Transcriptional activation of lysosomal exocytosis promotes cellular clearance

The transcription factor TFEB links mTORC1 signaling to transcriptional control of lysosome homeostasis

Lysosomes are the major cellular site for clearance of defective organelles and digestion of internalized material. Demand on lysosomal capacity can vary greatly, and lysosomal function must be adjusted to maintain cellular homeostasis. Here, we identified an interaction between the lysosome-localized mechanistic target of rapamycin complex 1 (mTORC1) and the transcription factor TFEB (transcription factor EB), which promotes lysosome biogenesis. When lysosomal activity was adequate, mTOR-dependent phosphorylation of TFEB on Ser(211) triggered the binding of 14-3-3 proteins to TFEB, resulting in retention of the transcription factor in the cytoplasm. Inhibition of lysosomal function reduced the mTOR-dependent phosphorylation of TFEB, resulting in diminished interactions between TFEB and 14-3-3 proteins and the translocation of TFEB into the nucleus, where it could stimulate genes involved in lysosomal biogenesis. These results identify TFEB as a target of mTOR and suggest a mechanism for matching the transcriptional regulation of genes encoding proteins of autophagosomes and lysosomes to cellular need. The closely related transcription factors MITF (microphthalmia transcription factor) and TFE3 (transcription factor E3) also localized to lysosomes and accumulated in the nucleus when lysosome function was inhibited, thus broadening the range of physiological contexts under which this regulatory mechanism may prove important.

Status of mTOR activity may phenotypically differentiate senescence and quiescence

SA β-Gal activity is a key marker of cellular senescence. The origin of this activity is the lysosomal β-galactosidase, whose activity has increased high enough to be detected at suboptimal pH. SA β-Gal is also expressed in the cells in quiescence driven by serum-starvation or a high confluency, and it has been hypothesized that SA β-Gal positivity is rather a surrogate marker of high lysosome content or activity. In this study, it was determined how SA β-Gal activity is expressed in quiescence and how lysosome content and activities are differently maintained in senescence and quiescence using DNA damage-induced senescence and serum starvation-induced quiescence as study models. Lysosome content increased to facilitate SA β-Gal expression in both the conditions but with a big difference in the levels of the change. Lipofuscins whose accumulation leads to an increase in residual bodies also increased but with a smaller difference between the two conditions. Meanwhile, lysosome biogenesis was actively ongoing only in senescence progression, indicating that the difference in the lysosome contents may largely be due to lysosome biogenesis. Further, the cells undergoing senescence progression but not the ones in quiescence maintained high mTOR and low autophagy activities. Overall, the results indicate that, although SA β-Gal is expressed due to the elevated lysosome content in both cellular senescence and quiescence, senescence differs from quiescence with high lysosome biogenesis and low autophagy activity, and mTOR activity might be involved in these differences.

Drug-drug interactions involving lysosomes: mechanisms and potential clinical implications

INTRODUCTION

Many commercially available, weakly basic drugs have been shown to be lysosomotropic, meaning they are subject to extensive sequestration in lysosomes through an ion trapping-type mechanism. The extent of lysosomal trapping of a drug is an important therapeutic consideration because it can influence both activity and pharmacokinetic disposition. The administration of certain drugs can alter lysosomes such that their accumulation capacity for co-administered and/or secondarily administered drugs is altered.

AREAS COVERED

In this review the authors explore what is known regarding the mechanistic basis for drug-drug interactions involving lysosomes. Specifically, the authors address the influence of drugs on lysosomal pH, volume and lipid processing.

EXPERT OPINION

Many drugs are known to extensively accumulate in lysosomes and significantly alter their structure and function; however, the therapeutic and toxicological implications of this remain controversial. The authors propose that drug-drug interactions involving lysosomes represent an important potential source of variability in drug activity and pharmacokinetics. Most evaluations of drug-drug interactions involving lysosomes have been performed in cultured cells and isolated tissues. More comprehensive in vivo evaluations are needed to fully explore the impact of this drug-drug interaction pathway on therapeutic outcomes.

