RCHY1 and OPTN are required for melanophagy, selective autophagy of melanosomes.

Melanosomes are specific organelles dedicated to melanin synthesis and accumulation in melanocytes. Autophagy is suggestively involved in melanosome degradation, although the potential underlying molecular mechanisms remain elusive. In selective autophagy, autophagy receptors and E3-ligases are the key factors conferring cargo selectivity. In B16F10 cells, ß-mangostin efficiently induced melanosome degradation without affecting other organelles such as mitochondria, peroxisomes, and the endoplasmic reticulum. Among various autophagy receptors, optineurin (OPTN) contributes TANK-binding kinase 1 (TBK1)-dependently to melanosome degradation and its knockdown inhibited ß-mangostin-mediated melanosome degradation. OPTN translocation to melanosomes was dependent on its ubiquitin-binding domain. Moreover, OPTN-mediated TBK1 activation and subsequent TBK1-mediated S187 OPTN phosphorylation were essential for melanosome degradation. ß-mangostin increased K63-linked melanosome ubiquitination. Finally, the E3-ligase RCHY1 knockdown inhibited the melanosome ubiquitination required for OPTN- and TBK1-phosphorylation as well as melanosome degradation. This study suggests that melanophagy, melanosome-selective autophagy, contributes to melanosome degradation, and OPTN and RCHY1 are an essential autophagy receptor and a E3-ligase, respectively, conferring cargo selectivity in melanophagy.

ERK3 Increases Snail Protein Stability by Inhibiting FBXO11-Mediated Snail Ubiquitination.

Snail is a key regulator of the epithelial-mesenchymal transition (EMT), the key step in the tumorigenesis and metastasis of tumors. Although induction of Snail transcription precedes the induction of EMT, the post-translational regulation of Snail is also important in determining Snail protein levels, stability, and its ability to induce EMT. Several kinases are known to enhance the stability of the Snail protein by preventing its ubiquitination; however, the precise molecular mechanisms by which these kinases prevent Snail ubiquitination remain unclear. Here, we identified ERK3 as a novel kinase that interacts with Snail and enhances its protein stability. Although ERK3 could not directly phosphorylate Snail, Erk3 increased Snail protein stability by inhibiting the binding of FBXO11, an E3 ubiquitin ligase that can induce Snail ubiquitination and degradation, to Snail. Importantly, functional studies and analysis of clinical samples indicated the crucial role of ERK3 in the regulation of Snail protein stability in pancreatic cancer. Therefore, we conclude that ERK3 is a key regulator for enhancing Snail protein stability in pancreatic cancer cells by inhibiting the interaction between Snail and FBXO11.