Characterization of an element within the rat parathyroid hormone/parathyroid hormone-related peptide receptor gene promoter that enhances expression in osteoblastic osteosarcoma 17/2.8 cells.

Parathyroid hormone (PTH) and PTH-related peptide (PTHrP) mediate their actions via a common G-protein-coupled receptor. High levels of PTH/PTHrP receptor expression have been detected in many tissues including bone and kidney. This study has demonstrated specific PTH/PTHrP receptor expression from the U3 promoter in the osteoblastic osteosarcoma ROS 17/2.8 cell line, which expresses the endogenous PTH/PTHrP receptor, compared to rat 2 fibroblasts which do not express the endogenous PTH/PTHrP receptor gene. Transient transfection studies revealed cell-specific expression of a construct containing 4391 bp of DNA upstream of exon U3 of the PTH/PTHrP receptor gene fused to a luciferase reporter gene. Deletion mapping of the 5' region of U3 revealed that a construct containing 206 bp upstream of U3 confers cell-specific expression. These data suggest that cell-specific expression in ROS 17/2.8 involves cell-specific elements within the PTH/PTHrP receptor promoter.

Type I collagen influence on gene expression in UMR106-06 osteoblast-like cells is inhibited by genistein.

We have previously shown that an exogenous type I collagen matrix can regulate expression of mRNA for parathyroid hormone (PTH)-related protein (PTHrP) and its receptor, the PTH/PTHrP receptor, in the UMR106-06 osteogenic sarcoma cell line, which is considered to be representative of a relatively mature osteoblast phenotype. Consistent with those data, we show here that growth of UMR106-06 cells on type I collagen increased PTH/PTHrP receptor-binding capacity. Analysis of the binding data showed that the number of PTH/PTHrP receptors expressed by cells cultured on collagen was at least 2-fold greater than that of cells cultured on plastic. Expression of mRNA encoding alkaline phosphatase (ALP) and osteopontin (OP) was also upregulated in cells cultured on collagen, suggesting that interaction with collagen promotes the osteoblast phenotype in this cell line. Retinoic acid (RA), which has also been shown to promote osteoblastic differentiation, synergized with type I collagen to cause super-induction of OP mRNA. In contrast, RA abolished the collagen-induced increase in ALP mRNA and PTH/PTHrP receptor mRNA. The collagen-mediated increase in the expression of OP and PTH/PTHrP receptor mRNA, but not that of ALP, was perturbed by prior covalent modification of the collagen by non-enzymatic glycation. The collagen effects did not occur via interaction with RGD amino acid domains in type I collagen, but evidence was obtained for involvement of the DGEA amino acid cell-binding domain. The mechanism by which plating of UMR106-06 cells on a type I collagen substrate affects PTH/PTHrP receptor mRNA levels was investigated. Inhibition of cytoskeletal organization using cytochalasin D, and inhibitors of protein phosphatases, protein kinase C, phospholipase C and cyclooxygenase, did not abrogate the collagen-mediated effects. In contrast, treatment of cells with the protein tyrosine kinase inhibitor genistein, but not herbimycin A, dose-dependently abolished the collagen effects on the expression of PTH/PTHrP receptor, ALP and OP mRNA. These results show that a type I collagen substrate influences the expression of osteoblast-associated genes in a cell model of mature osteoblasts and suggests that this involves, at least in part, changes in intracellular tyrosine phosphorylation.

Differential regulation of the parathyroid hormone-related protein gene P1 and P3 promoters by cAMP.

The role of calcitonin, and other agonists which activate the cAMP pathway, in regulating transcription of the human parathyroid hormone-related protein (PTHrP) gene was investigated in a human lung cancer cell line (BEN). Both calcitonin and forskolin caused a 5-6-fold increase in transcription initiated from both the P1 and P3 promoters, but with no observed effect on the P2 promoter. Maximal 6-fold activation of the P1 promoter occurred at 16 h post-stimulation and effects of calcitonin were observed within the pM range. The PKC agonist, phorbol 12-myristate 13-acetate diester (PMA), did not modulate transcription initiated from the P1 promoter. The ionophore ionomycin had a small effect on transcription of the P1 promoter, and transcriptional control may involve an interaction between the cAMP and intracellular calcium second messenger pathways. Deletion mapping studies indicated that increases in transcription of the human PTHrP gene is being mediated via a CRE element situated at -3313 to -3306 upstream of the P1 promoter. Mutational analysis of this CRE element confirmed a role for this sequence in mediating the increase in transcription effected by cAMP. Consistent with these transfection studies, RT-PCR of PTHrP mRNA also indicated a significant increase in transcripts generated from the P1 promoter. Gel retardation assays utilising a fragment of the P1 promoter region, encompassing the putative CRE, determined that nuclear proteins were binding to this region. Competition binding studies with labelled probe and cold competitors determined that the binding was specific for this sequence. A wild-type CRE consensus oligonucleotide also competed for binding with this sequence.