MTORC1 functions as a transcriptional regulator of autophagy by preventing nuclear transport of TFEB

The mammalian target of rapamycin (MTOR) protein kinase complex is a key component of a pathway that regulates cell growth and proliferation in response to energy levels, hypoxia, nutrients and insulin. Inhibition of MTORC1 strongly induces autophagy by regulating the activity of the ULK protein kinase complex that is required for the formation of autophagosomes. However, the participation of MTORC1 in the expression of autophagy genes has not been characterized. Here we show that MTORC1 regulates nuclear localization and activity of the transcription factor EB (TFEB), a member of the bHLH leucine-zipper family of transcription factors that drives expression of autophagy and lysosomal genes. Under normal nutrient conditions, TFEB is phosphorylated in Ser211 in an MTORC1-dependent manner. This phosphorylation promotes association of TFEB with members of the YWHA (14-3-3) family of proteins and retention of the transcription factor in the cytosol. Pharmacological or genetic inhibition of MTORC1 causes dissociation of the TFEB/YWHA complex and rapid transport of TFEB to the nucleus where it increases transcription of multiple genes implicated in autophagy and lysosomal function. Active TFEB also associates with late endosomal/lysosomal membranes through interaction with the LAMTOR/RRAG/MTORC1 complex. Our results unveil a novel role for MTORC1 in the maintenance of cellular homeostasis by regulating autophagy at the transcriptional level.

Localized AL amyloidosis: a suicidal neoplasm?

Although AL amyloidosis usually is a systemic disease, strictly localized AL deposits are not exceptionally rare. Such case reports form a considerable body of published articles. Although both AL amyloidosis types are formed from an N-terminal segment of a monoclonal immunoglobulin light chain, a typical localized AL amyloid differs from the systemic counterpart by the morphological appearance of the amyloid, and presence of clonal plasma cells and of giant cells. In this article it is pointed out that localized AL amyloidosis ('amyloidoma') represents a true plasma cell neoplasm and not a pseudotumor. The pathogenesis of localized AL amyloidosis may differ from that of the systemic type, a suggestion underlined by the fact that localized AL amyloidosis of kappa type is as common as that of lambda origin, in contrast to the systemic form where lambda chains constitute the overwhelming majority of cases. It is suggested that oligomeric assemblies of the produced immunoglobulin light chain are toxic to plasma cells, which in this way commit suicide.

Autophagy in lysosomal storage disorders

Lysosomes are ubiquitous intracellular organelles that have an acidic internal pH, and play crucial roles in cellular clearance. Numerous functions depend on normal lysosomes, including the turnover of cellular constituents, cholesterol homeostasis, downregulation of surface receptors, inactivation of pathogenic organisms, repair of the plasma membrane and bone remodeling. Lysosomal storage disorders (LSDs) are characterized by progressive accumulation of undigested macromolecules within the cell due to lysosomal dysfunction. As a consequence, many tissues and organ systems are affected, including brain, viscera, bone and cartilage. The progressive nature of phenotype development is one of the hallmarks of LSDs. In recent years biochemical and cell biology studies of LSDs have revealed an ample spectrum of abnormalities in a variety of cellular functions. These include defects in signaling pathways, calcium homeostasis, lipid biosynthesis and degradation and intracellular trafficking. Lysosomes also play a fundamental role in the autophagic pathway by fusing with autophagosomes and digesting their content. Considering the highly integrated function of lysosomes and autophagosomes it was reasonable to expect that lysosomal storage in LSDs would have an impact upon autophagy. The goal of this review is to provide readers with an overview of recent findings that have been obtained through analysis of the autophagic pathway in several types of LSDs, supporting the idea that LSDs could be seen primarily as "autophagy disorders."

A lysosome-to-nucleus signalling mechanism senses and regulates the lysosome via mTOR and TFEB

The lysosome plays a key role in cellular homeostasis by controlling both cellular clearance and energy production to respond to environmental cues. However, the mechanisms mediating lysosomal adaptation are largely unknown. Here, we show that the Transcription Factor EB (TFEB), a master regulator of lysosomal biogenesis, colocalizes with master growth regulator mTOR complex 1 (mTORC1) on the lysosomal membrane. When nutrients are present, phosphorylation of TFEB by mTORC1 inhibits TFEB activity. Conversely, pharmacological inhibition of mTORC1, as well as starvation and lysosomal disruption, activates TFEB by promoting its nuclear translocation. In addition, the transcriptional response of lysosomal and autophagic genes to either lysosomal dysfunction or pharmacological inhibition of mTORC1 is suppressed in TFEB-/- cells. Interestingly, the Rag GTPase complex, which senses lysosomal amino acids and activates mTORC1, is both necessary and sufficient to regulate starvation- and stress-induced nuclear translocation of TFEB. These data indicate that the lysosome senses its content and regulates its own biogenesis by a lysosome-to-nucleus signalling mechanism that involves TFEB and mTOR.