The Sphingosine Kinase 2 Inhibitor ABC294640 Reduces the Growth of Prostate Cancer Cells and Results in Accumulation of Dihydroceramides In Vitro and In Vivo

Abstract

Despite recent advances in the development of novel therapies against castration resistant prostate cancer, the advanced form of the disease remains a major treatment challenge. Aberrant sphingolipid signaling through sphingosine kinases and their product sphingosine-1-phosphate can promote proliferation, drug resistance, angiogenesis and inflammation. The sphingosine kinase 2 inhibitor ABC294640 is undergoing clinical testing in cancer patients, and in this study we investigated the effects this first-in-class inhibitor in castration resistant prostate cancer. In vitro, ABC294640 decreased prostate cancer cell viability as well as the expression of c-Myc and the androgen receptor while lysosomal acidification increased. ABC294640 also induced a greater than 3-fold increase in dihydroceramides that inversely correlated with inhibition of dihydroceramide desaturase (DEGS) activity. Expression of sphingosine kinase 2 was dispensable for the ABC294640-mediated increase in dihydroceramides. In vivo, ABC294640 diminished the growth rate of TRAMP-C2 xenografts in syngeneic hosts and elevated dihydroceramides within tumors as visualized by MALDI imaging mass spectroscopy. The plasma of ABC294640 treated mice contained significantly higher levels of C16- and C24:1-ceramides (but not dihydro-C16-ceramide) compared to vehicle treated mice. In summary, our results suggest that ABC294640 may reduce the proliferative capacity of castration resistant prostate cancer cells through both, inhibition of sphingosine kinase 2 and dihydroceramide desaturase, which provides a foundation for future exploration of this small molecule inhibitor for the treatment of advanced disease.

Sphingosine Kinase 1: A Potential Therapeutic Target in Pulmonary Arterial Hypertension?

“…Indeed, we have shown that proteasomal degradation of SphK1 by 'indirect' inducers occurs via additional inhibition of dihydroceramide desaturase (Degs1) (an enzyme within the de novo ceramide synthesis pathway). In this regard, SKi inhibited Degs1 activity in Jurkat and prostate cancer cells [44,45]. We have proposed that Degs1 might function to limit protein degradation flux through the proteasome [44] and therefore its inhibition by SKi promotes proteasomal degradation of SphK1.…”

Recent advances in the development of sphingosine kinase inhibitors

“…It was recently found that ABC294640 acts as a weak estrogen receptor (ERα) antagonist with low micromolar affinity (100-fold higher than estrogen and 200-fold higher than tamoxifen) [113]. However, the recent discovery that ABC294640 inhibits Des1, leading to a 3-fold increase in dihydroceramide species [53,80] means that earlier studies using this inhibitor may need some re-interpretation.…”

“…More recent studies with ABC294640 have implied roles for SK2 in regulating autophagy [119], transcription of key cancer-promoting genes such as c-Myc [36,98] and pro-inflammatory mediators through NF-κB [120][121][122][123]. ABC294640 also induced proteasomal degradation of SK1 and Des1 [80], Myc and other targets such as androgen receptor [53,98] and Mcl-1 [37] through an as yet undetermined mechanism that appears to be related to its ability to inhibit Des1 [80].…”

“…The findings that a number of potent SK1 inhibitors fail to induce cancer cell death despite effectively reducing cellular S1P levels [45][46][47], as well as discrepancies with SK2-selective inhibitors either increasing [48,49] or decreasing [50,51] circulating S1P levels in mice have necessitated re-evaluation of how rigorously inhibitors need to be assessed for on-and off-target effects, particularly within the sphingolipid pathway. Indeed, important off-targets have been recently reported for a number of commonly-used SK inhibitors [52,53], meaning re-evaluation of previously published work is required in light of this.…”

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“…This might represent an early event in ER stress via deployment of News ER-associated degradation (ERAD) that is then switched to CHOP/caspase-3-induced apoptosis in the presence of bortezomib. Second, ABC294640 also inhibits dihydroceramide desaturase (Des1) [6,7] that we have suggested might be a 'gate-keeper' of the proteasome [6]. Third, we have also shown that the dual SK1/SK2 and Des1 inhibitor, SKi (4-([4-(4-chlorophenyl)thiazol-2-yl] amino) phenol) induces an ER stress/UPR in T-cell lymphoblastic leukemic (T-ALL) cells that results in a protective cell survival autophagy [8].…”