Increased Assembly of Clathrin Occurs in Response to Mitogenic Activation of Murine Lymphocytes

Recruitment of activated G protein-coupled receptors to pre-existing clathrin-coated pits in living cells

The process of clathrin-mediated endocytosis tightly regulates signaling of the superfamily of seven-transmembrane G protein-coupled receptors (GPCRs). A fundamental question in the cell biology of membrane receptor endocytosis is whether activated receptors can initiate the formation of clathrin-coated pits (CPs) or whether they are simply mobilized to pre-existing CPs. Here, using various approaches, including a dynamic assay to monitor the distribution of CPs and GPCR-beta-arrestin complexes in live HeLa cells, we demonstrate for the first time that activated GPCRs do not initiate the de novo formation of CPs but instead are targeted to pre-existing CPs.

Lipopolysaccharide potentiates the effect of hepatocyte growth factor upon replication in lung, thyroid, spleen, and colon in rats in vivo

Induction of replication may potentiate in vivo gene therapy, as some viral vectors only transduce dividing cells. Hepatocyte growth factor (HGF) increases the percentage of replicating hepatocytes to 18-fold that in normal rats, and lipopolysaccharide (LPS) modestly potentiates this effect. In this study, the effect of iv HGF upon replication in other organs was determined. HGF at 10 mg/kg resulted in replication that was < or =3-fold that of normal rats in alveolar and proximal renal tubular cells. HGF alone had no effect upon replication of epithelial cells from the bronchi, thyroid, pancreas, or colon or upon cells from the muscle, pancreatic islets, spleen, blood vessels, or thymus. HGF and LPS at 5 mg/kg resulted in replication that was 9-fold that of normal rats in alveolar cells, 25-fold in bronchial epithelial cells, 4-fold in thyroid epithelial cells, 1.5-fold in the red pulp of the spleen, and 2-fold in colonic epithelial cells. The synergistic effect may be due to the fact that LPS upregulated the HGF receptor c-met in thyroid, spleen, and colon. We conclude that iv administration of HGF alone is relatively specific for inducing hepatocyte replication and would allow selective gene transfer into the liver.

Endocytic clathrin-coated pit formation is independent of receptor internalization signal levels

The mechanisms responsible for coated pit formation in cells remain unknown, but indirect evidence has argued both for and against a critical role of receptor cytoplasmic domains in the process. If the endocytic motifs of receptors are responsible for recruiting AP2 to the plasma membrane, thereby driving coated pit formation, then the level of constitutively internalized receptors at the membrane would be expected to govern the steady-state level of coated pits in cells. Here we directly test this hypothesis for broad classes of receptors containing three distinct constitutive internalization signals. Chimeric proteins consisting of an integral membrane reporter protein (Tac) coupled to cytoplasmic domains bearing tyrosine-, di-leucine-, or acidic cluster/casein kinase II-based internalization signals were overexpressed to levels that saturated the internalization pathway. Quantitative confocal immunofluorescence microscopy indicated that the number of plasma membrane clathrin-coated pits and the concentration of their structural components were invariant when comparing cells expressing saturating levels of the chimeric receptors to nonexpressing cells or to cells expressing only the Tac reporter lacking cytoplasmic internalization signals. Biochemical analysis showed that the distribution of coat proteins between assembled coated pits and soluble pools was also not altered by receptor overexpression. Finally, the cellular localizations of AP2 and AP1 were similarly unaffected. These results provide a clear indication that receptor endocytic signals do not determine coated pit levels by directly recruiting AP2 molecules. Rather, the findings support a model in which coated pit formation proceeds through recruitment and activation of AP2, likely through a limited number of regulated docking sites that act independently of endocytic signals.

Richter's syndrome associated with loss of response to transforming growth factor-beta

A patient with chronic lymphocytic leukemia developed a large cell lymphoma apparently derived from the same neoplastic B-cell clone (Richter's syndrome). At the same time, mitogen-stimulated proliferation of the patient's circulating leukemic B-cells was no longer inhibited by the regulatory cytokine transforming growth factor-beta (TGF-beta), suggesting that such loss of inhibition might be contributing to the clinical and biological progression of the disease.

Characterization of L-[3H]glutamate binding sites in bovine brain coated vesicles

Clathrin-coated vesicles isolated from bovine brain exhibit an L-[3H]glutamate-specific binding. Coated vesicles were purified from bovine brain by differential centrifugation and gel filtration. High purity of coated vesicles was established previously by several enzyme markers and electron microscopy. The binding activity was performed in the absence of Na+, Ca2+, and Cl- ions to avoid binding and/or uptake to uptake sites. Coated vesicles were frozen, thawed, treated with 0.04% Triton X-100 and washed before incubation with L-[3H]glutamate. Saturation binding experiments revealed a single binding site with a Kd = 439 +/- 87 nM and a Bmax = 11.74 +/- 3.4 pmol/mg protein, consistent with kinetics characteristic for glutamate receptors. The glutamate-specific binding was stereospecific for glutamate and aspartate, showing higher affinity for L-forms than D-forms. Pharmacological characterization indicated that specific binding was sensitive to quisqualate and almost insensitive to kainate and N-methyl-D-aspartate. 200 microM guanosine triphosphate (GTP) produced a decrease of 50% in L-[3H]glutamate binding activity and competition experiments produced an affinity shift to the right of the glutamate dose-response curve. These results support the evidence that glutamate receptors are present in bovine brain coated vesicles and, at least in part, are associated to a G-protein.