GFI1-Dependent Repression of SGPP1 Increases Multiple Myeloma Cell Survival

Abstract

Multiple myeloma (MM) remains incurable for most patients due to the emergence of drug resistant clones. Here we report a p53-independent mechanism responsible for Growth Factor Independence-1 (GFI1) support of MM cell survival by its modulation of sphingolipid metabolism to increase the sphingosine-1-phosphate (S1P) level regardless of the p53 status. We found that expression of enzymes that control S1P biosynthesis, SphK1, dephosphorylation, and SGPP1 were differentially correlated with GFI1 levels in MM cells. We detected GFI1 occupancy on the SGGP1 gene in MM cells in a predicted enhancer region at the 5’ end of intron 1, which correlated with decreased SGGP1 expression and increased S1P levels in GFI1 overexpressing cells, regardless of their p53 status. The high S1P:Ceramide intracellular ratio in MM cells protected c-Myc protein stability in a PP2A-dependent manner. The decreased MM viability by SphK1 inhibition was dependent on the induction of autophagy in both p53WT and p53mut MM. An autophagic blockade prevented GFI1 support for viability only in p53mut MM, demonstrating that GFI1 increases MM cell survival via both p53WT inhibition and upregulation of S1P independently. Therefore, GFI1 may be a key therapeutic target for all types of MM that may significantly benefit patients that are highly resistant to current therapies.

Role of Sphingolipids in Multiple Myeloma Progression, Drug Resistance, and Their Potential as Therapeutic Targets

“…S1P can be produced and released into the BM microenvironment by many cell types such as: osteoclast-lineage cells ( 64 , 65 ), osteoblasts ( 66 ), mesenchymal stromal cells ( 67 ), and endothelial cells ( 68 ). Other important contributors of S1P in the BM are erythrocytes and platelets ( 69 ), but also MM cells themselves, as we have recently shown ( 36 ). Importantly, S1P can also be released from dying cells (necrotic or apoptotic) and damaged tissues ( 70 , 71 ), and might create an unwanted pro-metastatic environment as a side effect of chemotherapy ( 71 ).…”

“…Although S1P lyase activity represents the unique exit point of the sphingolipid pathway by breaking down S1P into non-sphingolipid molecules, hexadecenal can be reduced to palmitate and subsequently reincorporated into lipid metabolic pathways ( 41 ). Intracellularly, S1P acts as a signaling molecule that modulates calcium mobilization ( 42 ), mitochondrial respiration via interaction with PHB2 ( 39 ), proteasomal degradation of TERT ( 38 ) and c-Myc ( 36 , 37 ), and gene expression in multiple ways: via direct activation of PPARγ ( 33 ), activation of NF-κB via direct interaction with TRAF2 ( 35 ) or chaperones HSP90 and GPR94 during ER stress ( 34 ), and direct inhibition of HDAC1/2 that alters chromatin structure ( 43 ) ( Figure 2B ). Extracellular S1P signals through binding to one of five G-protein-coupled receptors (S1PR1-S1PR5) thus participating in regulation of several cell processes such as cell survival, proliferation and migration ( Figure 2A ) ( 44 ).…”

“…S1P has also been reported to interact with TRAF2 during TNF signaling to activate NF-κB via RIP1 ( 35 ). S1P promotes cell viability by indirectly stabilizing cMyc by preventing dephosphorylation by protein phosphatase 2A (PP2A) via increasing interaction with the PP2A inhibitor I2PP2A ( 36 , 37 ). S1P also increases cell longevity by directly interacting with telomerase reverse transcriptase (hTERT) to prevent interaction with MKRN1 and nuclear export to the proteasome ( 38 ).…”

“…Since there are no identified activating mutations of SphK1, it has been suggested that tumors become dependent on SphK1 overexpression and/or increased activity which may induce a non-oncogene addiction ( 145 ). Our group recently showed that IL-6 and adhesive interactions with stromal cells simultaneously increased SphK1 and c-Myc mRNA and protein levels in several MM cell lines ( 36 ).…”

“…As mentioned above, S1P effects on a given cell can be receptor-mediated (autocrine, paracrine and signal amplification loops) and intracellular target protein mediated. Our group showed that exogenous S1P rapidly increases c-Myc protein levels in MM cells, the major contributor to the MM malignant phenotype ( 36 ). Extracellular S1P effects are mediated by binding to one or more of the five G protein-coupled S1P receptors (S1PRs): S1PR1/EDG-1, S1PR2/EDG-5, S1PR3/EDG-3, S1PR4/EDG-6, and S1PR5/EDG-8 ( Figure 2A ).…”