Meihua Shan

Nitrogen starvation is a universal stimulus of autophagy. At present, little is known about the relationship between carbon metabolism and autophagy under nitrogen starvation. Here, we show that yeast cells continue to consume glucose and downregulate fermentation under nitrogen starvation. Storage lipid production is increased, with concurrent proliferation of lipid droplets. Furthermore, we provide evidence that triacylglycerol synthesis is crucial for autophagosome biogenesis. It is involved in a step downstream of PAS (phagophore assembly site) scaffold assembly, and upstream of the recruitment of Atg1, Atg14, Atg5 and Atg8. Finally, we demonstrate that lipid droplets transiently interact with Atg8-containing membranes. Our study reveals a novel connection linking neutral lipid metabolism, lipid droplets and autophagy.

Eukaryotes initiate autophagy when facing environmental changes such as a lack of external nutrients. However, the mechanisms of autophagy initiation are still not fully elucidated. Here, we showed that deacetylation of ATG4B plays a key role in starvation-induced autophagy initiation. Specifically, we demonstrated that ATG4B is activated during starvation through deacetylation at K39 by the deacetylase SIRT2. Moreover, starvation triggers SIRT2 dephosphorylation and activation in a cyclin E/CDK2 suppression–dependent manner. Meanwhile, starvation down-regulates p300, leading to a decrease in ATG4B acetylation at K39. K39 deacetylation also enhances the interaction of ATG4B with pro-LC3, which promotes LC3-II formation. Furthermore, an in vivo experiment using Sirt2 knockout mice also confirmed that SIRT2-mediated ATG4B deacetylation at K39 promotes starvation-induced autophagy initiation. In summary, this study reveals an acetylation-dependent regulatory mechanism that controls the role of ATG4B in autophagy initiation in response to nutritional deficiency.

Fluorescence microscopy of live cells is instrumental in deciphering the molecular details of autophagy. To facilitate the routine examination of yeast Atg proteins under diverse conditions, here we provide a comprehensive tool set, including (1) plasmids for the expression of GFP chimeras at endogenous levels for most Atg proteins, (2) RFP-Atg8 constructs with improved properties as a PAS marker, and (3) plasmids for the complementation of common yeast auxotrophic markers. We hope that the availability of this tool set will further accelerate yeast autophagy research.

N6-methyladenosine (m6A), a dynamically reversible modification in eukaryotic RNAs, modulates gene expression and pathological processes in various tumors. KIAA1429, the largest component of the m6A methyltransferase complex, plays an important role in m6A modification. However, the underlying mechanism of KIAA1429 in hepatocellular carcinoma (HCC) remains largely unknown. Immunohistochemical assay was performed to examine the expression of KIAA1429 in HCC tissues. Transwell, wound healing and animal experiments were used to investigate the influence of KIAA1429 on cell migration and invasion. The mRNA high-throughput sequencing (RNA-seq) and methylated RNA immunoprecipitation sequencing (MeRIP-seq) were performed to screen the downstream target of KIAA1429. RNA stability assays, RNA immunoprecipitation assay (RIP), MeRIP-qPCR and luciferase assay were used to evaluate the relationship between KIAA1429 and the m6A-modified genes. Results showed that the expression level of KIAA1429 was significantly higher in HCC tissues than in adjacent tissues, and the upregulation of KIAA1429 could promote HCC metastasis in vitro and in vivo. Mechanistically, we confirmed that KIAA1429 negatively regulated the tumor suppressor, Rho family GTPase 3 (RND3), by decreasing its mRNA stability in coordination with the m6A reader YTHDC1. Moreover, we demonstrated that KIAA1429 could regulate the m6A modification of RND3 mRNA via its RNA binding domain. Our data indicated that KIAA1429 exerted its oncogenic role by inhibiting RND3 expression in an m6A-dependent manner, suggesting that KIAA1429 might be a potential prognostic biomarker and therapeutic target in HCC.