LXRß controls glioblastoma cell growth, lipid balance, and immune modulation independently of ABCA1.

Cholesterol is a critical component of membranes and a precursor for hormones and other signaling molecules. Previously, we showed that unlike astrocytes, glioblastoma cells do not downregulate cholesterol synthesis when plated at high density. In this report, we show that high cell density induces ABCA1 expression in glioblastoma cells, enabling them to get rid of excess cholesterol generated by an activated cholesterol biosynthesis pathway. Because oxysterols are agonists for Liver X Receptors (LXRs), we investigated whether increased cholesterol activates LXRs to maintain cholesterol homeostasis in highly-dense glioblastoma cells. We observed that dense cells had increased oxysterols, which activated LXRß to upregulate ABCA1. Cells with CRISPR-mediated knockdown of LXRß, but not ABCA1, had decreased cell cycle progression and cell survival, and decreased feedback repression of the mevalonate pathway in densely-plated glioma cells. LXRß gene expression poorly correlates with ABCA1 in glioblastoma patients, and expression of each gene correlates with poor patient prognosis in different prognostic subtypes. Finally, gene expression and lipidomics analyses cells revealed that LXRß regulates the expression of immune response gene sets and lipids known to be involved in immune modulation. Thus, therapeutic targeting of LXRß in glioblastoma might be effective through diverse mechanisms.

p75 Is Required for the Establishment of Postnatal Sensory Neuron Diversity by Potentiating Ret Signaling.

Producing the neuronal diversity required to adequately discriminate all elements of somatosensation is a complex task during organogenesis. The mechanisms guiding this process during dorsal root ganglion (DRG) sensory neuron specification remain poorly understood. Here, we show that the p75 neurotrophin receptor interacts with Ret and its GFRα co-receptor upon stimulation with glial cell line-derived neurotrophic factor (GDNF). Furthermore, we demonstrate that p75 is required for GDNF-mediated Ret activation, survival, and cell surface localization of Ret in DRG neurons. In mice in which p75 is deleted specifically within sensory neurons beginning at E12.5, we observe that approximately 20% of neurons are lost between P14 and adulthood, and these losses selectively occur within a subpopulation of Ret+ nonpeptidergic nociceptors, with neurons expressing low levels of Ret impacted most heavily. These results suggest that p75 is required for the development of the nonpeptidergic nociceptor lineage by fine-tuning Ret-mediated trophic support.

Disabled cell density sensing leads to dysregulated cholesterol synthesis in glioblastoma.

A hallmark of cellular transformation is the evasion of contact-dependent inhibition of growth. To find new therapeutic targets for glioblastoma, we looked for pathways that are inhibited by high cell density in astrocytes but not in glioma cells. Here we report that glioma cells have disabled the normal controls on cholesterol synthesis. At high cell density, astrocytes turn off cholesterol synthesis genes and have low cholesterol levels, but glioma cells keep this pathway on and maintain high cholesterol. Correspondingly, cholesterol pathway upregulation is associated with poor prognosis in glioblastoma patients. Densely-plated glioma cells increase oxygen consumption, aerobic glycolysis, and the pentose phosphate pathway to synthesize cholesterol, resulting in a decrease in reactive oxygen species, TCA cycle intermediates, and ATP. This constitutive cholesterol synthesis is controlled by the cell cycle, as it can be turned off by cyclin-dependent kinase inhibitors and it correlates with disabled cell cycle control though loss of p53 and RB. Finally, glioma cells, but not astrocytes, are sensitive to cholesterol synthesis inhibition downstream of the mevalonate pathway, suggesting that specifically targeting cholesterol synthesis might be an effective treatment for glioblastoma.