Dr. Daniel Raben
Associate Professor of Physiology and Oncology
Biochemistry and chemistry of lipids and lipid metabolizing enzymes involved in signaling cascades It is now apparent that agonist-induced lipid metabolism results in the production of a number of second messengers. This metabolism principally involves hydrolysis or phosporylation of specific lipids. Currently, a major effort in our laboratory focuses on understanding the regulation of these enzymes. Studies include identifying proteins involved in regulating the enzymes, the molecular aspects of the regulation including the domains involved in protein-protein and protein-lipid interactions and the sub-cellular compartments in which these enzymes and regulatory factors reside. Additionally, structure/function studies are underway to understand the catalytic mechanisms of the lipid metabolizing enzymes at a molecular level.
Specifically, we are particularly interested in nuclear signaling cascades that involve agonist-induced lipid metabolism resulting in the production of lipid second messengers at the nucleus. One of the model systems we use is the stimulation of quiescent fibroblasts with á-thrombin; a potent fibroblast mitogen. This stimulation results in an increase in the level of nuclear diacylglycerol (DG), a known lipid second messenger. In addition to this increase, our laboratory showed that á-thrombin stimulates an increase in activity of a DG metabolizing enzyme, DG kinase-è (DGK-è), at the nucleus. Our recent studies indicate that different isoforms of another lipid metabolizing enzyme, phosphatidylinositol 3-kinase (PI 3-K), have specific roles in regulating nuclear DGK-è. In addition to this enzyme, one of the other nuclear enzymes whos activity increases following á-thrombin treatment is a phosphatidylcholine-specific phospholipase D, PC-PLD1â, leading to the production of another lipid second messenger, phosphatidic acid (PA). Stimulation of this nuclear enzyme is dependent on the translocation of a small molecular weight GTPase, RhoA. Interestingly, in contrast to this effect on nuclear PC-PLD1â, RhoA inhibits nuclear DGK-è. Current studies are in progress to identify molecular aspects involved in agonist-induced regulation of these two nuclear enzymes as well as discover their physiological consequences. Additionally, in collaboration with Dr. Al Mildvan, a major effort focuses on elucidating the catalytic mechanism(s) of DGK-è, and other DGK isoforms.
In another project, we are taking advantage of our data indicating that the âæ subunits of a heterotrimeric G-protein is involved in regulating an apparent eukaryotic PC-specific phospholipase C (PC-PLC). Ablation of the á subunit of Go in fibroblasts leads to the elevation of the âæ subunits and results in the constitutive activation of the apparent PC-PLC activity. Interestingly, the cells are transformed implicating a novel "tumor suppressor" role for the Ga subunit. We are dedicating some effort to understand this apparent PC-PLC activity, identify the molecular aspects of its regulation and determine its role in cellular transformation.
Cheng,J., Baldassare, J.J., and Raben, D.M. (1999) Dual coupling of the á-thrombin receptor to signal-transduction pathways involving phosphatidylinositol and phosphatidylcholine metabolism. Biochem. J. 337: 97-104.
Phillips-Mason, P.J., Raben D.M., Baldassare, J.J. (2000) Phosphatidylinositol 3-kinase activity regulates á-thrombin-stimulated G1 progression by its effect on Cyclin D1 expression and CDK4 activity. J. Biol. Chem. 275:18046-18053.
Raben, D.M. and Baldassare, J.J. (2000) Phospholipid metabolism and nuclear envelope signaling. Advances in Enzyme Regulation 40:97-123.
Raben, D.M. (2000) Signal Transduction. McGraw-Hill Encyclopedia of Science and Technology 9th Edition. in press.
Raben, D.M., and Baldassare, J.J. (2000) Nuclear Envelope Signaling-Role of Phospholipid Metabolism. European Journal of Histochemistry 44: 67-80.
Baldassare, J.J., Klaus, J., Phillips, P.J., and Raben, D.M. (2001) HamPLD1b in IIC9 Fibroblasts is Selectively Activated in the Nucleus But Not in the Golgi Apparatus. Cell Biol. Int. 25: 1207-1212.
Bregoli, L., Baldassare, J.J., Raben, D.M. (2001), Nuclear diacylglycerol kinase-è is Activated in Response to á-Thrombin. J. Biol. Chem. 276: 23288-23295.
Gardner, A, Phillips-Mason, P.J., Raben, D.M., and Baldassare, J.J. (2002) A Novel Role for Gq alpha in á-thrombin-mediated Mitogenic Signaling Pathways. Cellular Signaling 14:499-507.Bregoli, L, Tu-Sekine, B., and Raben, D.M. (2002)DGK and nuclear signaling nuclear diacylglycerol kinases in IIC9 cells. Advances in Enzyme Regulation 42, 213-26.
Reema, G. Phillips-Mason, P. J., Raben, D.M., and Baldassare, J.J. (2002) á-Thrombin Induces Rapid and Sustained Akt Phosphorylation by â-Arrestin1-dependent and -independent Mechanisms, and Only the Sustained Akt Phosphorylation is Essential for G1 phase Progression. J. Biol. Chem. in press.