Background: Humanin, a secreted bioactive peptide, suppresses a variety of cell toxicities. Results: An intracellular protein SH3BP5, which directly inhibits JNK activity, is identified as an effector of Humanin. Conclusion: SH3BP5 is a physiological inhibitor of JNK and the firstly identified effector of Humanin. Significance: The discovery provides a novel mechanistic insight into the actions of JNK and Humanin.

Humanin, a short bioactive peptide, inhibits a variety of cell deaths. Humanin-mediated inhibition of neuronal cell death, caused by an Alzheimer's disease (AD)-linked mutant gene occurs via binding of Humanin to its heterotrimeric Humanin receptor (htHNR), which results in the activation of the Janus-associated kinases (JAKs) and signal transducer and activator and transcription 3 (STAT3) signaling pathway. A previous study demonstrated that the Humanin-induced activation of the htHNR/JAK2/STAT3 signaling pathway leads to increased expression of SH3 domain-binding protein 5 (SH3BP5), which is an essential effector of Humanin's anti-cell death activity in some cultured neuronal cells. However, it remains unknown whether SH3BP5 is the sole effector of the Humanin signaling pathway via htHNR/JAKs/STAT3. Here we show that the Humanin signaling pathway via htHNR/JAKs/STAT3 increased the expression levels of mRNA and protein of Apollon/Bruce, an unusual member of the inhibitors of apoptosis proteins, and that overexpression of Apollon/Bruce inhibits neuronal death, caused by a London-type familial AD-linked mutant (V642I) of amyloid β precursor protein. Overall, the results indicate that expression of Apollon/Bruce is upregulated by Humanin, and Apollon/Bruce could be an effector of Humanin in a context-dependent manner.

We suggest that our assay technology might be of clinical value in treating patients with cancer, cerebral ischemia, or patients who need a prompt and predictive diagnosis for adequate treatment.