SPHINGOLIPID SIGNALING AND DISEASE: THE KEY ROLE OF CERAMIDE TRAFFIC IN CELL FATE REGULATION

Cancer and diabetes are among the most common diseases in western societies. Sphingolipids, a class of lipids ubiquitously present in eukaryotic membranes, play a key role in the regulation of different signal transduction pathways involved in the modulation of many cellular functions [1, 2]. The past two decades have seen increased interest in the bioactive sphingolipids ceramide (Cer) and sphingosine-1 phosphate (S1P). Cer, a central molecule of sphingolipid metabolism, is involved in the control of many cell-stress responses, including growth arrest, senescence and cell death [3]. On the other hand, several studies have proposed a crucial role of S1P in cell growth and survival, cell migration, angiogenesis, and inflamma¬tion [3]. Thus, Cer and S1P can differentially regulate cell death and survival by controlling opposing signaling pathways [4]. So far, a large amount of studies has clarified the complexity of the interplay between S1P, Cer and sphingolipid metabolism and these implications in the etiology of several human diseases. Effectively, deregulation of sphingolipid metabolism is implicated in numerous diseases, and accumulating evidence has shown a clear indication that sphingolipids have important role in the pathogenesis of diabetes and cancer [5]. Very recently, it is emerging a pivotal role of Cer traffic from the Endoplasmic Reticulum (ER) to the Golgi apparatus in the regulation of sphingolipid metabolism. In fact, Cer transport is a highly regulated step in the sphingolipid biosynthesis. Two main mechanisms are involved in Cer transport from ER to the Golgi apparatus: a protein-mediated transport, operated by CERT [6, 7] and a CERT-independent vesicular transport. Moreover, several studies reported that ER is a critical intracellular organelle involved in the control of cell fate [8, 9]. In this light, Cer accumulation in the ER appears to be a key element in the promotion of cell death in different human diseases, as well as in glioblastoma and diabetes. In light of these findings, in my Ph.D. course I wanted to evaluate in cellular models of glioblastoma (GBM) and type 2 diabetes (T2D) the role of Cer transport in the regulation of cell fate. Part 1: Glucolipotoxicity impairs ceramide flow from the ER to the Golgi apparatus in INS-1 -cells T2D is the most common form of diabetes characterized by insulin resistance and β-cell dysfunction. The etiology of T2D is not well established but it is know that loss of insulin secretion is directly linked to a loss of function and death of pancreatic β-cells [10]. Glucolipotoxicity is a condition determined by the combined action of elevated glucose and free fatty acids (FFAs) levels that exerts deleterious effects on pancreatic -cell function and survival. Several mechanisms have been proposed for glucolipotoxicity-induced β-cell dysfunction, and, among them, ER stress and elevations of the proapoptotic sphingolipid Cer appear to play key roles. Moreover, Cer accumulation due to glucolipotoxicity can be associated to the induction of ER stress. As in other cells, in the INS-1 insulinoma cell line, two main mechanisms are involved in Cer transport from the ER to the Golgi apparatus: a CERT-mediated transport [6, 11], and a CERT-independent vesicular transport. Glucolipotoxic conditions, obtained with 0.4 mM palmitate and 30 mM glucose administration, strongly affected both of them by reducing CERT protein synthesis and activity, and the rate of Cer vesicular traffic, thus promoting Cer accumulation in the ER. [11, 12]. The two Cer transport mechanisms are effectively involved in the control of sphingolipid metabolism, participating in the regulation of Cer accumulation in the ER involved in pancreatic -cell function and death during T2D. Part 2: The stimulation of Cer traffic from the ER to the Golgi apparatus S1P-dependent as a survival factor in T98G glioma cells. GBM is the most common malignant primary brain tumor in adults and one of the most lethal human cancers [13]. S1P and Cer have emerged as bio-effectors molecules, involved in both glioblastoma development and resistance [3, 14-17]. Several lines of evidence indicated that, different anticancer drugs, including etoposide, exert cytotoxic effects also promoting accumulation of Cer in the ER. Thus, the transport of Cer from ER to Golgi apparatus could be represent a key pathway for limiting Cer accumulation in the ER and escape from cell death. Moreover, we recently demonstrated that vesicular-mediated Cer transport is positively regulated by the pro-survival pathway phosphatidylinositol 3-phosphate kinase (PI3K)/Akt [18], known effectors of extracellular S1P. In T98G glioma cells, S1P treatment was able to increase vesicular Cer transport from the ER to the Golgi apparatus by PI3K/Akt pathway activation, inducing Cer conversion to SM and glycosphingolipids (GSL). In these cells, the cytotoxic Cer accumulation in the ER promoted by the anticancer drug etoposide was strongly reduced after S1P administration. S1P, by enhancing Cer traffic, act as pro-oncogenic factor, favoring both the reduction of the pro-apoptotic Cer at the ER and the synthesis of complex sphingolipids. By this mechanism S1P also stimulates the generation of new membranes, functional to cell growth and consequently to the tumor survival and progression.