Inhibition of astroglia-induced endothelial differentiation by inorganic lead: a role for protein kinase C

In vitro techniques for the assessment of neurotoxicity

Risk assessment is a process often divided into the following steps: a) hazard identification, b) dose-response assessment, c) exposure assessment, and d) risk characterization. Regulatory toxicity studies usually are aimed at providing data for the first two steps. Human case reports, environmental research, and in vitro studies may also be used to identify or to further characterize a toxic hazard. In this report the strengths and limitations of in vitro techniques are discussed in light of their usefulness to identify neurotoxic hazards, as well as for the subsequent dose-response assessment. Because of the complexity of the nervous system, multiple functions of individual cells, and our limited knowledge of biochemical processes involved in neurotoxicity, it is not known how well any in vitro system would recapitulate the in vivo system. Thus, it would be difficult to design an in vitro test battery to replace in vivo test systems. In vitro systems are well suited to the study of biological processes in a more isolated context and have been most successfully used to elucidate mechanisms of toxicity, identify target cells of neurotoxicity, and delineate the development and intricate cellular changes induced by neurotoxicants. Both biochemical and morphological end points can be used, but many of the end points used can be altered by pharmacological actions as well as toxicity. Therefore, for many of these end points it is difficult or impossible to set a criterion that allows one to differentiate between a pharmacological and a neurotoxic effect. For the process of risk assessment such a discrimination is central. Therefore, end points used to determine potential neurotoxicity of a compound have to be carefully selected and evaluated with respect to their potential to discriminate between an adverse neurotoxic effect and a pharmacologic effect. It is obvious that for in vitro neurotoxicity studies the primary end points that can be used are those affected through specific mechanisms of neurotoxicity. For example, in vitro systems may be useful for certain structurally defined compounds and mechanisms of toxicity, such as organophosphorus compounds and delayed neuropathy, for which target cells and the biochemical processes involved in the neurotoxicity are well known. For other compounds and the different types of neurotoxicity, a mechanism of toxicity needs to be identified first. Once identified, by either in vivo or in vitro methods, a system can be developed to detect and to evaluate predictive ability for the type of in vivo neurotoxicity produced. Therefore, in vitro tests have their greatest potential in providing information on basic mechanistic processes in order to refine specific experimental questions to be addressed in the whole animal.

An in vitro three-dimensional coculture model of cerebral microvascular angiogenesis and differentiation

The microvasculature of the developing brain is plastic and responds differently to the many insults associated with preterm birth. We developed three-dimensional in vitro culture models for the study of the responses of the developing cerebral microvasculature. Beagle brain microvascular endothelial cells (BBMEC) were isolated by differential centrifugation from newborn beagle pups on postnatal Day 1 and placed in three-dimensional culture dispersed in a collagen gel. Alternatively, BBMEC were placed in a three-dimensional coculture with neonatal rat forebrain astrocytes. Cultures were analyzed for extracellular matrix components at 1 and 6 d, and total RNA was extracted for Northern analyses. Urokinase plasminogen activator activity was assayed in both mono- and cocultures of the two cell types. Studies of three-dimensional BBMEC/astrocyte cocultures demonstrated progressive tube formation with only low levels of endothelial proliferation. By 6 d in three-dimensional coculture, the BBMEC formed capillarylike tubes with a wrapping of glial processes, and basement membrane protein synthesis was noted. Urokinase plasminogen zymography suggested intercellular signaling by the two cell types. These data suggest that the three-dimensional beagle brain germinal matrix microvascular endothelial cell/neonatal rat astrocyte coculture provides a good model for the investigation of microvascular responses in the developing brain.

The influence of Pb2+ on expression of acetylcholinesterase and the acetylcholine receptor

This paper examines the influence of inorganic lead (Pb2+) on the presence of acetylcholinesterase (AchE) molecular forms and the acetylcholine receptor (AchR) in two types of excitable tissue, primary cultures of skeletal muscle and neural retina from embryonic chick. Treatment of skeletal muscle with Pb2+ is observed to cause reductions in the 5/7S and 19S but not the 11.4S molecular forms of AchE. The reductions are dose-dependent, requiring submicromolar concentrations, slow in onset, requiring incubation times greater than 24 hr, and tissue specific, being pronounced in skeletal muscle but absent from neural retina. Significantly, the reductions in AchE occur without corresponding reductions in amounts of AchR and without reduction in activity of protein kinase C (PKC). These studies illustrate a tissue-specific action of inorganic lead that is not mediated through PKC.

Lead induced rise in intracellular free calcium is mediated through activation of protein kinase C in osteoblastic bone cells

