Biomedical subjects
Publications and source records attributed to T J Weber.
This article discusses the important secondary causes of osteoporosis that contribute significantly to bone loss and that seem to increase fracture risk, including hypogonadism, endogenous and exogenous thyroxine excess, hyperparathyroidism, malignancies, gastrointestinal diseases, medications, vices, and connective tissue diseases.
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Experiments have been conducted to examine the impact of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on growth-related signaling in vascular smooth muscle cells (SMCs). A 40% reduction of peak DNA synthesis was observed in SMCs only when TCDD was added during the G0/G1 transition of the cell cycle. Enhanced phosphorylation of several endogenous proteins during this period was coincident with increased tyrosine kinase activity as early as 15 min following TCDD challenge. No changes in protein phosphorylation status occurred in cells treated with TCDD during the G1/S transition or during S phase. Cotreatment of quiescent SMCs with 10 nM TCDD and serum for 3 h reduced serum-inducible binding activity to a 12-O-tetradecanoyl phorbol 13-acetate responsive element (TRE) by approximately 40%. No alterations of constitutive TRE binding were observed in quiescent SMCs treated with TCDD for up to 5 h. These data show that mitogen-related signaling in vascular SMCs is modulated by TCDD selectively during the G0/G1 transition, and these effects influence the growth behavior of these cells.
In view of the prominent role of protein kinase C (PKC) in the regulation of vascular smooth muscle cell (VSMC) growth and differentiation, the present studies were conducted to assess the impact of c-Ha-rasEJ transfection on PKC-dependent growth programming. PKC activity was elevated in the cytosolic and particulate compartments of c-Ha-rasEJ VSMC, relative to naive or pSV2neo vector controls. Constitutive and 12-O-tetradecanoyl phorbol 13-acetate (TPA)-inducible binding to a TPA-responsive element (TRE) was also enhanced in c-Ha-rasEJ VSMC. Fetal bovine serum (FBS) did not increase TRE-binding activity in serum-starved c-Ha-rasEJ VSMC but increased TRE-binding activity in pSV2neo VSMC. FBS-mediated TRE-binding activity was dramatically decreased in serum-starved pSV2neo VSMC pretreated with 100 ng/ml TPA for 24 h to downregulate PKC activity. c-Ha-rasEJ VSMC exhibited a marked proliferative advantage over controls under both restrictive and growth-permissive serum conditions. PKC downregulation did not influence the mitogenic response to serum in c-Ha-rasEJ VSMC but ablated [3H]thymidine incorporation into DNA in naive or pSV2neo vector counterparts. Western blot analysis demonstrated increased expression of extracellular signal-regulated kinase 2 (ERK2), but not ERK1, in c-Ha-rasEJ VSMC, relative to pSV2neo control. Immunoblots of serum-starved and PKC-depleted c-Ha-rasEJ VSMC demonstrated a dramatic increase in the phosphorylated form of ERK2, relative to pSV2neo controls. These data suggest that oncogenic c-Ha-rasEJ circumvents a requirement for a TPA-responsive PKC isoform(s) during mitogenic stimulation of VSMC.
Although the exact mechanism of prostaglandin E2 (PGE2)-mediated cytoprotection has not been elucidated, its ability to induce cytoprotection in cell culture suggests this action occurs at the cellular level. The present studies were conducted to determine whether PGE2 induces protection against 2,3,5-(trisglutathion-S-yl)-hydroquinone [2,3,5-(trisglutathion-S-yl)-HQ]-mediated cytotoxicity in a renal proximal tubule epithelial cell line (LLC-PK1) and to delineate the cellular and molecular mechanisms associated with this response. Pretreatment of LLC-PK1 cells with 0.01-40 microM PGE2 for 24 h fully protects against a moderately toxic concentration of 2,3,5-(trisglutathion-S-yl)-HQ. PGE2-mediated cytoprotection is observed in cells pretreated at pH 7.4 but not at pH 7.8. However, cytoprotection is observed in LLC-PK1 cells pretreated with the PGE2 analog, 11-deoxy-16,16-dimethyl PGE2 (DDM-PGE2) but not with the PGE2 receptor [E-prostanoid (EP)] agonists 17-phenyltrinor PGE2 (EP1), 11-deoxy PGE1 (EP2/EP4), sulprostone (EP1/EP3), PGE1, or PGA2. 12-O-tetradecanoylphorbol-13-acetate (TPA), a potent activator of protein kinase C (PKC), also induces cytoprotection, supporting a role for this pathway in the cytoprotective response. PGE2, DDM-PGE2, and TPA all induce the binding of nuclear proteins to a TPA responsive element (TRE), whereas analogs that did not induce cytoprotection (PGE1, 17-phenyltrinor PGE2, sulprostone) were without effect. DDM-PGE2- and TPA-mediated cytoprotection and TRE binding activity are inhibited by N-(2[[3-(4-bromophenyl)-2-propenyl]-amino]-ethyl)-5-isoquinolinesulfonam ide (H-89), a PKC inhibitor. These data suggest that cytoprotection by PGE2 and DDM-PGE2 in LLC-PK1 cells is mediated by a PKC-coupled receptor, which is pharmacologically distinct from the presently classified EP receptor subtypes.
