Spatially restricted expression of regulators of G-protein signaling in primary olfactory neurons.

The intracellular signal transduction machinery of heterotrimeric G-protein coupled odorant and putative pheromone receptors converts odorous information into a cellular response. We have investigated for the presence of 18 members of the family termed "regulators of G-protein signaling" (RGS) in primary olfactory sensory neurons of the main as well as the accessory (vomeronasal) system of the mouse. Unexpectedly, expression of a few RGS members show spatial restrictions correlating with the patterns described for G-protein coupled receptors in these two types of olfactory neurons. RGS3 was selectively coexpressed with the Galphai2 G-protein subunit in a subpopulation of vomeronasal neurons. The mutually exclusive spatial extents of RGS9 and RGSZ1 expression in main olfactory neurons corresponded precisely to that of certain odorant receptor zones. This renders these RGS members the first described intracellular signal transduction components with a potential role in the spatially organized sensory coding in the main olfactory system.

Odorant receptors: a plethora of G-protein-coupled receptors.

Odorant receptors (ORs) comprise the largest family of G-protein-coupled receptors (GPCRs). They are located in the nasal epithelium, at the ciliated surface of olfactory sensory neurones, where the initial steps of the olfactory transduction cascade occur. ORs are encoded by a large and diverse multi-gene family, which has been characterized in cyclostomes, teleosts, amphibia, birds and mammals, as well as in Drosophila and Caenorhabditis elegans. Here, the range of diversity in OR and chemoreceptor structure is examined, noting that their functions are fundamentally similar to those of many neurotransmitter or neurohormone receptors. It is argued that ORs have emerged directly from other GPCRs independently in many species. According to this view, there is no structural prerequisite for OR identity and any GPCR has the potential to be or become an OR at a given point in evolution.

Repression of dioxin signal transduction in fibroblasts. Identification Of a putative repressor associated with Arnt.

Heterodimeric complexes of basic helix-loop-helix/PAS transcription factors are involved in regulation of diverse physiological phenomena such as circadian rhythms, reaction to low oxygen tension, and detoxification. In fibroblasts, the basic helix-loop-helix/PAS heterodimer consisting of the ligand-inducible dioxin receptor and Arnt shows DNA-binding activity, and the receptor and Arnt are able to activate transcription when fused to a heterologous DNA-binding domain. However, fibroblasts are nonresponsive to dioxin with regard to induction mediated by the DNA response element recognized by the receptor and Arnt. Here we demonstrate that Arnt is associated with a fibroblast-specific factor, forming a complex that is capable of binding the dioxin response element. This factor may function as a repressor since negative regulation of target gene induction appears to be abolished by inhibition of histone deacetylase activity by trichostatin A. Finally, the negative regulatory function of this factor appears to be restricted for dioxin signaling since Arnt was able to mediate, together with hypoxia-inducible factor-1alpha, transcriptional activation in hypoxic cells. Taken together, these data suggest that fibroblast-specific inhibition of dioxin responsiveness involves recruitment by Arnt of a cell type- and signaling pathway-specific corepressor associated with a histone deacetylase.

A novel family of ancient vertebrate odorant receptors.

Vertebrate odorant receptor (OR) genes have been isolated and characterized in several taxa, including bony fish and mammals. However, the search for more ancient vertebrate OR genes has been unsuccessful to date, indicating that these ancient genes share little sequence identity with previously isolated ORs. The lamprey (Lampetra fluviatilis) olfactory epithelium does not appear to express any of the modern vertebrate ORs previously identified in bony fish and mammals. We have isolated and characterized an ancient family of vertebrate membrane receptors from the olfactory epithelium of the lamprey. Sequence analysis reveals similarities with other Class A (rhodopsin-like) G protein-coupled receptors such as serotonin, dopamine, and histamine receptors, but the expression patterns of members of the new family, as well as certain conserved motifs, strongly suggest that the sequences encode ORs. Sequence similarity within the lamprey OR family is low, and Southern blot analysis suggests reduced-sized subfamilies. This novel vertebrate OR gene family, the most ancient isolated to date, is proposed to be involved in the detection of water-borne molecules in jawless fishes. Lamprey OR genes therefore represent a new level of diversity within the vertebrate OR gene family, but also provide clues as to how vertebrate ORs might have emerged.

