Runx1 Orchestrates Sphingolipid Metabolism and Glucocorticoid Resistance in Lymphomagenesis

The three‐membered RUNX gene family includes RUNX1, a major mutational target in human leukemias, and displays hallmarks of both tumor suppressors and oncogenes. In mouse models, the Runx genes appear to act as conditional oncogenes, as ectopic expression is growth suppressive in normal cells but drives lymphoma development potently when combined with over‐expressed Myc or loss of p53. Clues to underlying mechanisms emerged previously from murine fibroblasts where ectopic expression of any of the Runx genes promotes survival through direct and indirect regulation of key enzymes in sphingolipid metabolism associated with a shift in the “sphingolipid rheostat” from ceramide to sphingosine‐1‐phosphate (S1P). Testing of this relationship in lymphoma cells was therefore a high priority. We find that ectopic expression of Runx1 in lymphoma cells consistently perturbs the sphingolipid rheostat, whereas an essential physiological role for Runx1 is revealed by reduced S1P levels in normal spleen after partial Cre‐mediated excision. Furthermore, we show that ectopic Runx1 expression confers increased resistance of lymphoma cells to glucocorticoid‐mediated apoptosis, and elucidate the mechanism of cross‐talk between glucocorticoid and sphingolipid metabolism through Sgpp1. Dexamethasone potently induces expression of Sgpp1 in T‐lymphoma cells and drives cell death which is reduced by partial knockdown of Sgpp1 with shRNA or direct transcriptional repression of Sgpp1 by ectopic Runx1. Together these data show that Runx1 plays a role in regulating the sphingolipid rheostat in normal development and that perturbation of this cell fate regulator contributes to Runx‐driven lymphomagenesis. J. Cell. Biochem. 118: 1432–1441, 2017. © 2016 The Authors. Journal of Cellular Biochemistry Published by Wiley Periodicals, Inc.

CD23/FcεRII: molecular multi-tasking

CD23 is the low-affinity receptor for immunoglobulin (Ig)E and plays important roles in the regulation of IgE responses. CD23 can be cleaved from cell surfaces to yield a range of soluble CD23 (sCD23) proteins that have pleiotropic cytokine-like activities. The regions of CD23 responsible for interaction with many of its known ligands, including IgE, CD21, major histocompatibility complex (MHC) class II and integrins, have been identified and help to explain the structure-function relationships within the CD23 protein. Translational studies of CD23 underline its credibility as a target for therapeutic intervention strategies and illustrate its involvement in mediating therapeutic effects of antibodies directed at other targets.

Tissue inhibitor of metalloproteinases-3 inhibits shedding of L-selectin from leukocytes

Although the enzyme or enzymes mediating shedding of L-selectin have not yet been identified, this activity can be blocked by synthetic hydroxamic acid-based inhibitors of metalloproteinases such as Ro 31-9790. However, the endogenous matrix metalloproteinase inhibitor tissue inhibitor of metalloproteinases (TIMP)-1 does not block L-selectin shedding. Here, we report that TIMP-3, but not TIMP-2, inhibits L-selectin shedding from mouse and human lymphocytes, Jurkat T cells, and human monocytes. TIMP-3 has an IC50 of 0.3-0.4 microM on these cell types compared with 0.7-4.8 microM for Ro 31-9790. A metalloproteinase (tumor necrosis factor-alpha (TNF-alpha)-converting enzyme; ADAM17) has recently been identified which cleaves the pro-form of TNF-alpha to produce soluble cytokine. We compared inhibition of L-selectin shedding by TIMPs and Ro 31-9790 with inhibition of TNF-alpha shedding from human monocytes. TIMP-3 inhibited TNF-alpha shedding (IC50 of 0.1 microM), as did Ro 31-9790 (IC50 of 0.4 microM). TIMP-2 had a partial effect, and TIMP-1 did not inhibit. This study confirms that L-selectin sheddase is a metalloproteinase, but not a matrix metalloproteinase, and investigates the relationship between shedding of L-selectin and TNF-alpha.