Autophagy is a highly conserved catabolic process that degrades and recycles intracellular components through the lysosomes. Atg9 is the only integral membrane protein among autophagy-related (Atg) proteins thought to carry the membrane source for forming autophagosomes. Here we show that Drosophila Atg9 interacts with Drosophila tumor necrosis factor receptor-associated factor 2 (dTRAF2) to regulate the c-Jun N-terminal kinase (JNK) signaling pathway. Significantly, depletion of Atg9 and dTRAF2 compromised JNK-mediated intestinal stem cell proliferation and autophagy induction upon bacterial infection and oxidative stress stimulation. In mammalian cells, mAtg9 interacts with TRAF6, the homolog of dTRAF2, and plays an essential role in regulating oxidative stress-induced JNK activation. Moreover, we found that ROS-induced autophagy acts as a negative feedback regulator of JNK activity by dissociating Atg9/mAtg9 from dTRAF2/TRAF6 in Drosophila and mammalian cells, respectively. Our findings indicate a dual role for Atg9 in the regulation of JNK signaling and autophagy under oxidative stress conditions.

Keywords: Ape1, Atg19, autophagy, Cvt, X-ray crystallographyAbbreviations: 5 0 -IAF, 5-iodoacetamidofluorescein; Ape1, aminopeptidase I; Atg, autophagy-related; AUC, analytical ultracentrifugation; Cvt, cytoplasm-to-vacuole targeting; DDM, n-dodecyl-b-D-maltopyranoside; EM, electron microscopy; FM 4-64, N-(3-triethylammoniumpropyl)-4-(6-(4-(diethylamino) phenyl) hexatrienyl) pyridinium dibromide; FP, fluorescence polarization; GFP, green fluorescent protein; MBP, maltose binding protein; Ni-NTA, nickel-nitrilotriacetic acid; PAS, phagophore assembly site; TEV, tobacco etch virus.In Saccharomyces cerevisiae, a constitutive biosynthetic transport pathway, termed the cytoplasm-to-vacuole targeting (Cvt) pathway, sequesters precursor aminopeptidase I (prApe1) dodecamers in the form of a large complex into a Cvt vesicle using autophagic machinery, targeting it into the vacuole (the yeast lysosome) where it is proteolytically processed into its mature form, Ape1, by removal of an amino-terminal 45-amino acid propeptide. prApe1 is thought to serve as a scaffolding cargo critical for the assembly of the Cvt vesicle by presenting the propeptide to mediate higher-ordered complex formation and autophagic receptor recognition. Here we report the Xray crystal structure of Ape1 at 2.5 A resolution and reveal its dodecameric architecture consisting of dimeric and trimeric units, which associate to form a large tetrahedron. The propeptide of prApe1 exhibits concentration-dependent oligomerization and forms a stable tetramer. Structure-based mutagenesis demonstrates that disruption of the intersubunit interface prevents dodecameric assembly and vacuolar targeting in vivo despite the presence of the propeptide. Furthermore, by examining the vacuolar import of propeptide-fused exogenous protein assemblies with different quaternary structures, we found that 3-dimensional spatial distribution of propeptides presented by a scaffolding cargo is essential for the assembly of the Cvt vesicle for vacuolar delivery. This study describes a molecular framework for understanding the mechanism of Cvt or autophagosomal biogenesis in selective macroautophagy.

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The rapid removal of larval midgut is a critical developmental process directed by molting hormone ecdysone during Drosophila metamorphosis. To date, it remains unclear how the stepwise events can link the onset of ecdysone signaling to the destruction of larval midgut. This study investigated whether ecdysone-induced expression of receptor protein tyrosine phosphatase PTP52F regulates this process. The mutation of the Ptp52F gene caused significant delay in larval midgut degradation. Transitional endoplasmic reticulum ATPase (TER94), a regulator of ubiquitin proteasome system, was identified as a substrate and downstream effector of PTP52F in the ecdysone signaling. The inducible expression of PTP52F at the puparium formation stage resulted in dephosphorylation of TER94 on its Y800 residue, ensuring the rapid degradation of ubiquitylated proteins. One of the proteins targeted by dephosphorylated TER94 was found to be Drosophila inhibitor of apoptosis 1 (DIAP1), which was rapidly proteolyzed in cells with significant expression of PTP52F. Importantly, the reduced level of DIAP1 in response to inducible PTP52F was essential not only for the onset of apoptosis but also for the initiation of autophagy. This study demonstrates a novel function of PTP52F in regulating ecdysone-directed metamorphosis via enhancement of autophagic and apoptotic cell death in doomed Drosophila midguts.

Macroautophagy/autophagy is an evolutionarily conserved intracellular pathway for the degradation of cytoplasmic materials. Under stress conditions, autophagy is upregulated and double-membrane autophagosomes are formed by the expansion of phagophores. The ATG16L1 precursor fusion contributes to development of phagophore structures and is critical for the biogenesis of autophagosomes. Here, we discovered a novel role of the protein tyrosine phosphatase PTPN9 in the regulation of homotypic ATG16L1 vesicle fusion and early autophagosome formation. Depletion of PTPN9 and its Drosophila homolog Ptpmeg2 impaired autophagosome formation and autophagic flux. PTPN9 colocalized with ATG16L1 and was essential for homotypic fusion of ATG16L1 + vesicles during starvation-induced autophagy. We further identified the Q-SNARE VTI1B as a substrate target of PTPN9 phosphatase. Like PTPN9, the VTI1B nonphosphorylatable mutant but not the phosphomimetic mutant enhanced SNARE complex assembly and autophagic flux. Our findings highlight the important role of PTPN9 in the regulation of ATG16L1 + autophagosome precursor fusion and autophagosome biogenesis through modulation of VTI1B phosphorylation status.

Tumor infiltrating lymphocytes (TILs) are frequently detected in a variety of malignancies, including gastric cancer (GC). TILs are known to produce high level of interferon-gamma (IFN-γ) in GC, but fail to suppress tumor growth. We have previously shown that Rhotekin (RTKN), the gene encoding the Rho effector RTKN, is overexpressed in human GC, and its expression is correlated with disease progression. In this study, we show that RTKN expression efficiently blocks IFN-γ-mediated anti-growth and immunologic responses, suggesting a role of RTKN in attenuating tumor immunosurveillance during GC pathogenesis. We show that RTKN attenuates STAT1 phosphorylation in response to IFN-γ, and confers increased resistance to IFN-γ-mediated growth suppression in GC cells through inhibiting the expression of STAT1-dependent downstream target genes, including cyclin-dependent kinase inhibitor p21 and antiapoptotic genes BCL-2 and BCL-xL. Conversely, elimination of RTKN expression by knockdown approach facilitates IFN-γ-mediated STAT1 phosphorylation and promotes IFN-γ-mediated growth arrest and apoptosis. We further show that RTKN-mediated inhibition of STAT1 phosphorylation can be reversed by the treatment of sodium orthovanadate, a tyrosine phosphatase inhibitor, suggesting the involvement of tyrosine phosphatases in RTKN-mediated process. In support, our data show that RTKN suppresses IFN-γ-mediated STAT1 phosphorylation partly through facilitating the interactions of SHP2 and STAT1. In summary, our data suggest that RTKN overexpression confers growth advantages to GC by evading immunosurveillance through suppression of IFN-γ-mediated anti-tumor activity. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4435. doi:1538-7445.AM2012-4435