Abstract
The resistance mechanisms that limit the efficacy of retinoid therapy in cancer are poorly understood. Sphingosine kinase 2 (SphK2) is a highly conserved enzyme that is mainly located in the nucleus and endoplasmic reticulum. Unlike well-studied sphingosine kinase 1 (SphK1) located in the cytosol, little has yet understood the functions of SphK2. Here we show that SphK2 overexpression contributes to the resistance of all-trans retinoic acid (ATRA) therapy in colon cancer through rapid degradation of cytoplasmic retinoid X receptor α (RXRα) by lysine 48 (K48)- and lysine 63 (K63)-based polyubiquitination. Human colonic adenocarcinoma HCT-116 cells transfected with SphK2 (HCT-116Sphk2 cells) demonstrate resistance to ATRA therapy as determined by in vitro and in vivo assays. Sphk2 overexpression increases the ATRA-induced nuclear RXRα export to cytoplasm and then rapidly degrades RXRα through the polyubiquitination pathway. We further show that Sphk2 activates the ubiquitin-proteasome system through the signal mechanisms of (1) K48-linked proteosomal degradation and (2) K63-linked ubiquitin-dependent autophagic degradation. These results provide new insights into the biological functions of Sphk2 and the molecular mechanisms that underlie the Sphk2-mediated resistance to retinoid therapy.
Highlights
Retinoid therapy improves the outcomes of many cancers
Cytoplasmic retinoid X receptor α (RXRα) is rapidly degraded in HCT-116Sphk2 cells, as shown by the weaker green fluorescence of RXRα beginning at 2 h post all-trans retinoic acid (ATRA) exposure (Figure 1D–IV)
The fluorescence of RXRα became weaker from 6 h and most RXRα fluorescence had disappeared by 12 h post ATRA treatment. These results indicate that cytoplasmic RXRα is rapidly degraded in HCT-116Sphk2 cells upon ATRA treatment (Figure 4B)
Summary
Retinoid therapy improves the outcomes of many cancers. The biological function of retinoids is achieved through binding with their nuclear DNA-binding receptors retinoic acid receptors (RARs) and retinoid X receptors (RXRs). In response to retinoid therapy, RARs and RXRs act as RXRs-RARs heterodimers that bind to a variety of retinoic acid response elements (RARE) sequences which regulate gene transcription through recruitment of corepressors and coactivators [1, 2]. RXRα dimerizes either with other nuclear receptors to form heterodimers or with itself to form homodimers that bind to specific DNA-response elements in the promoter regions of target genes to release corepressors and recruit coactivators, permitting the multiprotein transcriptional machinery to initiate transcription [3, 4]. The translocation of RXRα from the nucleus to the cytoplasm is highly regulated by its dimerization and ligand binding. The translocation www.impactjournals.com/oncotarget of RXRα/Nur heterodimer from the nucleus to the cytoplasm leads to cancer apoptosis [7]. Only one-third of cancer patients respond to retinoid therapy and the resistance mechanisms that limit the efficacy of retinoids in cancer are poorly understood [10]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