The nuclear hormone receptor family comprises a group of structurally related transcriptional regulators that mediate the actions of diverse ligands, including steroid hormones, thyroid hormone, vitamin D, and retinoids. The nuclear receptor family also contains members for which activating ligands have not been identified-the orphan nuclear receptors. One of these orphan nuclear receptors, steroidogenic factor 1 (SF-1), has emerged as an essential regulator of steroidogenic cell function within the adrenal cortex and gonads; SF-1 also plays important roles in reproduction at all three levels of the hypothalamic-pituitary-gonadal axis. First identified as a tissue-specific regulator of the transcription of the cytochrome P450 steroid hydroxylases, considerably broader roles for SF-1 were revealed by genetic studies in mice lacking SF-1 due to targeted gene disruption. These SF-1-knockout mice had agenesis of their adrenal glands and gonads, male-to-female sex reversal of their internal and external genitalia, impaired gonadotrope function, and agenesis of the ventromedial hypothalamic nucleus. These studies delineated essential roles of SF-1 in regulating endocrine differentiation and function at multiple levels. Despite these insights into roles of SF-1, the precise mechanisms by which SF-1 exerts its multiple effects remain to be determined. This review highlights experiments that have established SF-1 as a pivotal determinant of endocrine differentiation and function and identifies areas in which additional studies are needed to expand our understanding of SF-1 action.
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The modulation of protein kinase C (PKC)-mediated protein phosphorylation in quiescent vascular smooth muscle cells (SMCs) by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exhibits a discrete temporal pattern in which early reductions of kinase activity are followed by marked increases in activity. This profile may be accounted for by transcriptional- and/or cell cycle-related effects of TCDD. To test this hypothesis, experiments were conducted to examine the influence of TCDD on PKC activity during different phases of the cell cycle in vascular (aortic) SMCs. Increased PKC activity was observed in the cytosolic and particulate fractions of randomly cycling SMC cultures derived from female rats treated in vivo with 10 microgram/kg TCDD relative to corn oil. Treatment of cycling naive SMC cultures with TCDD (0.1 to 1000 nM) for 0.5 h caused a concentration-dependent increase of particulate PKC activity and no changes in cytosolic counterparts. Extended challenge of SMCs with TCDD for 24 h increased PKC activity in both cellular fractions. Incubation of SMCs with various concentrations of fetal bovine serum for 72 h to differentially regulate cell cycling followed by challenge with 10 nM TCDD for 24 h reduced cytosolic and particulate PKC activity in quiescent cells, but enhanced activity in cycling cells. To determine if this serum-related profile was strictly dependent upon cell cycle-related events, SMCs were synchronized in the G0 phase and then pulsed with 10 nM TCDD during different phases of the cell cycle. Differential profiles were observed where reduced C-kinase activity occurred during the G0/G1 transition followed by increases during G1/S and no changes during S. Western blot analysis confirmed the patterns of PKC activity observed during the G0/G1 and G1/S transitions. PKCalpha, beta II, and delta isoforms were reduced during G0/G1, while only PKCbetaII and delta were increased during G1/S. These data show that modulation of PKC by TCDD in vascular SMCs exhibits cell cycle dependence and isoform specificity.
