Mechanism of cytosol phospholipase C and sphingomyelinase-induced lysosome destabilization

Sensitive Fluorescent Biosensor Reveals Differential Subcellular Regulation of PKC

The protein kinase C (PKC) family of serine/threonine kinases, which consist of three distinctly regulated subfamilies, have long been established as critical for a variety of cellular functions. However, how PKC enzymes are regulated at different subcellular locations, particularly at emerging signaling hubs such as the ER, lysosome, and Par signaling complexes, is unclear. Here, we present a sensitive Excitation Ratiometric (ExRai) C Kinase Activity Reporter (ExRai-CKAR2) that enables the detection of minute changes in subcellular PKC activity. Using ExRai-CKAR2 in conjunction with an enhanced diacylglycerol (DAG) biosensor capable of detecting intracellular DAG dynamics, we uncover the differential regulation of PKC isoforms at distinct subcellular locations. We find that G-protein coupled receptor (GPCR) stimulation triggers sustained PKC activity at the ER and lysosomes, primarily mediated by Ca2+ sensitive conventional PKC (cPKC) and novel PKC (nPKC), respectively, with nPKC showing high basal activity due to elevated basal DAG levels on lysosome membranes. The high sensitivity of ExRai-CKAR2, targeted to either the cytosol or Par-complexes, further enabled us to detect previously inaccessible endogenous atypical PKC (aPKC) activity in 3D organoids. Taken together, ExRai-CKAR2 is a powerful tool for interrogating PKC regulation in response to physiological stimuli.

Tamoxifen Activates Transcription Factor EB and Triggers Protective Autophagy in Breast Cancer Cells by Inducing Lysosomal Calcium Release: A Gateway to the Onset of Endocrine Resistance

Among the several mechanisms accounting for endocrine resistance in breast cancer, autophagy has emerged as an important player. Previous reports have evidenced that tamoxifen (Tam) induces autophagy and activates transcription factor EB (TFEB), which regulates the expression of genes controlling autophagy and lysosomal biogenesis. However, the mechanisms by which this occurs have not been elucidated as yet. This investigation aims at dissecting how TFEB is activated and contributes to Tam resistance in luminal A breast cancer cells. TFEB was overexpressed and prominently nuclear in Tam-resistant MCF7 cells (MCF7-TamR) compared with their parental counterpart, and this was not dependent on alterations of its nucleo-cytoplasmic shuttling. Tam promoted the release of lysosomal Ca2+ through the major transient receptor potential cation channel mucolipin subfamily member 1 (TRPML1) and two-pore channels (TPCs), which caused the nuclear translocation and activation of TFEB. Consistently, inhibiting lysosomal calcium release restored the susceptibility of MCF7-TamR cells to Tam. Our findings demonstrate that Tam drives the nuclear relocation and transcriptional activation of TFEB by triggering the release of Ca2+ from the acidic compartment, and they suggest that lysosomal Ca2+ channels may represent new druggable targets to counteract the onset of autophagy-mediated endocrine resistance in luminal A breast cancer cells.

Oxidative damage of lysosomes in regulated cell death systems: Pathophysiology and pharmacologic interventions

Lysosomes are small specialized organelles containing a variety of different hydrolase enzymes that are responsible for degradation of all macromolecules, entering the cells through the endosomal system or originated from the internal sources. This allows for transport and recycling of nutrients and internalization of surface proteins for antigen presentation as well as maintaining cellular homeostasis. Lysosomes are also important storage compartments for metal ions and nutrients. The integrity of lysosomal membrane is central to maintaining their normal function, but like other cellular membranes, lysosomal membrane is subject to damage mediated by reactive oxygen species. This results in spillage of lysosomal enzymes into the cytoplasm, leading to proteolytic damage to cellular systems and organelles. Several forms of lysosomal dependent cell death have been identified in diseases. Examination of these events are important for finding treatment strategies relevant to cancer or neurodegenerative diseases as well as autoimmune deficiencies. In this review, we have examined the current literature on involvement of lysosomes in induction of programed cell death and have provided an extensive list of therapeutic approaches that can modulate cell death. Exploitation of these mechanisms can lead to novel therapies for cancer and neurodegenerative diseases.

Activation of TRPML1 clears intraneuronal Aβ in preclinical models of HIV infection

Antiretroviral therapy extends the lifespan of human immunodeficiency virus (HIV)-infected patients, but many survivors develop premature impairments in cognition. These residual cognitive impairments may involve aberrant deposition of amyloid β-peptides (Aβ). By unknown mechanisms, Aβ accumulates in the lysosomal and autophagic compartments of neurons in the HIV-infected brain. Here we identify the molecular events evoked by the HIV coat protein gp120 that facilitate the intraneuronal accumulation of Aβ. We created a triple transgenic gp120/APP/PS1 mouse that recapitulates intraneuronal deposition of Aβ in a manner reminiscent of the HIV-infected brain. In cultured neurons, we found that the HIV coat protein gp120 increased the transcriptional expression of BACE1 through repression of PPARγ, and increased APP expression by promoting interaction of the translation-activating RBP heterogeneous nuclear ribonucleoprotein C with APP mRNA. APP and BACE1 were colocalized into stabilized membrane microdomains, where the β-cleavage of APP and Aβ formation were enhanced. Aβ-peptides became localized to lysosomes that were engorged with sphingomyelin and calcium. Stimulating calcium efflux from lysosomes with a TRPM1 agonist promoted calcium efflux, luminal acidification, and cleared both sphingomyelin and Aβ from lysosomes. These findings suggest that therapeutics targeted to reduce lysosomal pH in neurodegenerative conditions may protect neurons by facilitating the clearance of accumulated sphingolipids and Aβ-peptides.

