Effects of stereochemistry, saturation, and hydrocarbon chain length on the ability of synthetic constrained azacyclic sphingolipids to trigger nutrient transporter down-regulation, vacuolation, and cell death

Simultaneous inhibition of endocytic recycling and lysosomal fusion sensitizes cells and tissues to oligonucleotide therapeutics

Inefficient endosomal escape remains the primary barrier to the broad application of oligonucleotide therapeutics. Liver uptake after systemic administration is sufficiently robust that a therapeutic effect can be achieved but targeting extrahepatic tissues remains challenging. Prior attempts to improve oligonucleotide activity using small molecules that increase the leakiness of endosomes have failed due to unacceptable toxicity. Here, we show that the well-tolerated and orally bioavailable synthetic sphingolipid analog, SH-BC-893, increases the activity of antisense oligonucleotides (ASOs) and small interfering RNAs (siRNAs) up to 200-fold in vitro without permeabilizing endosomes. SH-BC-893 treatment trapped endocytosed oligonucleotides within extra-lysosomal compartments thought to be more permeable due to frequent membrane fission and fusion events. Simultaneous disruption of ARF6-dependent endocytic recycling and PIKfyve-dependent lysosomal fusion was necessary and sufficient for SH-BC-893 to increase non-lysosomal oligonucleotide levels and enhance their activity. In mice, oral administration of SH-BC-893 increased ASO potency in the liver by 15-fold without toxicity. More importantly, SH-BC-893 enabled target RNA knockdown in the CNS and lungs of mice treated subcutaneously with cholesterol-functionalized duplexed oligonucleotides or unmodified ASOs, respectively. Together, these results establish the feasibility of using a small molecule that disrupts endolysosomal trafficking to improve the activity of oligonucleotides in extrahepatic tissues.

TFEB phosphorylation on Serine 211 is induced by autophagy in human synovial fibroblasts and by p62/SQSTM1 overexpression in HEK293 cells

Autophagy receptor p62/SQSTM1 signals a complex network that links autophagy-lysosomal system to proteasome. Phosphorylation of p62 on Serine 349 (P-Ser349 p62) is involved in a cell protective, antioxidant pathway. We have shown previously that P-Ser349 p62 occurs and is rapidly degraded during human synovial fibroblasts autophagy. In this work we observed that fingolimod (FTY720), used as a medication for multiple sclerosis, induced coordinated expression of p62, P-Ser349 p62 and inhibitory TFEB form, phosphorylated on Serine 211 (P-Ser211 TFEB), in human synovial fibroblasts. These effects were mimicked and potentiated by proteasome inhibitor MG132. In addition, FTY720 induced autophagic flux, LC3B-II up-regulation, Akt phosphorylation inhibition on Serine 473 but down-regulated TFEB, suggesting stalled autophagy. FTY720 decreased cytoplasmic fraction contained TFEB but induced TFEB in nuclear fraction. FTY720-induced P-Ser211 TFEB was mainly found in membrane fraction. Autophagy and VPS34 kinase inhibitor, autophinib, further increased FTY720-induced P-Ser349 p62 but inhibited concomitant expression of P-Ser211 TFEB. These results suggested that P-Ser211 TFEB expression depends on autophagy. Overexpression of GFP tagged TFEB in HEK293 cells showed concomitant expression of its phosphorylated form on Serine 211, that was down-regulated by autophinib. These results suggested that autophagy might be autoregulated through P-Ser211 TFEB as a negative feedback loop. Of interest, overexpression of p62, p62 phosphorylation mimetic (S349E) mutant and phosphorylation deficient mutant (S349A) in HEK293 cells markedly induced P-Ser211 TFEB. These results showed that p62 is involved in regulation of TFEB phosphorylation on Serine 211 but that this involvement does not depend on p62 phosphorylation on Serine 349.

Synthetic Sphingolipids with 1,2-Pyridazine Appendages Improve Antiproliferative Activity in Human Cancer Cell Lines

A synthetic sphingolipid related to a ring-constrained hydroxymethyl pyrrolidine analog of FTY720 that was known to starve cancer cells to death was chemically modified to include a series of alkoxy-tethered 3,6-substituted 1,2-pyridazines. These derivatives exhibited excellent antiproliferative activity against eight human cancer cell lines from four different cancer types. A 2.5- to 9-fold reduction in IC₅₀ in these cell lines was observed relative to the lead compound, which lacked the appended heterocycle.

FTY720 induces non-canonical phosphatidylserine externalization and cell death in acute myeloid leukemia

FTY720 (fingolimod) is a FDA-approved sphingosine analog that is phosphorylated in vivo to modulate sphingosine-1-phosphate receptor (S1PR) signaling for immunosuppression in patients with refractory multiple sclerosis. FTY720 also exhibits promising anticancer efficacy in several preclinical models. While FTY720-induced cytotoxicity is not due to S1PR signaling, the mechanism remains unclear and is reported to occur through various cell death pathways. Here, we performed a systematic, mechanistic study of FTY720-induced cell death in acute myeloid leukemia (AML). We found that FTY720 induced cell death in a panel of genetically diverse AML cell lines that was accompanied by rapid phosphatidylserine (PS) externalization. Importantly, FTY720-induced PS exposure was not due to any direct effects on plasma membrane integrity and was independent of canonical signaling by regulated cell death pathways known to activate lipid flip-flop, including caspase-dependent apoptosis/pyroptosis, necroptosis, ferroptosis, and reactive oxygen species-mediated cell death. Notably, PS exposure required cellular vacuolization induced by defects in endocytic trafficking and was suppressed by the inhibition of PP2A and shedding of Annexin V-positive subcellular particles. Collectively, our studies reveal a non-canonical pathway underlying PS externalization and cell death in AML to provide mechanistic insight into the antitumor properties of FTY720.

