Sphingomyelin Metabolism Is a Regulator of K-Ras Function

Abstract

KRAS must localize to the plasma membrane (PM) for biological activity. We show here that multiple acid sphingomyelinase (ASM) inhibitors, including tricyclic antidepressants, mislocalized phosphatidylserine (PtdSer) and KRASG12V from the PM; resulting in abrogation of KRASG12V signaling and potent, selective growth inhibition of mutant KRAS transformed cancer cells. Concordantly, in nude mice, the ASM inhibitor fendiline decreased the rate of growth of oncogenic KRAS-expressing MiaPaCa-2 tumors, but had no effect on the growth of the wild-type KRAS-expressing BxPC-3 tumors. ASM inhibitors also inhibited activated LET-60 (a KRAS ortholog) signaling in as evidenced by suppression of the induced multi-vulva phenotype. Using RNAis against genes encoding other enzymes in the sphingomyelin (SM) biosynthetic pathway, we identified 14 enzymes whose knockdown strongly or moderately suppressed the LET-60 multi-vulva phenotype. In mammalian cells, pharmacological agents that target these enzymes all depleted PtdSer from the PM and caused KRASG12V mislocalization. These effects correlated with changes in SM levels or subcellular distribution. Selected compounds, including sphingosine kinase inhibitors, potently inhibited the proliferation of oncogenic KRAS-expressing pancreatic cancer cells. In conclusion, these results show that normal SM metabolism is critical for KRAS function, which may present therapeutic options for the treatment of KRAS-driven cancers.

HRAS‐driven cancer cells are vulnerable to TRPML1 inhibition

“…Other endolysosomal transporters and channels have also been implicated in cancer progression [65]. Our findings are also reminiscent of the reports that inhibition of lysosomal acid sphingomyelinase attenuates ERK signaling and tumorigenesis [8,10,11]. The overlap in the phenotypes associated with inhibition of acid sphingomyelinase and TRPML1 raises the question of whether these proteins experience functional interdependence.…”

“…For example, molecular mechanisms that permit cancer-specific reorganization of cellular metabolism constitute pathways that could be targeted to deter tumorigenesis with exquisite sensitivity and specificity [4][5][6]. In this context, components of the autophagic and endolysosomal system represent actionable targets [7][8][9][10][11]. Indeed, arresting autophagy and lysosomal degradation via dissipation of the endolysosomal pH gradient using chloroquine is beneficial in some preclinical cancer models, although it is not clear whether the sensitivity to chloroquine correlates with RAS mutations [12,13].…”

Deciphering lipid codes: K‐Ras as a paradigm

“…Thus, interfering with PtdSer transport via targeting ASM is a promising strategy for inhibiting K-Ras function. Many other tricyclic anti-depressants, such as desipramine, imipramine and amitriptyline, are also effective ASM inhibitors and preliminary data suggest that all of these drugs mis-localize PtdSer and K-Ras from the PM and disrupt K-Ras nanoclustering 62 . The mechanism of action of another pharmacological agent staurosporine, that also depletes PtdSer and hence K-Ras from the PM 44 , has recently been described 63 .…”

“…Staurosporine depletes cells of ORMDL3 a negative regulator of the first step in SM biosynthesis, in consequence cells become loaded with SM, which again induces lysosomal dysfunction and PtdSer loss from the PM 63 . Following on from these studies, recent work suggests that there are many other critical enzymes in the SM metabolic pathway that are required to maintain PM PtdSer levels and hence K-Ras function 62 .…”

“…Many other tricyclic anti-depressants, such as desipramine, imipramine and amitriptyline, are also effective ASM inhibitors and preliminary data suggest that all of these drugs mis-localize PtdSer and K-Ras from the PM and disrupt K-Ras nanoclustering. 62 The mechanism of action of another pharmacological agent staurosporine, that also depletes PtdSer and hence K-Ras from the PM, 44 has recently been described. 63 Staurosporine depletes cells of ORMDL3, a negative regulator of the first step in SM biosynthesis, in consequence cells become loaded with SM, which again induces lysosomal dysfunction and PtdSer loss from the PM.…”