Mutations of the orphan nuclear receptors, steroidogenic factor 1 (SF-1) and DAX-1, cause complex endocrine phenotypes that include impaired adrenal development and hypogonadotrophic hypogonadism. These similar phenotypes suggest that SF-1 and DAX-1 act in the same pathway(s) of endocrine development. To explore this model, we now compare directly their sites of expression. In mouse embryos, SF-1 expression in the urogenital ridge and brain either preceded or coincided with Dax-1 expression, with coordinate expression thereafter in the adrenal cortex, testis, ovary, hypothalamus, and anterior pituitary. The striking colocalization of SF-1 and Dax-1 supports the model that they are intimately linked in a common pathway of endocrine development. The slightly earlier onset of SF-1 expression and its ability to bind specifically to a conserved sequence in the Dax-1 5'-flanking region suggested that SF-1 may activate Dax-1 expression. However, promoter activity of Dax-1 5'-flanking sequences did not require this potential SF-1-responsive element, and Dax-1 expression was unimpaired in knockout mice lacking SF-1, establishing that SF-1 is not required for Dax-1 gene expression in these settings. Although the precise mechanisms remain to be established and may be multifactorial, our results strongly suggest that these two orphan nuclear receptors interact in a common pathway of endocrine development.
Recent studies have suggested that aromatic hydrocarbons can initiate glomerular mesangial cell (GMC) injury and contribute to the onset of renal disease. The present studies were conducted to assess the impact of benzo[a]pyrene (BaP), a ubiquitous polycyclic aromatic hydrocarbon, on the proliferation of GMCs. Challenge of cultured GMCs with BaP (0.3-30 microM) for 24 h was associated with concentration-dependent decreases in DNA synthesis, a response mediated by selective interference with early G1 cell cycle progression. One cycle of sequential treatment with 3 microM BaP for 24 h followed by challenge with 10 microM methoxamine (MeoA), a growth-promoting alpha 1-adrenergic agonist, for an additional 24 h attenuated the inhibitory response elicited by BaP alone. Following three rounds of sequential treatment with BaP and MeoA, GMCs exposed to BaP alone or BaP/MeoA exhibited enhanced proliferation rates relative to controls. BaP/MeoA cells acquired the greatest proliferative enhancement and exhibited unregulated c-jun and c-fos gene expression under growth-arrested and serum-stimulated conditions. Marked increases in specific AP-1 binding to a synthetic oligonucleotide were observed upon serum stimulation of quiescent cultures of BaP/MeoA cells relative to controls or any of the other treatment groups. These data demonstrate that sequential treatment with BaP in combination with MeoA is associated with induction of highly proliferative phenotypes in GMCs characterized by differential expression of growth-related protooncogenes.
Elucidation of the mechanisms involved in the deregulation of vascular smooth muscle cell (SMC) growth and differentiation during the course of atherogenesis and the putative role of toxic injury in this process have been a subject of considerable interest in recent years. In this regard, we have recently shown that in vitro exposure of vascular (aortic) SMCs to benzo[a]pyrene (BaP), an atherogenic polycyclic aromatic hydrocarbon, initially delays cell cycle progression and inhibits cell proliferation and then causes permanent modulation to a highly proliferative state. To define the molecular basis of this response, we have examined critical components of the protein kinase C (PKC) signal transduction system upon exposure to BaP. Marked inhibition of serum-stimulated inositol phospholipid turnover was observed in growth-arrested SMC cultures challenged with 30 microM BaP for 24 h and then stimulated with 10% fetal bovine serum for 120 or 1800 s. Benzo[a]pyrene inhibited PKC-mediated phosphorylation of exogenous and endogenous proteins in the cytosolic and particulate fraction of cycling, as well as quiescent cultures. The PKC inhibitory response was observed as early as 0.5 h following BaP treatment and maintained for at least 5 days. Exposure of quiescent SMCs to 30 microM BaP inhibited the ability of serum to induce c-fos mRNA expression and decreased AP-1 binding to a 12-O-tetradecanoyl phorbol-13-acetate responsive element. Inhibition of PKC-related signal transduction was not due to generalized interference with cell cycle events since peak expression of the c-myc and c-Ha-ras protooncogenes following serum stimulation of quiescent cultures was unchanged, or slightly enhanced, by 30 microM BaP. Collectively, these data suggest that the ability of BaP to modulate growth and differentiation programs in vascular SMCs involves early interference with PKC-related mitogenic signal transduction.