Design, synthesis and anticancer activity of constrained sphingolipid-phenoxazine/phenothiazine hybrid constructs targeting protein phosphatase 2A

Inspired by the cytotoxicity of perphenazine toward cancer cells and its ability to activate the serine/threonine protein phosphatase 2A (PP2A), we prepared series of ether-carbon linked analogs of a constrained synthetic sphingolipid analog 3, known for its cytotoxicity, nutrient transporter down-regulation and vacuolation properties, incorporating the tricyclic neuroleptics phenoxazine and phenothiazine to represent hybrid structures with possible synergistic cytotoxic activity. While the original activity of the lead compound 3 was diminished by fusion with the phenoxazine or phenothiazine tethered moieties, the corresponding 3-pyridyltetryl ether analog 10 showed cytotoxicity and nutrient transporter down-regulation similar to the lead compound 3, although it separated these PP2A-dependent phenotypes from that of vacuolation.

Protein phosphatase 2A as a therapeutic target in inflammation and neurodegeneration

Protein phosphatase 2A (PP2A) is a highly complex heterotrimeric enzyme that catalyzes the selective removal of phosphate groups from protein serine and threonine residues. Emerging evidence suggests that it functions as a tumor suppressor by constraining phosphorylation-dependent signalling pathways that regulate cellular transformation and metastasis. Therefore, PP2A-activating drugs (PADs) are being actively sought and investigated as potential novel anti-cancer treatments. Here we explore the concept that PP2A also constrains inflammatory responses through its inhibitory effects on various signalling pathways, suggesting that PADs may be effective in the treatment of inflammation-mediated pathologies.

Dynamic Phosphoproteomics Uncovers Signaling Pathways Modulated by Anti-oncogenic Sphingolipid Analogs

The anti-neoplastic sphingolipid analog SH-BC-893 starves cancer cells to death by down-regulating cell surface nutrient transporters and blocking lysosomal trafficking events. These effects are mediated by the activation of protein phosphatase 2A (PP2A). To identify putative PP2A substrates, we used quantitative phosphoproteomics to profile the temporal changes in protein phosphorylation in FL5.12 cells following incubation with SH-BC-893 or the specific PP2A inhibitor LB-100. These analyses enabled the profiling of more than 15,000 phosphorylation sites, of which 958 sites on 644 proteins were dynamically regulated. We identified 114 putative PP2A substrates including several nutrient transporter proteins, GTPase regulators (e.g. Agap2, Git1), and proteins associated with actin cytoskeletal remodeling (e.g. Vim, Pxn). To identify SH-BC-893-induced cell signaling events that disrupt lysosomal trafficking, we compared phosphorylation profiles in cells treated with SH-BC-893 or C2-ceramide, a non-vacuolating sphingolipid that does not impair lysosomal fusion. These analyses combined with functional assays uncovered the differential regulation of Akt and Gsk3b by SH-BC-893 (vacuolating) and C2-ceramide (non-vacuolating). Dynamic phosphoproteomics of cells treated with compounds affecting PP2A activity thus enabled the correlation of cell signaling with phenotypes to rationalize their mode of action.

In search of constrained FTY720 and phytosphingosine analogs as dual acting anticancer agents targeting metabolic and epigenetic pathways

A series of compounds containing pyrrolidine and pyrrolizidine cores with appended hydrophobic substituents were prepared as constrained analogs of FTY720 and phytosphingosine. The effect of these compounds on the viability of cancer cells, on downregulation of the nutrient transport systems, and on their ability to cause vacuolation was studied. An attempt to inhibit HDACs with some phosphate esters of our analogs was thwarted by our failure to reproduce the reported inhibitory action of FTY720-phosphate.

Sphingolipids inhibit endosomal recycling of nutrient transporters by inactivating ARF6

Endogenous sphingolipids (ceramide) and related synthetic molecules (FTY720, SH-BC-893) reduce nutrient access by decreasing cell surface expression of a subset of nutrient transporter proteins. Here, we report that these sphingolipids disrupt endocytic recycling by inactivating the small GTPase ARF6. Consistent with reported roles for ARF6 in maintaining the tubular recycling endosome, MICAL-L1-positive tubules were lost from sphingolipid-treated cells. We propose that ARF6 inactivation may occur downstream of PP2A activation since: (1) sphingolipids that fail to activate PP2A did not reduce ARF6-GTP levels; (2) a structurally unrelated PP2A activator disrupted tubular recycling endosome morphology and transporter localization; and (3) overexpression of a phosphomimetic mutant of the ARF6 GEF GRP1 prevented nutrient transporter loss. ARF6 inhibition alone was not toxic; however, the ARF6 inhibitors SecinH3 and NAV2729 dramatically enhanced the killing of cancer cells by SH-BC-893 without increasing toxicity to peripheral blood mononuclear cells, suggesting that ARF6 inactivation contributes to the anti-neoplastic actions of sphingolipids. Taken together, these studies provide mechanistic insight into how ceramide and sphingolipid-like molecules limit nutrient access and suppress tumor cell growth and survival.