Lyn, an Src-type tyrosine kinase, is associated with the interleukin (IL)-5 receptor in eosinophils. The mechanism of its activation is unknown. Through yeast twohybrid screening we have cloned and characterized a new signaling molecule, Unc119, that associates with IL-5R␣ and Src family tyrosine kinases. Unc119 induces the catalytic activity of these kinases through interaction with Src homology 2 and 3 domains. IL-5 stimulation of eosinophils increases Unc119 association with Lyn and induces its catalytic activity. Lyn is important for eosinophil survival. Eosinophils that are transduced with Unc119 have increased Lyn activity and demonstrate prolonged survival in the absence of IL-5. Inhibition of Unc119 down-regulates eosinophil survival. To our knowledge Unc119 is the first receptor-associated activator of Src family tyrosine kinases.One of the fundamental goals of cell biology is to understand the mechanism of signal generation by receptors. Many receptors rely upon kinases, especially tyrosine kinases, for receptor phosphorylation and activation of signaling cascades. The Src family tyrosine kinases (SrcTKs) 1 frequently serve as the trigger mechanism for cytosolic signals (1). Receptor-associated SrcTKs exist in a non-active conformation and become transiently activated following ligand binding and receptor oligomerization (2). How receptor oligomerization leads to the activation of SrcTKs is unknown.Two intramolecular interactions tightly regulate structural conformation and enzymatic activity of SrcTKs. One is through Src homology 2 (SH2) and the other is through the SH3 domain. All cellular SrcTKs have a C-terminal regulatory tyrosine residue (Tyr 527 for Src). Phosphorylated Tyr 527 forms an intramolecular interaction with the SH2 domain of the kinase (3). This interaction decreases the kinase activity. Dephosphorylation of this tyrosine residue by phosphatases such as CD45 is required but is not sufficient for kinase activation (4). Receptor-bound SrcTKs are frequently found to be dephosphorylated under basal conditions, yet lack appreciable catalytic activity (5). CD45-negative cells have variable effects on SrcTKs. CD45 negatively regulates Lyn by dephosphorylating both its negative and positive regulatory tyrosine residues (6). The activity of Lyn and Hck is increased in macrophages (7) and B cells (8) lacking CD45.Regulation of SrcTK activity through other intramolecular interactions was realized after the crystal structures of Src (9) and Hck (10) were solved. The linker region that is located between the SH2 and the kinase domains binds to the SH3 domain of the kinase rendering a non-active conformation. Furthermore, a short amino acid stretch connecting the SH2 and SH3 domains has recently been identified as an additional negative regulator of Hck and Src activation (11). The SH3 domain of SrcTKs prefers to bind to the RXXPXXP motif (a motif is defined as a short stretch of amino acid residues that binds to a signaling domain) (12, 13). Although the linkers of SrcTK bind to the SH3 domain, ...

Recently it was discovered that a transient activation of transcription factor NF-κB can give cells properties essential for invasiveness and cancer initiating potential. In contrast, most oncogenes to date were characterized on the basis of mutations or by their constitutive overexpression. Study of NF-κB actually leads to a far more dynamic perspective on cancer: tumors caused by diverse oncogenes apparently evolve into cancer after loss of feedback regulation for NF-κB. This event alters the cellular phenotype and the expression of hormonal mediators, modifying signals between diverse cell types in a tissue. The result is a disruption of stem cell hierarchy in the tissue, and pervasive changes in the microenvironment and immune response to the malignant cells.

Cancer cells have altered metabolism and, in some cases, an increased demand for cholesterol. It is important to identify novel, rational treatments based on biology, and cellular cholesterol metabolism as a potential target for cancer is an innovative approach. Toward this end, we focused on diffuse large B-cell lymphoma (DLBCL) as a model because there is differential cholesterol biosynthesis driven by B-cell receptor (BCR) signaling in germinal center (GC) versus activated B-cell (ABC) DLBCL. To specifically target cellular cholesterol homeostasis, we employed high-density lipoprotein-like nanoparticles (HDL NP) that can generally reduce cellular cholesterol by targeting and blocking cholesterol uptake through the high-affinity HDL receptor, scavenger receptor type B-1 (SCARB1). As we previously reported, GC DLBCL are exquisitely sensitive to HDL NP as monotherapy while ABC DLBCL are less sensitive. Herein, we report that enhanced BCR signaling and resultant de novo cholesterol synthesis in ABC DLBCL drastically reduces the ability of HDL NPs to reduce cellular cholesterol and induce cell death. Therefore, we combined HDL NP with the BCR signaling inhibitor ibrutinib and the SYK inhibitor R406. By targeting both cellular cholesterol uptake and BCR-associated de novo cholesterol synthesis, we achieved cellular cholesterol reduction and induced apoptosis in otherwise resistant ABC DLBCL cell lines. These results in lymphoma demonstrate that reduction of cellular cholesterol is a powerful mechanism to induce apoptosis. Cells rich in cholesterol require HDL NP therapy to reduce uptake and molecularly targeted agents that inhibit upstream pathways that stimulate de novo cholesterol synthesis, thus, providing a new paradigm for rationally targeting cholesterol metabolism as therapy for cancer.