Manipulation Of The Notch Pathway By Gamma-Secretase Inhibitors As A Novel Therapeutic Approach In Multiple Myeloma.

Abstract Notch receptors expressed on hematopoietic stem cells interact with their ligands on bone marrow stromal cells. Thereby they control cell fate decisions and survival. We recently demonstrated that Notch has a pathogenetic role in multiple myeloma (MM), where tight interactions between neoplastic plasma cells and their microenvironment are essential for tumor cell growth (Blood.2004; 103:3511–3515). Our data provided evidence that Notch1 and Notch2 were highly expressed in cultured and primary MM cells, whereas nonneoplastic counterparts showed low to undetectable levels of Notch. Furthermore, our functional data indicated that activating the Notch pathway in MM cells by the Notch ligand Jagged1 potently induced tumor cell growth and suggested that these interactions contribute to myelomagenesis in vivo. In this study, we blocked Notch by novel γ-secretase inhibitors in cultured MM cells. γ-secretase catalyzes the release of the intracellular domain of Notch that subsequently translocates to the nucleus to activate expression of downstream target genes. Inhibition of γ-secretase activity is currently investigated as a therapeutic strategy in Alzheimer’s disease, because γ-secretase similarly cleaves amyloid precursor proteins to release Aβ peptides, accumulation of which is causally related to Alzheimer’s disease. To identify novel γ-secretase inhibitors that might block Notch in MM cells, we analyzed two million compounds by 3D in silico screening. Thereby structurally known inhibitors were compared with compounds from data banks. Novel structurally related compounds (27) were tested by in vitro assays. Specific inhibition of Notch signaling was controlled by RT-PCR of the down-regulated Notch target gene Hes-1 in MM cells. In addition, tumor cell proliferation of MM cells was efficiently blocked by 3 out of 27 newly identified inhibitors in a dose-dependent manner. Currently, we investigate whether these compounds also have potent anti-tumor activity against MM cells in mouse models in vivo. If so, interruption of Notch signaling by newly identified inhibitors might be a novel therapeutic principle to control the proliferation capacity of MM.