Lead characteristically perturbs processes linked to the calcium messenger system. This study was undertaken to determine the role of PKC in the Pb2+ induced rise of [Ca2+]i. [Ca2+]i was measured using the divalent cation indicator, 1,2-bis(2-amino-5-fluorophenoxy) ethane N, N,N',N'-tetraacetic acid (5F-BAPTA) and 19F-NMR in the osteoblast cell line, ROS 17/2.8. Treatment of cells with Pb2+ at 1 and 5 microM produced a rise in [Ca2+]i from a basal level of 125 nM to 170 nM and 230 nM, respectively, while treatment with phorbol 12-myristate 13-acetate (PMA) (10 microM), an activator of PKC, produced a rise in [Ca2+]i to 210 nM. Pretreatment with calphostin C, a potent and highly selective inhibitor of PKC activation failed to produce a change in basal [Ca2+]i and prevented any rise in [Ca2+]i in response to Pb2+. To determine whether Pb2+ acts directly on PKC, we measured the Pb2(+)-dependent activation of phosphatidylserine/diolein-dependent incorporation of 32P from ATP into histone and endogenous TCA precipitable proteins in the 100,000 X g supernatant from homogenized ROS 17/2.8 cells. The free concentrations of Pb2+ and Ca2+ were set using 5F-BAPTA; and [Ca2+] and [Pb2+] in the PKC reaction mixtures were confirmed by 19F-NMR. We found that Pb2+ activates PKC in the range of 10(-11)-10(-7) M, with an activation constant of 1.1 X 10(-10) M, whereas Ca2+ activates PKC in the range from 10(-8) to 10(-3) M, with an activation constant of 3.6 X 10(-7) M. These data suggest that Pb2+ activates PKC in ROS 17/2.8 cells and that Pb2+ activation of PKC mediates the documented rise in [Ca2+]i and, perhaps, other toxic effects of Pb2+.

Distinct mechanisms of neurotransmitter release from PC 12 cells exposed to lead

Two enzymes, protein kinase C and microsomal Ca(2+)-ATPase help regulate levels of Ca2+ in many types of cells. Since proteins that regulate Ca2+ often influence sensitivity to Pb2+, we determined the possible roles played by protein kinase C and microsomal Ca(2+)-ATPase for the Pb(2+)-evoked release of norepinephrine (NOR) in PC cells. NOR release was observed at 10 microM Pb2+ when PC 12 cells were stimulated with inhibitors of microsomal Ca(2+)-ATPase such as thapsigargin, cyclopiazonic acid, or 2,5-di-(t-butyl)-hydroquinone. At 5 microM, Pb2+ evoked the release of NOR in PC 12 cells stimulated with activators of protein kinase C such as phorbol 12-myristate 13-acetate (PMA) or (-)-7-octylindolactam. NOR release was observed at 1 microM Pb2+ in the presence of both PMA and thapsigargin. Ni2+ and Cd2+ blocked NOR release stimulated by Pb2+ in the presence of thapsigargin but not by PMA. NOR released by thapsigargin stimulation was not altered in PC 12 cells depleted of protein kinase C. Two proteins found in vesicles, chromogranin B and secretogranin-II were released with NOR. Our results indicate that in PC 12 cells, PB(2+)-evokes the release of neurotransmitters. Furthermore, thapsigargin and PMA increase the cell's sensitivity to Pb2+ by different pathways.

Phosphorylation of membrane proteins in erythrocytes treated with lead

In immature rat microvessels, endothelial cells and glioma cells, exposure to lead results in an increase in the level of protein kinase C in membranes. In this paper we have extended these studies to human erythrocytes and, in addition, studied the phosphorylation of membrane proteins. A significant increase in the phosphorylation of membrane cytoskeletal proteins of molecular mass 120, 80, 52 and 45 kDa was observed in human erythrocytes treated for 60 min with lead acetate at concentrations greater than 100 nM. These same proteins were phosphorylated when erythrocytes were treated for 10 min with 50 nM phorbol 12-myristate 13-acetate (PMA). Similarly, protein kinase C activity was elevated and an increase in the amount of protein kinase C-alpha was observed in membranes from erythrocytes exposed to concentrations of lead acetate above 100 nM. No changes, however, in the activities of cAMP-dependent protein kinase, protein phosphatases I and IIA or casein kinase were observed. Phosphorylation of these membrane proteins stimulated by lead acetate or by PMA was not observed in erythrocytes depleted of protein kinase C by a 72-h treatment with 500 nM phorbol 12,13-dibutyrate. Finally, no changes in the levels of calcium or diacylglycerol were observed in erythrocytes stimulated with 100 nM lead acetate. These results indicate that, in erythrocytes, lead acetate stimulates the phosphorylation of membrane cytoskeletal proteins by a mechanism dependent on protein kinase C. Since levels of calcium or diacylglycerols did not increase, it appears that lead may activate the enzyme by a direct interaction.

Regulation of in vitro glia-induced microvessel morphogenesis by urokinase

Plasminogen activators (PAs) regulate a variety of processes involved in tissue morphogenesis and differentiation. We used a coculture system in which microvascular endothelial cells are induced by glial cells to form capillary-like structures in order to examine the role of urokinase-type PA (uPA) during microvessel morphogenesis within the central nervous system (CNS). Endothelia-derived uPA activity decreased sevenfold within glial-endothelial cocultures when capillary-like structures were formed. Incubation of cocultures with concentrations of phorbol 12-myristate 13-acetate (0.1 and 1.0 nM) that induced endothelial uPA activity (45-210%) inhibited endothelial differentiation (25-70%). Furthermore, incubation of cocultures with proteolytically active low molecular weight uPA (5-500 IU/ml) inhibited endothelial differentiation (37-75%), whereas the amino terminal cell-binding fragment of uPA had minimal effect. Inhibition of plasminogen activation in cocultures with the serine protease/plasmin inhibitors aprotinin and soybean trypsin inhibitor increased glia-induced capillary-like structure formation (96-98%). These findings establish a paracrine/autocrine function for urokinase and its inhibitors in regulating endothelial responses to perivascular glia and provide insight into mechanisms of microvascular reactions to CNS pathology.