Evidence for distinct signaling mechanisms in two mammalian olfactory sense organs.

In mammals, olfactory stimuli are detected by sensory neurons at two distinct sites: the olfactory epithelium (OE) of the nasal cavity and the neuroepithelium of the vomeronasal organ (VNO). While the OE can detect volatile chemicals released from numerous sources, the VNO appears to be specialized to detect pheromones that are emitted by other animals and that convey information of behavioral or physiological importance. The mechanisms underlying sensory transduction in the OE have been well studied and a number of components of the transduction cascade have been cloned. Here, we investigated sensory transduction in the VNO by asking whether VNO neurons express molecules that have been implicated in sensory transduction in the OE. Using in situ hybridization and Northern blot analyses, we found that most of the olfactory transduction components examined, including the guanine nucleotide binding protein alpha subunit (G-alpha-olf), adenylyl cyclase type III, and an olfactory cyclic nucleotide-gated (CNG) channel subunit (oCNC1), are not expressed by VNO sensory neurons. In contrast, VNO neurons do express a second olfactory CNG channel subunit (oCNC2). These results indicate that VNO sensory transduction is distinct from that in the OE but raise the possibility that, like OE sensory transduction, sensory transduction in the VNO might involve cyclic nucleotide-gated ion channels.

Sensory transduction in vomeronasal neurons: evidence for G alpha o, G alpha i2, and adenylyl cyclase II as major components of a pheromone signaling cascade.

The mammalian vomeronasal organ (VNO) is an accessory olfactory structure implicated in the sensing of pheromones. Although virtually nothing is known about sensory transduction in the mammalian VNO, recent findings have raised the possibility that it proceeds via a G-protein-coupled mechanism and involves a cyclic nucleotide-gated ion channel as in the nasal olfactory epithelium. To investigate this possibility, we cloned G-protein alpha subunits, adenylyl cyclases, and guanylyl cyclases that are expressed in the VNO and examined their patterns of expression. Of seven G alpha subunits identified as being expressed in the VNO, we found that mRNAs encoding only two, G alpha o and G alpha i2, are highly expressed in VNO neurons. Moreover, G alpha o and G alpha i2 are highly expressed by separate subsets of neurons that are located in different regions of the VNO neuroepithelium. Immunohistochemical studies show that both G alpha o and G alpha i2 are enriched in VNO microvilli, suggesting that G-proteins containing both of these alpha subunits may be involved in VNO sensory transduction. Of the adenylyl and guanylyl cyclases that we cloned, we found that only one, adenylyl cyclase type II, is highly expressed in VNO neurons; furthermore, it is expressed by both G alpha o+ and G alpha i2+ subsets. Our findings suggest that spatially segregated subsets of VNO neurons may use different, but related, sensory transduction pathways in which G-proteins and an adenylyl cyclase play major roles.

Differential effects of a topoisomerase I inhibitor on dioxin inducibility and high-level expression of the cytochrome P450IA1 gene.

The basic helix-loop-helix containing dioxin receptor mediates dioxin signal transduction. The ligand-activated receptor complex binds to specific sequences termed xenobiotic response elements and regulates transcription of target genes such as the gene for cytochrome P450IA1. This study demonstrates that induction of cytochrome P450IA1 and P450IB1 gene expression by a dioxin receptor ligand is repressed by camptothecin, an inhibitor of the topoisomerase I enzyme. However, a transiently transfected reporter construct under control of an xenobiotic response element-containing promoter was not affected by the topoisomerase inhibitor. In agreement with this observation, ligand-dependent activation of the dioxin receptor to its DNA-binding form is not altered by camptothecin as analyzed by electrophoretic mobility shift assay. Moreover, the inhibitory effect of camptothecin cannot be exerted once the P450IA1 gene has been activated. These results imply that topoisomerase I activity is necessary for the primary P450IA1 induction response, possibly involving dioxin-dependent alterations in chromatin structure of the P450IA1 promoter.

