Biology subjects

Schott, M. B.

Publications and source records attributed to Schott, M. B..

3 recordsLinked to original sources

bioRxiv2025

cAMP promotes acute lysosome biogenesis through TFEB nuclear import-export dynamics.

Cyclic adenosine monophosphate (cAMP) signaling is a major stimulus for lipid and glucose catabolism, yet catabolic processes like these can also coordinate with lysosome-dependent degradation. However, the impact of cAMP signaling on lysosomal dynamics remains unclear. Transcription factor EB (TFEB), a master regulator of lysosomal biogenesis, is regulated by stimulus-dependent nuclear-cytoplasmic shuttling through a variety of phosphorylation events. Here, we find that elevating intracellular cAMP with forskolin and IBMX induces rapid nuclear import of TFEB-GFP within 30 minutes and coincides with a transient upregulation of TFEB target lysosome genes. By 8 hours, TFEB returns to the cytoplasm, accompanied by transcriptional downregulation. Inhibition of cAMP-dependent protein kinase A (PKA) using H89 did not block nuclear import but unexpectedly caused sustained nuclear accumulation, indicating that PKA promotes TFEB nuclear export. Consistent with this, phosphoproteomic profiling revealed increased phosphorylation of a PKA-consensus motif (RRxS) during the export phase. These findings suggest that cAMP-PKA signaling plays a novel role in temporally "tuning" lysosomal gene expression by regulating TFEB nuclear-cytoplasmic shuttling. SummaryThis study reveals that cAMP signaling dynamically regulates TFEB subcellular localization, promoting transient, calcium-dependent nuclear import as well as downstream, PKA-dependent export and phosphorylation at serines 466/467. These findings uncover a novel mechanism by which cAMP stimulation fine-tunes lysosomal gene expression by regulating TFEB nuclear import and export.

cell biology↗

bioRxiv2025

The ESCRT-0 protein HRS regulates hepatocellular lipid droplet catabolism

Lipid droplets (LDs) are dynamic organelles that regulate lipid storage and metabolism pathways central to metabolic liver disease. LD turnover occurs in part through lysosomal catabolism (i.e. lipophagy) whereby LDs are thought to follow two distinct trafficking pathways: autophagosome-dependent macrolipophagy and the autophagosome-independent microlipophagy. However, the molecular machinery that regulates these two distinct pathways, especially that of microlipophagy in mammalian cells, is poorly understood. In yeast, microlipophagy has been shown to rely on a protein family known as the endosomal sorting complex required for transport (ESCRT). Here, we used an ESCRT-specific RNAi library in hepatocytes which identified the ESCRT-0 protein hepatocyte growth factor receptor substrate (HRS) as a critical regulator of LD homeostasis. HRS depletion leads to significant LD accumulation which is not due to increased LD formation but from impaired LD catabolism. HRS-deficient cells retain lipolysis activity; however, they exhibit decreased LD targeting via microlipophagy, accompanied by compensatory increases in autophagosome targeting to LDs. In agreement with these findings, HRS knockdown suppressed mTOR signaling, boosted autophagosome formation, and reduced the degradation of autophagic cargo. Despite maintaining lysosome numbers, HRS knockdown raised lysosomal pH causing decreased autophagic degradative capacity and contributing to LD accumulation. Overall, these findings identify HRS as a modulator of LD turnover in mammalian cells, regulating lipophagy through lysosomal function. Significance StatementO_LIThe regulatory molecular mechanisms of lipophagy are not clearly defined. This study identifies novel ESCRT proteins as regulators of LD homeostasis in several cell lines. C_LIO_LIIn hepatocytes, we identified HRS specifically regulates LD catabolism, whereby HRS-dependent regulation of LDs is dual-faceted, affecting LD-lysosomal targeting and lysosomal function. C_LIO_LIOur findings are significant because they provide mechanistic insights into the role of ESCRT proteins in LD metabolism. Elucidating ESCRT-mediated lipophagy can potentially aid in developing novel targets to prevent aberrant lipid trafficking and utilization, particularly in the liver where LDs can accumulate and cause irreversible liver damage. C_LI

biochemistry↗

bioRxiv2025

RAB5 NUCLEOTIDE BINDING PROMOTES β-OXIDATION TO FUEL HEPATOCELLULAR CARCINOMA CELL PROLIFERATION

Altered lipid metabolism and lipid droplet (LD) dynamics are hallmark features of hepatocellular carcinoma (HCC) subtypes, but the molecular mechanisms governing LD trafficking and catabolism in HCC cells remain unclear. The small GTPase Rab5, a key regulator of early endosomal dynamics, has been observed to localize to the surface of LDs, suggesting it may play a role in LD turnover. However, the regulation of Rab5-LD interactions and its functional consequences in HCC cell metabolism and proliferation have not been elucidated. In this study, we explored the role of Rab5 in governing LD homeostasis and its impact on HCC cell proliferation. We found that the GTP-bound (Q79L), active form of Rab5 exhibited increased association with LDs compared to the GDP-bound, inactive mutant (S34N). Nutrient starvation enhanced Rab5 GTP-loading and its recruitment to LDs, indicating that Rab5s GTPase cycle regulates its LD localization. Importantly, inhibition of Rab5 GTP-binding impaired LD catabolism, reduced mitochondrial oxidative phosphorylation, and significantly impaired HCC cell proliferation. Transcriptomic analyses further revealed that RAB5 is significantly overexpressed in HCC patient samples, and this overexpression correlated with poorer overall survival. These findings demonstrate that Rab5s GTPase cycle is a critical regulator of LD dynamics in HCC cells, governing LD turnover to sustain mitochondrial energy production and support cancer cell proliferation. Targeting the Rab5-mediated regulation of LD metabolism may represent a novel therapeutic strategy to disrupt the metabolic adaptations that fuel liver cancer progression.

cancer biology↗