A type I collagen substrate increases PTH/PTHrP receptor mRNA expression and suppresses PTHrP mRNA expression in UMR106-06 osteoblast-like cells.

We have previously shown that the response of osteoblasts to parathyroid hormone (PTH) can be influenced at the receptor level by growth on the physiological substrate, type I collagen, or by treatment with retinoic acid. We have also shown differential expression of genes when cells of the osteoblast lineage are grown on type I collagen. The aim of this study was therefore to examine the effect of retinoic acid and growth on type I collagen on PTH/PTH-related protein (PTHrP) receptor mRNA expression in the osteosarcoma osteoblast-like cell line UMR 106-06. PTH/PTHrP receptor mRNA levels, as assessed by Northern blot, of cells grown on collagen were increased up to 2-fold compared with cells on plastic and in a concentration-dependent manner with respect to collagen. An increase was seen as early as 6 h and was maintained over a 24 h period. This was not due to increased mRNA stability. Retinoic acid decreased the level of receptor mRNA on both plastic and collagen at each time but did not alter mRNA stability. For all treatments PTH/PTHrP receptor mRNA abundance, relative to glyceraldehyde-3-phosphate dehydrogenase, increased steadily over 24 h after subculture of cells. In contrast, PTHrP mRNA levels were reduced in cells on collagen, compared with plastic. PTH-stimulated cAMP levels of cells grown on collagen were increased compared with plastic at 24 h, but not earlier. Consistent with the mRNA data, retinoic acid decreased the amplitude of cAMP responses in cells on plastic and collagen. There was no evidence for changes in adenylate cyclase per se, since forskolin-induced cAMP levels did not change with either treatment. This study shows that known modulators of osteoblast maturation also affect signal transduction in these cells by regulating gene expression of the PTH/PTHrP receptor as well as the PTHrP ligand.

Transcriptional control and the regulation of endocrine genes.

1. Endocrine genes are regulated at a number of levels during their expression. Regulation can occur during transcription, mRNA splicing, mRNA degradation, translation, or post-translational processing of protein precursors. 2. Transcription is controlled by an increasingly well studied and enlarging family of transcription factors that bind to basal control DNA sequences (promoters) and transcriptional activator sequences (enhancers). 3. Steroid receptors act as transcription factors, as do the proteins involved in the gene regulation by cyclic AMP. Parathyroid hormone related protein is typical of many endocrine genes in that it is regulated by multiple agonists including glucocorticoids and hormones activating the cyclic AMP cascade.

Calcitonin increases transcription of parathyroid hormone-related protein via cAMP.

Transcriptional regulation of the human parathyroid hormone-related protein (PTHrP) gene by calcitonin was examined in a lung cancer line (BEN cells). Northern analysis demonstrated that calcitonin caused a rapid 4.5-fold elevation in PTHrP mRNA. Transient transfection of a construct containing 1119 base pairs of the human PTHrP gene 5' to the ATG start site of translation, fused to the CAT reporter sequence, was used to demonstrate a five-fold increase in transcription by calcitonin. Similar increases were also observed when transfected cells were exposed to a number of cAMP agonists including forskolin, as well as isobutyl-methylxanthine. A putative cAMP responsive element (5'-TGACTTCA-3') present within exon 4 was placed upstream of the heterologous SV40 promoter. Expression of this construct was elevated 4.5-fold in response to calcitonin and 7-fold in response to forskolin. Similar responses to calcitonin occurred with a smaller construct (pZMR30) containing 530 bp of sequence upstream of the ATG start site. Thus we postulate that calcitonin acts at least partially via cAMP through this element in exon 4 of the human PTHrP gene.

