The Drug Transporter P-Glycoprotein and Its Impact on Ceramide Metabolism-An Unconventional Ally in Cancer Treatment

Abstract

The tumor-suppressor sphingolipid ceramide is recognized as a key participant in the cytotoxic mechanism of action of many types of chemotherapy drugs, including anthracyclines, Vinca alkaloids, the podophyllotoxin etoposide, taxanes, and the platinum drug oxaliplatin. These drugs can activate de novo synthesis of ceramide or stimulate the production of ceramide via sphingomyelinases to limit cancer cell survival. On the contrary, dysfunctional sphingolipid metabolism, a prominent factor in cancer survival and therapy resistance, blunts the anticancer properties of ceramide-orchestrated cell death pathways, especially apoptosis. Although P-glycoprotein (P-gp) is famous for its role in chemotherapy resistance, herein, we propose alternate interpretations and discuss the capacity of this multidrug transporter as a "ceramide neutralizer", an unwelcome event, highlighting yet another facet of P-gp's versatility in drug resistance. We introduce sphingolipid metabolism and its dysfunctional regulation in cancer, present a summary of factors that contribute to chemotherapy resistance, explain how P-gp "neutralizes" ceramide by hastening its glycosylation, and consider therapeutic applications of the P-gp-ceramide connection in the treatment of cancer.

Keywords: P-glycoprotein; cancer drug resistance; ceramide; sphingolipids.

Figure 1

Ceramide metabolism and metabolism of higher sphingolipids. Ceramide is synthesized in the endoplasmic reticulum. The de novo biosynthesis starts with the condensation of serine and palmitoyl-CoA, catalyzed by serine palmitoyltransferase. The product, 3-ketosphinganine, contains 18 carbons and is reduced to sphinganine by 3-ketosphinganine reductase. The next step generates the saturated ceramide precursor, dihydroceramide, via the action of ceramide synthases, of which there are six isoforms (CerS1-6) that ultimately give rise to a multitude of molecular species with distinct roles. Finally, although dihydroceramide is nearly identical in structure to ceramide, it lacks the 4,5-trans double bond, which is inserted by dihydroceramide desaturase to form ceramide. Ceramide can also be produced by the action of specialized phospholipases, known as sphingomyelinases. Sphingomyelinases, which are characterized according to their pH optimum and subcellular locations, cleave sphingomyelin at the phosphodiester bond that is proximal to ceramide, producing ceramide and choline phosphate. Ceramide can also be formed by the action of ceramide-1-phosphate (C1-P) phosphatase and by glucosylcerebrosidase. Once produced, ceramide can be hydrolyzed by ceramidases, glycosylated by glucosylceramide synthase (GCS), used to generate sphingomyelin by sphingomyelin synthases, or phosphorylated by ceramide kinase producing C1-P. Strategic points in de novo synthesis and in subsequent ceramide metabolism can be activated or inhibited, providing useful avenues to define ceramide-regulated events, such as cell fate. Enzyme inhibitors and P-glycoprotein (P-gp) antagonists are often used to amplify the induction of cell death by ceramide. Enzymes that deplete intracellular ceramide either by catabolism (e.g., ceramidase activity) or anabolism (e.g., glycosylation, sphingomyelin synthesis, phosphorylation) can contribute to cancer cell growth.

Figure 2

Factors contributing to chemotherapy resistance. The inclusion of dysfunctional sphingolipid metabolism marks an important biology that contributes to chemotherapy resistance. As noted in the text, modifications in sphingolipid metabolism work in concert with other contributors, such as the ABC transporter, P-gp, which directs chemotherapy efflux and also participates in ceramide metabolism. P-gp, P-glycoprotein. Created with BioRender.com.

Figure 3

The impact of chemotherapy selection pressure on sphingolipid metabolism. Cancer cells grown to acquire resistance to anticancer agents display prominent elevations in the expression and activity of some of the key enzymes in sphingolipid metabolism, including GCS, AC, and SPHK1 as indicated by the upward arrows. Through glycosylation, GCS converts proapoptotic ceramide to GlcCer, a detoxified product. Likewise, AC hydrolyzes ceramide to disable its cytotoxic impact. SPHK1 utilizes sphingosine, a product of AC, to produce S1-P and elicit mitogenic responses. GCS, glucosylceramide synthase; AC, acid ceramidase; SPHK1, sphingosine kinase 1; So, sphingosine.

Figure 4

P-glycoprotein antagonists enhance efficacy of anticancer agents that employ ceramides in mechanism of action. (A) Anticancer drugs (e.g., CNL, daunorubicin, etoposide, cytarabine, fenretinide, imatinib) can increase intracellular ceramide levels via de novo synthesis, ceramide synthase activation, and sphingomyelinase activation. (B) The resulting ceramide deluge converges on mitochondria to elicit the intrinsic pathway of apoptosis (pro-death). Upregulated activities of (C) GCS and (D) P-gp, characteristic in multidrug resistance, hijack this ceramide-governed cell death pathway by hyper-conversion of ceramide to GlcCer. (E) P-gp antagonists block drug efflux to circumvent efflux-mediated resistance mechanisms. (F) GCS inhibitors (e.g., NB-DNJ) and (G) P-gp inhibitors (e.g., zosuquidar, tamoxifen, tariquidar, elacridar) can block ceramide glycosylation directly at GCS or indirectly at Golgi-resident P-gp by blocking GlcCer entry into the Golgi, promoting buildup of GlcCer and product inhibition of GCS. (H) The inhibitory symbol from GlcCer to GCS indicates product inhibition. GCS, glucosylceramide synthase; P-gp, P-glycoprotein; GlcCer, glucosylceramide; NB-DNJ, 1N-Butyldeoxynojirimycin; LacCer, lactosylceramide; CNL, ceramide nanoliposome. Created with BioRender.com.