Characterization of activating mutations of NOTCH3 in T-cell acute lymphoblastic leukemia and anti-leukemic activity of NOTCH3 inhibitory antibodies

Notch receptors have been implicated as oncogenic drivers in several cancers, the most notable example being NOTCH1 in T-cell acute lymphoblastic leukemia (T-ALL). To characterize the role of activated NOTCH3 in cancer, we generated an antibody that detects the neo-epitope created upon gamma-secretase cleavage of NOTCH3 to release its intracellular domain (ICD3), and sequenced the negative regulatory region (NRR) and PEST domain coding regions of NOTCH3 in a panel of cell lines. We also characterize NOTCH3 tumor-associated mutations that result in activation of signaling and report new inhibitory antibodies. We determined the structural basis for receptor inhibition by obtaining the first co-crystal structure of a NOTCH3 antibody with the NRR protein and defined two distinct epitopes for NRR antibodies. The antibodies exhibit potent anti-leukemic activity in cell lines and tumor xenografts harboring NOTCH3 activating mutations. Screening of primary T-ALL samples reveals that two of 40 tumors examined show active NOTCH3 signaling. We also identified evidence of NOTCH3 activation in 12 of 24 patient-derived orthotopic xenograft models, two of which exhibit activation of NOTCH3 without activation of NOTCH1. Our studies provide additional insights into NOTCH3 activation and offer a path forward for identification of cancers that are likely to respond to therapy with NOTCH3 selective inhibitory antibodies.

Lysophospholipids--receptor revelations

Upon cell activation, membrane phospholipids are metabolized into potent lysophospholipid (LP) mediators, such as sphingosine 1-phosphate and lysophosphatidic acid. LPs fulfill signaling roles in organisms as diverse as yeast and humans. The recent discovery of G protein-coupled receptors for LPs in higher eukaryotes, and their involvement in regulating diverse processes such as angiogenesis, cardiac development, neuronal survival, and immunity, has stimulated growing interest in these lipid mediators. LP receptor biology has generated insights into fundamental cellular mechanisms and may provide therapeutic targets for drug development.

Role of the sphingosine 1-phosphate receptor EDG-1 in vascular smooth muscle cell proliferation and migration

Sphingosine 1-phosphate (S1P), a platelet-derived ligand for the EDG-1 family of G protein-coupled receptors (GPCRs), has recently emerged as a regulator of vascular development. Although S1P has potent effects on endothelial cells and vascular smooth muscle cells (VSMCs), the functions of the specific S1P receptors in the latter cell type are not known. Here we show that pup-intimal VSMCs express higher levels of EDG-1 mRNA than adult-medial VSMCs. Stable transfection of EDG-1 into adult-medial VSMCs enhanced their proliferative response to S1P, concomitant with induction of p70 S6 kinase activity and expression of cyclin D1. Pertussis toxin treatment inhibited S1P-induced p70 S6 kinase activation, cyclin D1 expression and proliferation, suggesting that EDG-1-coupling to the G(i) pathway is critical. Furthermore, blocking p70 S6 kinase phosphorylation with rapamycin inhibited cyclin D1 expression and proliferation, suggesting that activation of p70 S6 kinase is critical in EDG-1/G(i)-mediated cell proliferation. EDG-1 expression also profoundly enhanced the migratory response of adult-medial VSMCs to S1P. S1P-induced migration of adult-medial VSMCs expressing exogenous EDG-1 required G(i) activation but not p70 S6 kinase. These results suggest that enhanced expression of EDG-1 in VSMCs dramatically stimulates both the proliferative and migratory responses to S1P. Since EDG-1 is expressed in the pup-intimal phenotype of VSMCs, S1P signaling via EDG-1 may play a role in vascular diseases in which the proliferation and migration of VSMCs are dysregulated.