Distribution of catecholamine uptake sites in human brain as determined by quantitative [3H] mazindol autoradiography.

Because of the importance of the catecholamine system in Parkinson's disease and its relevance to a variety of clinical movement disorders, catecholamine uptake sites were mapped in the human brain using [3H] mazindol autoradiography. Displacement studies with known dopamine (DA) and noradrenaline (NA) uptake blockers showed that binding in the striatum was to dopamine uptake sites; binding in the locus coeruleus was to noradrenergic uptake sites. By using the selective noradrenergic uptake blocker desmethylimipramine (DMI), a comprehensive map of both DA and NA uptake sites was generated. In general, catecholamine uptake sites were better seen in terminals than in cells of origin or axonal projections. In some areas, such as the locus coeruleus, punctate binding could be seen over individual pigmented cells. A variegated pattern of binding was seen in caudate nucleus and putamen and some correspondence of patches of low binding with striosomes was observed in the caudate. The highest levels of binding to DA uptake sites was observed in the striatum, where regional differences in binding occurred. The most dense binding was seen in the ventral striatum, and a rostral-to-caudal decrement in binding levels in caudate nucleus and putamen was evident. Binding was more intense in the putamen compared to the caudate and within the caudate lower values were seen laterally. The highest levels of binding to noradrenergic uptake sites were in the locus coeruleus and dorsal raphé, although these sites may be on terminals from other projections. Whereas uptake sites were more often evident in known catecholamine pathways, [3H] mazindol binding was seen in some areas where catecholamine neurons or terminals had not been identified previously. These maps of the catecholamine uptake system add further information concerning the nature of the distribution of catecholamines in human brain and provide an important baseline for the study of disease and ageing processes.

Evidence for plasticity of the dopaminergic system in parkinsonism.

A series of compensatory mechanisms within the dopaminergic system have been shown to maintain clinical function in the presence of dopamine loss. Experimental evidence for increased presynaptic dopamine turnover owing to increased dopamine synthesis, release, and reduced reuptake exists. Direct evidence that these mechanisms maintain extracellular dopamine levels is provided by intracerebral microdialysis techniques. Postsynaptic denervation supersensitivity clearly occurs with D2 dopamine receptors, although this is less evident with D1 receptors. Similarly, mechanisms of plasticity have been shown to be relevant in human postmortem and Positron Emission Tomographic studies of patients with Parkinson's disease. However, although presynaptic increases in dopamine turnover are well documented, postsynaptic D1 and D2 receptor changes have been more difficult to establish, mainly because of methodological difficulties. D2, but not D1, receptor increases have been documented in drug naive Parkinsonian patients with PET techniques. In transplantation of adrenal gland to striatum in animal models and patients with Parkinsonism where clinical improvement occurs, plasticity of host response may be as important as plasticity of the graft. Although some elements of the compensatory mechanism of dopamine plasticity may be deleterious, such as dyskinesias owing to dopamine receptor supersensitivity, the overall effect of delay and minimization of the clinical expression of disease is advantageous. An even greater understanding of the mechanisms involved may assist in developing future therapeutic strategies.

Catecholamine uptake sites in mouse brain: distribution determined by quantitative [3H]mazindol autoradiography.

Because of the importance of the mouse brain catecholamine system in the study of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and because little information is available concerning the chemical neuroanatomy of the mouse, catecholamine uptake sites were mapped in C57 black mouse brain using [3H]mazindol autoradiography. Displacement studies with known dopamine (DA) and noradrenaline (NA) uptake blockers showed that binding in the striatum was entirely to DA uptake sites, while binding in the locus coeruleus was to NA uptake sites only. By using the selective noradrenergic uptake blocker desmethylimipramine (DMI), a complete map of both DA and NA uptake sites was generated. The mesostriatal DA system was the most clearly labelled and uptake sites were seen better in striatal terminals than the substantia nigra. Within the noradrenergic system, highest binding levels were seen over the locus coeruleus, although it was unclear whether these uptake sites were on cell bodies or terminals from the lateral tegmental noradrenergic system. These maps of the catecholamine uptake system in mouse brain provide a baseline for study of newly discovered neurotoxins and ageing processes.