A network of substrates of the E3 ubiquitin ligases MDM2 and HUWE1 control apoptosis independently of p53.

Abstract	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.
Authors	Manabu Kurokawa, Jiyeon Kim, Joseph Geradts, Kenkyo Matsuura, Liu Liu, Xu Ran, Wenle Xia, Thomas J Ribar, Ricardo Henao, Mark W Dewhirst, Wun-Jae Kim, Joseph E Lucas, Shaomeng Wang, Neil L Spector, Sally Kornbluth
Journal	Science signaling (Sci Signal) Vol. 6 Issue 274 Pg. ra32 (May 07 2013) ISSN: 1937-9145 [Electronic] United States
PMID	23652204 (Publication Type: Journal Article, Research Support, N.I.H., Extramural, Research Support, Non-U.S. Gov't)
Chemical References	Imidazoles MCL1 protein, human Myeloid Cell Leukemia Sequence 1 Protein Nuclear Proteins Piperazines Protein Kinase Inhibitors Quinazolines Tumor Suppressor Protein p53 Tumor Suppressor Proteins Lapatinib nutlin 3 HUWE1 protein, human Proto-Oncogene Proteins c-mdm2 Ubiquitin-Protein Ligases Receptor, ErbB-2 Phosphoprotein Phosphatases protein phosphatase 5
Topics	Animals Apoptosis Breast Neoplasms (drug therapy, metabolism, pathology) Cell Line, Tumor Drug Resistance, Neoplasm (drug effects) Female Humans Imidazoles (pharmacology) Immunoblotting Lapatinib Mice Mice, Nude Myeloid Cell Leukemia Sequence 1 Protein (metabolism) Nuclear Proteins (metabolism) Phosphoprotein Phosphatases (metabolism) Piperazines (pharmacology) Protein Kinase Inhibitors (pharmacology) Proto-Oncogene Proteins c-mdm2 (antagonists & inhibitors, genetics, metabolism) Quinazolines (pharmacology) RNA Interference Receptor, ErbB-2 (antagonists & inhibitors, metabolism) Signal Transduction (drug effects) Substrate Specificity Tumor Suppressor Protein p53 (genetics, metabolism) Tumor Suppressor Proteins Ubiquitin-Protein Ligases (genetics, metabolism) Xenograft Model Antitumor Assays

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