Autocrine/paracrine sphingosine‐1‐phosphate fuels proliferative and stemness qualities of glioblastoma stem cells

Abstract

Accumulating reports suggest that human glioblastoma contains glioma stem-like cells (GSCs) which act as key determinants driving tumor growth, angiogenesis, and contributing to therapeutic resistance. The proliferative signals involved in GSC proliferation and progression remain unclear. Using GSC lines derived from human glioblastoma specimens with different proliferative index and stemness marker expression, we assessed the hypothesis that sphingosine-1-phosphate (S1P) affects the proliferative and stemness properties of GSCs. The results of metabolic studies demonstrated that GSCs rapidly consume newly synthesized ceramide, and export S1P in the extracellular environment, both processes being enhanced in the cells exhibiting high proliferative index and stemness markers. Extracellular S1P levels reached nM concentrations in response to increased extracellular sphingosine. In addition, the presence of EGF and bFGF potentiated the constitutive capacity of GSCs to rapidly secrete newly synthesized S1P, suggesting that cooperation between S1P and these growth factors is of central importance in the maintenance and proliferation of GSCs. We also report for the first time that S1P is able to act as a proliferative and pro-stemness autocrine factor for GSCs, promoting both their cell cycle progression and stemness phenotypic profile. These results suggest for the first time that the GSC population is critically modulated by microenvironmental S1P, this bioactive lipid acting as an autocrine signal to maintain a pro-stemness environment and favoring GSC proliferation, survival and stem properties.

The Role and Function of Sphingolipids in Glioblastoma Multiforme

“…Interestingly, besides changing the growth pattern of GSCs, S1P induced elevated levels of different stem cell markers in GSCs, thus enhancing their stemness phenotype [ 86 ]. Since S1P did not induce stemness marker expression in non-stem primary GBM cells, it is likely that the stemness promoting action of S1P resides in its ability to favor the selective expansion of GSCs.…”

“…The enrichment of GSCs in perinecrotic regions [ 90 ], suggests that in these locations S1P biosynthesis and release occurs very rapidly and to a high extent in GSCs, providing these cells a favorable environment. Of interest, the proliferative and stemness properties of GSCs are related to a gain in extracellular S1P, paralleled by a down-regulation of ceramide levels due to its increased fl ux to complex sphingolipids [ 86 ], indicating that the imbalance of the sphingolipid rheostat, with the S1P domination in the GSC niche, provides an advantage to the stemness qualities of GSCs.…”

“…When exposed to exogenous S1P, GSCs switch to a faster growth mode implying that S1P can act an autocrine signal for GSCs to grow and proliferate. Indeed, S1P sends a proliferative signal to GSCs that is strong enough to promote cell cycle and progression into G2/M phase of GSCs [ 86 ]. A consistent body of studies indicates that EGF is an important factor in the GSC environment, supporting GSC growth and inhibiting their spontaneous differentiation [ 92 , 93 ].…”

“…The effi ciency of S1P export by GSCs is strictly dependent on the availability of its substrate sphingosine [ 86 ], that can be released from necrotic cells. The enrichment of GSCs in perinecrotic regions [ 90 ], suggests that in these locations S1P biosynthesis and release occurs very rapidly and to a high extent in GSCs, providing these cells a favorable environment.…”

“…Noteworthy, EGF receptor III variant level, particularly expressed in GSCs and crucial for their stemness and tumorigenic potential [ 92 , 94 ], is directly related to SphK1 activity, that in turn determines a proliferative and survival advantage [ 95 ]. Furthermore, EGF and bFGF, which are crucial factors supporting GSC growth and stemness, promote S1P export by GSCs [ 86 ]. S1P proliferative action in GSCs seems dependent on its receptor binding, as it is inhibited by FTY720, a sphingosine analogue that, after cellular internalization and phosphorylation, is released extracellularly and down-regulates S1P receptors, acting as functional antagonist of S1P [ 96 ].…”