6 in cellen, terwijl overexpressie een E3 ligase-deficiënte MDM2 of neerhalen endogene MDM2 toegenomen SIRT6 overvloed. Trastuzumab (bekend als Herceptin) is een geneesmiddel dat een specifieke receptor gebruikelijk in sommige borstkanker richt, en neerhalen SIRT6 verhoogde de overleving van borstkanker cel blootgesteld aan trastuzumab. Overexpressie van een nonphosphorylatable SIRT6 mutant verhoogde trastuzumab gevoeligheid in een resistente borstkanker cellijn. Zo stabiliserende SIRT6 kan een klinische strategie voor het overwinnen van trastuzumab resistentie bij patiënten met borstkanker zijn.

Umadevi Thirumurthi1,2, Jia Shen1,2,

Weiya Xia1,

Adam M. LaBaff1,2,*,

Yongkun Wei1,

Chia-Wei Li1,

Wei-Chao Chang3,

Chung-Hsuan Chen4,5,6,

Hui-Kuan Lin1,2,

Dihua Yu1,2, and

Mien-Chie Hung1,2,3,7,

+ Author Affiliations

†Corresponding author. E-mail: 

* Present address: Department of Cancer Biology, Wake Forest School of Medicine, Winston-Salem, NC 27157, USA.


Sirtuin 6 (SIRT6) is associated with longevity and is also a tumor suppressor. Identification of molecular regulators of SIRT6 might enable its activation therapeutically in cancer patients. In various breast cancer cell lines, we found that SIRT6 was phosphorylated at Ser338 by the kinase AKT1, which induced the interaction and ubiquitination of SIRT6 by MDM2, targeting SIRT6 for protease-dependent degradation. The survival of breast cancer patients positively correlated with the abundance of SIRT6 and inversely correlated with the phosphorylation of SIRT6 at Ser338. In a panel of breast tumor biopsies, SIRT6 abundance inversely correlated with the abundance of phosphorylated AKT. Inhibiting AKT or preventing SIRT6 phosphorylation by mutating Ser338 prevented the degradation of SIRT6 mediated by MDM2, suppressed the proliferation of breast cancer cells in culture, and inhibited the growth of breast tumor xenografts in mice. Overexpressing MDM2 decreased the abundance of SIRT6 in cells, whereas overexpressing an E3 ligase–deficient MDM2 or knocking down endogenous MDM2 increased SIRT6 abundance. Trastuzumab (known as Herceptin) is a drug that targets a specific receptor common in some breast cancers, and knocking down SIRT6 increased the survival of a breast cancer cell exposed to trastuzumab. Overexpression of a nonphosphorylatable SIRT6 mutant increased trastuzumab sensitivity in a resistant breast cancer cell line. Thus, stabilizing SIRT6 may be a clinical strategy for overcoming trastuzumab resistance in breast cancer patients.


U. Thirumurthi, J. Shen, W. Xia, A. M. LaBaff, Y. Wei, C. Li, W. Chang, C. Chen, H. Lin, D. Yu, and M. Hung, MDM2-mediated degradation of SIRT6 phosphorylated by AKT1 promotes tumorigenesis and trastuzumab resistance in breast cancer. Sci. Signal. 7, ra71 (2014).


A Network of Substrates of the E3 Ubiquitin Ligases MDM2 and HUWE1 Control Apoptosis Independently of p53

  1. Manabu Kurokawa1,*,
  2. Jiyeon Kim2,,
  3. Joseph Geradts3,
  4. Kenkyo Matsuura2,
  5. Liu Liu4,
  6. Xu Ran4,
  7. Wenle Xia5,
  8. Thomas J. Ribar2,
  9. Ricardo Henao6,
  10. Mark W. Dewhirst7,
  11. Wun-Jae Kim8,
  12. Joseph E. Lucas6,
  13. Shaomeng Wang4,
  14. Neil L. Spector5, and
  15. Sally Kornbluth2,*

+ Author Affiliations

  1. *Corresponding author. E-mail: (M.K.); (S.K.)
  • Present address: Children’s Medical Center Research Institute, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75390–8502, USA.


In the intrinsic pathway of apoptosis, cell-damaging signals promote the release of cytochrome c from mitochondria, triggering activation of the Apaf-1 and caspase-9 apoptosome. The ubiquitin E3 ligase MDM2 decreases the stability of the proapoptotic factor p53. We show that it also coordinated apoptotic events in a p53-independent manner by ubiquitylating the apoptosome activator CAS and the ubiquitin E3 ligase HUWE1. HUWE1 ubiquitylates the antiapoptotic factor Mcl-1, and we found that HUWE1 also ubiquitylated PP5 (protein phosphatase 5), which indirectly inhibited apoptosome activation. Breast cancers that are positive for the tyrosine receptor kinase HER2 (human epidermal growth factor receptor 2) tend to be highly aggressive. In HER2-positive breast cancer cells treated with the HER2 tyrosine kinase inhibitor lapatinib, MDM2 was degraded and HUWE1 was stabilized. In contrast, in breast cancer cells that acquired resistance to lapatinib, the abundance of MDM2 was not decreased and HUWE1 was degraded, which inhibited apoptosis, regardless of p53 status. MDM2 inhibition overcame lapatinib resistance in cells with either wild-type or mutant p53 and in xenograft models. These findings demonstrate broader, p53-independent roles for MDM2 and HUWE1 in apoptosis and specifically suggest the potential for therapy directed against MDM2 to overcome lapatinib resistance.


M. Kurokawa, J. Kim, J. Geradts, K. Matsuura, L. Liu, X. Ran, W. Xia, T. J. Ribar, R. Henao, M. W. Dewhirst, W. Kim, J. E. Lucas, S. Wang, N. L. Spector, and S. Kornbluth, A Network of Substrates of the E3 Ubiquitin Ligases MDM2 and HUWE1 Control Apoptosis Independently of p53. Sci. Signal. 6, ra32 (2013).

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