Type 1 Insulin-Like Growth Factor Receptor Nuclear Localization in High-Grade Glioma Cells Enhances Motility, Metabolism, and In Vivo Tumorigenesis

Gliomas are the most frequent solid tumors in children. Among these, high-grade gliomas are less common in children than in adults, though they are similar in their aggressive clinical behavior. In adults, glioblastoma is the most lethal tumor of the central nervous system. Insulin-like growth factor 1 receptor (IGF1R) plays an important role in cancer biology, and its nuclear localization has been described as an adverse prognostic factor in different tumors. Previously, we have demonstrated that, in pediatric gliomas, IGF1R nuclear localization is significantly associated with high-grade tumors, worst clinical outcome, and increased risk of death. Herein we explore the role of IGF1R intracellular localization by comparing two glioblastoma cell lines that differ only in their IGF1R capacity to translocate to the nucleus. In vitro, IGF1R nuclear localization enhances glioblastoma cell motility and metabolism without affecting their proliferation. In vivo, IGF1R has the capacity to translocate to the nucleus and allows not only a higher proliferation rate and the earlier development of tumors but also renders the cells sensitive to OSI906 therapy. With this work, we provide evidence supporting the implications of the presence of IGF1R in the nucleus of glioma cells and a potential therapeutic opportunity for patients harboring gliomas with IGF1R nuclear localization.

Macrobrachium borellii hepatopancreas contains a mitochondrial glycerol-3-phosphate acyltransferase which initiates triacylglycerol biosynthesis

Mammals express four isoforms of glycerol-3-phosphate acyltransferase (GPAT). The mitochondrial isoform GPAT1 may have been the acyltransferase that appeared first in evolution. The hepatopancreas of the crustacean Macrobrachium borellii has a high capacity for triacylglycerol (TAG) biosynthesis and storage. In order to understand the mechanism of glycerolipid biosynthesis in M. borellii, we investigated its hepatopancreas GPAT activity. In hepatopancreas mitochondria, we identified a GPAT activity with characteristics similar to those of mammalian GPAT1. The activity was resistant to inactivation by SH-reactive N-ethylmaleimide, it was activated by polymyxin-B, and its preferred substrate was palmitoyl-CoA. The reaction products were similar to those of mammalian GPAT1. A 70-kDa protein band immunoreacted with an anti-rat liver GPAT1 antibody. Surprisingly, we did not detect high GPAT specific activity in hepatopancreas microsomes. GPAT activity in microsomes was consistent with mitochondrial contamination, and its properties were similar to those of the mitochondrial activity. In microsomes, TAG synthesis was not dependent on the presence of glycerol-3 phosphate as a substrate, and the addition of monoacylglycerol as a substrate increased TAG synthesis 2-fold. We conclude that in M. borellii the de novo triacylglycerol biosynthetic pathway can be completed in the mitochondria. In contrast, TAG synthesis in the ER may function via the monoacylglycerol pathway.

Mitochondrial glycerol phosphate acyltransferase contains two transmembrane domains with the active site in the N-terminal domain facing the cytosol

The topography of mitochondrial glycerol-3-phosphate acyltransferase (GPAT) was determined using rat liver mitochondria and mutagenized recombinant rat GPAT (828 aa (amino acids)) expressed in CHO cells. Hydrophobicity analysis of GPAT predicts two transmembrane domains (TMDs), residues 472-493 and 576-592. Residues 224-323 correspond to the active site of the enzyme, which is believed to lie on the cytosolic face of the outer mitochondrial membrane. Protease treatment of rat liver mitochondria revealed that GPAT has a membrane-protected segment of 14 kDa that could correspond to the mass of the two predicted TMDs plus a loop between aa 494 and 575. Recombinant GPAT constructs containing tagged epitopes were transiently expressed in Chinese hamster ovary cells and immunolocalized. Both the C and N termini epitope tags could be detected after selective permeabilization of only the plasma membrane, indicating that both termini face the cytosol. A 6-8-fold increase in GPAT-specific activity in the transfected cells confirmed correct protein folding and orientation. When the C terminus and loop-tagged GPAT construct was immunoassayed, the epitope at the C terminus could be detected when the plasma membrane was permeabilized, but loop-epitope accessibility required disruption of the outer mitochondrial membrane. Similar results were observed when GPAT was truncated before the second TMD, again consistent with an orientation in which the loop faces the mitochondrial intermembrane space. Although protease digestion of the HA-tagged loop resulted in preservation of a 14-kDa fragment, consistent with a membrane protected loop domain, neither the truncated nor loop-tagged enzymes conferred GPAT activity when overexpressed, suggesting that the loop plays a critical structural or regulatory role for GPAT function. Based on these data, we propose a GPAT topography model with two transmembrane domains in which both the N (aa 1-471) and C (aa 593-end) termini face the cytosol and a single loop (aa 494-575) faces the intermembrane space.