Glycosidated phospholipids: uncoupling of signalling pathways at the plasma membrane

Abstract

Cell expansion and metastasis are considered hallmarks of tumour progression. Therefore, efforts have been made to develop novel anti-cancer drugs that inhibit both the proliferation and the motility of tumour cells. Synthetic alkylphospholipids, compounds with aliphatic side chains that are ether linked to a glycerol backbone, are structurally derived from platelet-activating factor and represent a new class of drugs with anti-proliferative properties in tumour cells. These compounds do not interfere with the DNA or mitotic spindle apparatus of the cell. Instead, they are incorporated into cell membranes, where they accumulate and interfere with lipid metabolism and lipid-dependent signalling pathways. Recently, it has been shown that the most commonly studied alkylphospholipids inhibit proliferation by inducing apoptosis in malignant cells while leaving normal cells unaffected. This review focuses on a novel group of synthetic alkylphospholipids, the glycosidated phospholipids, which contain carbohydrates or carbohydrate-related molecules at the sn-2 position of the glycerol backbone. Members of this subfamily also exhibit anti-proliferative capacity and modulate the cell adhesion, differentiation, and migration of tumour cells. Among this group, Ino-C2-PAF shows the highest efficacy and low cytotoxicity. Apart from its anti-proliferative effect, Ino-C2-PAF strongly reduces cell motility via its inhibitory effect on the phosphorylation of the cytosolic tyrosine kinases FAK and Src. Signalling pathways under the control of the FAK/Src complex are normally required for both migration and proliferation and play a prominent role in tumour progression. We intend to highlight the potential of glycosidated phospholipids, especially Ino-C2-PAF, as a promising new group of drugs for the treatment of hyperproliferative and migration-based skin diseases.

Figure 1

Structure of platelet-activating factor (PAF) and the common antitumour lipids edelfosine, miltefosine and perifosine.

Figure 2

Biosynthesis of phosphatidylcholine.

Figure 3

Intrinsic and extrinsic apoptotic pathways. The extrinsic pathway is initiated by binding of ligands (FasL, tumour necrosis factor, or other cytokines) to death receptors, which recruits the adaptor protein Fas-associated Death Domain (FADD), and, consequently, procaspase-8. The formation of the death-inducing-signaling-complex (DISC) results in cleavage and auto-activation of caspase-8. The activated caspase-8 can then proteolytically activate downstream caspases (particularly caspase-3), which induce cell shrinkage, membrane blebbing and DNA fragmentation. Apoptosis can also be triggered by various apoptotic stimuli, which results in the activation of the intrinsic pathway. Here, caspase-8 cleaves and activates Bid. Subsequently, t-Bid translocates to the mitochondria where it can induce the oligomerization of Bax and/or Bak in the outer mitochondrial membrane leading to cytochrome c release. Afterwards, Apaf-1 binds cytochrome c and recruits caspase-9 to form a large complex, called apoptosome, which is responsible for the following avtivation of caspase-3. Anti-apoptotic Bcl-2 and Bcl-xL oppose the effecst of Bid and Bax.

Figure 4

Integrin signalling at focal adhesions. After binding to the extracellular matrix (ECM), the cytoplasmic tail of integrin beta 1 recruits focal adhesion kinase (FAK), which is activated by auto-phosphorylation at the tyrosine 397, and thereby allow the binding and activation of Src by phosphorylation of tyrosine 418. The activated FAK/Src complex phosphorylates other kinases (i.e. p130Cas and PI3K), which in turn elicit a cascade of events that lead to cell proliferation, survival and migration. Alternatively, alpha subunits of certain integrins are able to recruit Fyn (a Src family member) via caveolin-1. This allows the activation of Shc, which combines with adaptor proteins Grb2 and SOS in order to activate the extracellular signal-regulated kinase (ERK)/MAP kinase pathway. As represented, the integrin pathway has reciprocal action with other signalling cascades triggered by several growth factor receptors.

Figure 5

Structure of the glycosidated phospholipids Ino-C2- platelet-activating factor (PAF), Glc-PAF, Glc-PC, glucosimide-PAF and glucosamine-PAF.

Figure 6

Treatment of HaCaT cells with Ino-C2-platelet-activating factor (PAF) attenuates phosphorylation of the cytoplasmic tyrosine kinases FAK and Src. HaCaT cells maintained under sub-confluent conditions and treated with either 5 µM Ino-C2-PAF or under control conditions were labelled for pSrc (Y418) and pFAK (Y397).

Figure 7

Inhibition of migration induced by Ino-C2-platelet-activating factor (PAF) can be rescued by transfection with a constitutively active variant of Rac1 or partially rescued by treatment with the pharmacological inhibitor Y27632 (RhoA signalling pathway inhibitor). Significant differences in migration compared with mock-transfected control cells are marked with an asterisk.