Conjugated linoleic acid-induced apoptosis in mouse mammary tumor cells is mediated by both G protein coupled receptor-dependent activation of the AMP-activated protein kinase pathway and by oxidative stress

Conjugated linoleic acid (CLA) has shown chemopreventive activity in several tumorigenesis models, in part through induction of apoptosis. We previously demonstrated that the t10,c12 isomer of CLA induced apoptosis of TM4t mouse mammary tumor cells through both mitochondrial and endoplasmic reticulum (ER) stress pathways, and that the AMP-activated protein kinase (AMPK) played a critical role in the apoptotic effect. In the current study, we focused on the upstream pathways by which AMPK was activated, and additionally evaluated the contributing role of oxidative stress to apoptosis. CLA-induced activation of AMPK and/or induction of apoptosis were inhibited by infection of TM4t cells with an adenovirus expressing a peptide which blocks the interaction between the G protein coupled receptor (GPCR) and Gα(q), by the phospholipase C (PLC) inhibitor U73122, by the inositol trisphosphate (IP(3)) receptor inhibitor 2-APB, by the calcium/calmodulin-dependent protein kinase kinase α (CaMKK) inhibitor STO-609 and by the intracellular Ca(2+) chelator BAPTA-AM. This suggests that t10,c12-CLA may exert its apoptotic effect by stimulating GPCR through Gα(q) signaling, activation of phosphatidylinositol-PLC, followed by binding of the PLC-generated IP(3) to its receptor on the ER, triggering Ca(2+) release from the ER and finally stimulating the CaMKK-AMPK pathway. t10,c12-CLA also increased oxidative stress and lipid peroxidation, and antioxidants blocked its apoptotic effect, as well as the CLA-induced activation of p38 MAPK, a downstream effector of AMPK. Together these data elucidate two major pathways by which t10,c12-CLA induces apoptosis, and suggest a point of intersection of the two pathways both upstream and downstream of AMPK.

Regulation of apoptosis-associated lysosomal membrane permeabilization

Lysosomal membrane permeabilization (LMP) occurs in response to a large variety of cell death stimuli causing release of cathepsins from the lysosomal lumen into the cytosol where they participate in apoptosis signaling. In some settings, apoptosis induction is dependent on an early release of cathepsins, while under other circumstances LMP occurs late in the cell death process and contributes to amplification of the death signal. The mechanism underlying LMP is still incompletely understood; however, a growing body of evidence suggests that LMP may be governed by several distinct mechanisms that are likely engaged in a death stimulus- and cell-type-dependent fashion. In this review, factors contributing to permeabilization of the lysosomal membrane including reactive oxygen species, lysosomal membrane lipid composition, proteases, p53, and Bcl-2 family proteins, are described. Potential mechanisms to safeguard lysosomal integrity and confer resistance to lysosome-dependent cell death are also discussed.

Proteomic analysis of enriched lysosomes at early phase of camptothecin-induced apoptosis in human U-937 cells

A lysosomal pathway, characterized by partial rupture or labilization of lysosomal membranes and cathepsin activation, is evoked during camptothecin-induced apoptosis in human cancer cells, including human histiocytic lymphoma U-937 cells. These lysosomal events begin rapidly and simultaneously with mitochondrial permeabilization and caspase activation within 3 h after drug treatment. In this study, comparative and quantitative proteome analyses were performed to identify early changes in lysosomal protein expression/localization from U-937 cells undergoing apoptosis. In 2 independent experiments, among a total of more than 538 proteins putatively identified and quantitated by iTRAQ isobaric labeling and LC-ESI-MS/MS, 18 proteins were found to be upregulated and 9 downregulated in lysosomes purified from early apoptotic compared to control cells. Protein expression was validated by Western blotting on enriched lysosome fractions, and protein localization confirmed by fluorescence confocal microscopy of representative protein candidates, whose functions are associated with lysosomal membrane fluidity and dynamics. These include sterol-4-alpha-carboxylate 3-dehydrogenase (NSDHL), prosaposin (PSAP) and protein kinase C delta (PKC-delta). This comparative proteome analysis provides the basis for novel hypothesis and rationale functional experimentation, where the 3 validated candidate proteins are associated with lysosomal membrane fluidity and dynamics, particularly cholesterol, sphingolipid and glycosphingolipid metabolism.

Loss of membrane cholesterol influences lysosomal permeability to potassium ions and protons

Cholesterol is an essential component of lysosomal membranes. In this study, we investigated the effects of membrane cholesterol on the permeability of rat liver lysosomes to K+ and H+, and the organelle stability. Through the measurements of lysosomal beta-hexosaminidase free activity, membrane potential, membrane fluidity, intra-lysosomal pH, and lysosomal proton leakage, we established that methyl-beta-cyclodextrin (MbetaCD)-produced loss of membrane cholesterol could increase the lysosomal permeability to both potassium ions and protons, and fluidize the lysosomal membranes. As a result, potassium ions entered the lysosomes through K+/H+ exchange, which produced osmotic imbalance across the membranes and osmotically destabilized the lysosomes. In addition, treatment of the lysosomes with MbetaCD caused leakage of the lysosomal protons and raised the intra-lysosomal pH. The results indicate that membrane cholesterol plays important roles in the maintenance of the lysosomal limited permeability to K+ and H+. Loss of this membrane sterol is critical for the organelle acidification and stability.