Daniel M. Raben

Cell surface-associated growth inhibitory proteins: Evidence for conservation between mouse and human cell lines☆

Abstract The ability of the normal human fibroblast line (IMR91) to exhibit density-dependent regulation of growth has been examined. The line exhibits density-dependent regulation of growth; saturation density in 15% fetal bovine serum is 2 × 10 5 cells/cm 2 . Membranes prepared from confluent monolayers of these cells contained growth inhibitory factors to both exponentially growing IMR91 and Swiss 3T3 cells. This factor(s) appears to be similar to a previously described factor found on the surface of Swiss 3T3 cells [14]. The inhibition of DNA synthesis in growing IMR91 cultures by membranes was both time- and concentration-dependent. The effect was reversible by high serum. Specificity experiments utilizing membranes prepared from Swiss 3T3 cells indicated some species specificity for inhibition by membranes, but this specificity was no longer exhibited by solubilized membrane preparations. These results are compatible with the suggestion that both the growth inhibitory factors and their receptors are conserved through evolution.

Effect of 3T3 plasma membranes on cells exposed to epidermal growth factor

Epidermal growth factor (EGF) induced DNA synthesis in non-confluent, G0-arrested Swiss 3T3 fibroblasts is partially blocked by plasma membranes isolated from the EGF receptor deficient NR-6 Swiss 3T3 cell line. This inhibition could be due to either a steric block of the receptor by the membranes, a membrane induced down regulation of the EGF receptor, or a signal generated by membrane binding which is antagonistic towards the mitogenic signal generated by EGF. Binding measurements utilizing 125I-labeled EGF demonstrated that membranes do not block either the EGF induced down regulation of the receptor or alter the number of receptors on the surface. These results suggest that the membranes exert their inhibitory effect via generation of a signal which is antagonistic to the EGF induced mitogenic signal, with the result expressed as a reduced mitogenic response.

Regulation of DGK-θ

Diacylglycerol kinases are important regulators of lipid signaling and, consequently, important regulators of many diglyceride‐dependent and PA‐dependent proteins. Research over the last twenty years has clearly demonstrated that individual DGK isoforms can be connected with disparate cellular processes, indicating the presence of a sophisticated regulatory network for diglyceride and phosphatidic acid signaling through the regulation of individual DGK isoforms. This review presents the progress on the characterization of a primarily neuronal isoform DGK‐θ, and examines current data on the primary structure, regulation and potential cellular functions of this enzyme. J. Cell. Physiol. 220: 548–552, 2009.

Phosphofructokinase-2/Fructose Bisphosphatase-2

6-Phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2/FBPase-2) is a bifunctional enzyme wherein a single polypeptide chain contains two enzymatic activities: PFK-2 and FBPase-2. PFK-2 catalyzes the transfer of phosphate from adenosine triphosphate to the second carbon (C-2) of fructose-6-phosphate (F6P), an intermediate in the glycolytic pathway, generating fructose-2,6-bisphosphate (F2,6BP). Conversely, FBPase-2 catalyzes the dephosphorylation of F2,6BP resulting in the liberation of free inorganic phosphate and regeneration of F6P. In this manner, these two physically linked enzymes function to modulate the cellular concentration of F2,6BP. The importance of this regulation cannot be understated as this sugar plays an essential signaling role in regulating glucose metabolism in a variety of tissues. Perhaps most importantly, it is largely responsible for regulating the balance between glycolysis and gluconeogenesis in the liver. This balance, critical to maintaining blood glucose levels within the normal range regardless of the nutritional state, principally results from the fact that F2,6BP activates a key regulatory glycolytic enzyme, 6-phosphofructo-1-kinase, and inhibits a regulatory enzyme in gluconeogenesis, fructose-1,6-bisphosphatase. In the liver, therefore, when F2,6BP levels are high, glycolysis predominates; when F2,6BP levels are low, gluconeogenesis predominates. In other tissues, F2,6BP levels mainly serve to help regulate glycolytic flux. By regulating the level of this phosphorylated sugar, PFK-2/FBPase-2 activities play a central role in maintaining glucose homeostasis and utilization.