Previous studies in this laboratory have shown that benzo(a)pyrene (BaP) modulates protein kinase C (PKC)-mediated phosphorylation of aortic smooth muscle cell (SMC) proteins. This observation is consistent with the ability of other aromatic hydrocarbons (AHs), such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), to modulate kinase activities in cells of hepatic, testicular, and thymic origin. Because all these chemicals share the ability to bind the aryl hydrocarbon receptor (AhR), the present studies were conducted to determine if changes in PKC activity by AHs conform with established structure-activity relationships. Experiments were conducted to examine the effects of TCDD, 2,3,7,8-tetrachlorodibenzofuran (TCDF), and 2,8-dichlorodibenzodioxin (DCDD) on the phosphorylation of exogenous histone type-III under basal and PKC-activating conditions. These congeners exhibit both high (TCDD and TCDF) and low (DCDD) AhR agonist activities. Measurements of kinase activity were conducted in the cytosolic and particulate fractions of growth-arrested (i.e., serum-deprived) cultured rat aortic SMCs incubated with 10 nM TCDD, TCDF, and DCDD for 0.5, 12, or 24 hours. No changes in basal kinase activity were induced by these chemicals at any of the times tested. Significant decreases in cytosolic and particulate PKC activity relative to controls were observed upon exposure of SMCs for 0.5 hours to 10 nM TCDD, TCDF, and DCDD. In contrast, SMCs exposed to TCDD and TCDF for 12 hours exhibited a significant increase in PKC activity in both cytosolic and particulate fractions. The PKC activity in cells exposed to DCDD for 12 hours was not altered.(ABSTRACT TRUNCATED AT 250 WORDS)
Over the past several decades emphasis has been given to the elucidation of mechanisms involved in the onset and progression of cardiovascular disorders. Stroke, hypertension, and atherosclerosis continue to rank as primary causes of death in the western world. In the case of atherosclerosis, the preferential localization of atheroma to large- and medium-sized blood vessels and the sequence of events leading to plaque development have been well defined. Damage to luminal endothelial and/or medial smooth muscle cells, migration of inflammatory cells, diffusion or local delivery of mediators within the vessel wall, proliferation of vascular smooth muscle cells, and cellular accumulation of lipids are now recognized as hallmarks of the pathologic process. Although these events have been established with a fair degree of certainty, the mechanisms responsible for initiation of the atherosclerotic process are not yet completely understood. Environmental chemicals have come under increasing scrutiny as evidence continues to accumulate suggesting that toxic insult plays an important role in the initiation and/or progression of atherosclerotic disorders. This review focuses on various aspects of xenobiotic-induced vascular injury with emphasis on the toxic effects of allylamine and benzo[a]pyrene in smooth muscle cells, the primary cellular component of atherosclerotic lesions. Both of these chemicals modulate growth and differentiation programs in aortic smooth muscle cells and have been implicated in the development of atherosclerotic-like lesions in laboratory animals. The major findings from recent studies examining the cellular and molecular basis of toxicant-induced phenotypic modulation of vascular smooth muscle cells to a proliferative state and the role of oxidative metabolism, phospholipid turnover, protein kinase C, ras-related signal transduction, and matrix interactions in the vasculotoxic response to allylamine and benzo[a]pyrene are discussed.
Repeated cycles of allylamine-induced aortic injury in vivo modulate the proliferative potential of smooth muscle cells (SMCs) during serial propagation in vitro. This modulation may be partly mediated by disturbances in polyphosphoinositide metabolism which afford allylamine-treated cells a growth advantage over control cells [Cox, Murphy and Ramos (1990) Exp. Mol. Pathol. 53, 52-63]. The present studies were conducted to further evaluate the mechanisms which mediate the enhanced proliferative potential of allylamine cells. Cellular growth and/or [3H]thymidine incorporation into DNA were evaluated in control and allylamine cells seeded on plastic culture dishes or glass coverslips in the presence of 0.1, 1 or 10% fetal bovine serum (FBS). On either substrate, incubation in 0.1% FBS for 48 h inhibited DNA synthesis in cultures of both cell types, but the inhibitory response was more pronounced in allylamine cells. Subsequent challenge with 10% FBS increased thymidine incorporation to a greater extent in allylamine cells. Interestingly, enhanced DNA synthesis of allylamine cells was associated with increased cell numbers only when seeded on a glass surface. The enhanced growth rate on glass was not due to increased plating efficiency since comparable attachment rates were observed for both cell types. Reseeding of control cells on glass substrates pre-coated by allylamine cells afforded control cells a growth advantage comparable with that observed for allylamine cultures. Conditioned media from growth-arrested, as well as cycling cultures, of allylamine cells stimulated DNA synthesis in cultures of either cell type to a greater extent than conditioned media from control counterparts. In addition, the responsiveness of allylamine cells to secreted products was enhanced relative to that of control cells. Metabolic labelling studies revealed that the synthesis and/or secretion of 52, 46, 33 and 28 kDa proteins was enhanced in allylamine cultures relative to controls, and that the expression of two proteins of 30 and 31 kDa only occurred in allylamine cultures. We conclude that the enhanced growth response of allylamine cells is associated with both altered protein secretion and differential extracellular matrix deposition.
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