A tyrosine kinase-dependent pathway regulates ligand-dependent activation of the dioxin receptor in human keratinocytes.

Signal transduction by dioxin is mediated by the intracellular basic helix-loop-helix dioxin receptor which, in its ligand-activated state, binds to target DNA as a heteromeric complex with the partner factor Arnt. In contrast, the repressed form of the receptor is a complex with hsp90 which appears to maintain the receptor in an inducible conformation. In human keratinocytes dioxin receptor activation has previously been shown to depend on phosphorylation processes. To further dissect mechanisms regulating dioxin receptor function the importance of tyrosine phosphorylation was investigated by the use of specific tyrosine kinase inhibitors. Here we report that the inhibitor genistein inhibited dioxin-dependent induction of expression of the target gene cytochrome P-450IA1. This effect was rapid and reversible and did not lead to altered levels of dioxin receptor protein. Analyses of dioxin receptor or Arnt fusion proteins that function independently of one another showed that the target for genistein action was the dioxin receptor, and, more specifically, a region of the receptor harboring its ligand-binding domain. In addition, function of an unrelated transactivator, the glucocorticoid receptor, was inhibited by genistein while a truncated form lacking the ligand-binding domain was not. A common denominator between the ligand-binding domains of both receptors is their ability to interact with hsp90. Importantly, co-immunoprecipitation experiments showed that genistein inhibited ligand-induced release of hsp90 from the glucocorticoid receptor. Thus, the interaction of these transactivators with hsp90 may be regulated by a tyrosine kinase-dependent pathway.

Nonresponsiveness of normal human fibroblasts to dioxin correlates with the presence of a constitutive xenobiotic response element-binding factor.

Polychlorinated aromatic hydrocarbons such as 2,3,7,8-tetrachlorodibenzofuran (TCDF) have been shown to induce transcription of the cytochrome P-450IA1 gene by activating an intracellular receptor protein (the Ah- or dioxin receptor) to bind to specific DNA sequences, termed xenobiotic response elements (XREs). However, the expression and inducibility of the cytochrome P-450IA1 activity exhibit tissue-specific differences. With regard to the TCDF induction response, we have examined three human cell types of endodermal (the hepatoma cell line HepG2), ectodermal (normal keratinocytes), and mesodermal origin (normal fibroblasts). DNase I hypersensitivity analysis of the 5' flank and first intron of the P-450IA1 gene showed that in the nonresponsive fibroblasts the chromatin structure lacked open regions while in the two responsive cell types (keratinocytes and HepG2) several constitutive hypersensitive sites as well as TCDF-induced alterations in the chromatin structure could be detected. This observation might correlate with the fact that the XRE, in either the context of the P-450IA1 gene sequences or in front of a heterologous promoter, was inefficient in directing a TCDF induction response in fibroblasts. In in vitro DNA binding studies, the dioxin receptor was activated to a DNA-binding nuclear form in all three cell types. However, in fibroblast nuclear extracts two novel constitutive protein-XRE complexes were detected. The fibroblast factor(s) were immunochemically distinct from the receptor but exhibited indistinguishable DNA binding specificity. These data are compatible with a model where the P-450IA1 is noninducible in fibroblasts due to the presence of a putative repressor(s) which may compete effectively with the receptor for binding to the response element as indicated by in vitro DNA-binding off-rate experiments.

Cross-coupling of signal transduction pathways: the dioxin receptor mediates induction of cytochrome P-450IA1 expression via a protein kinase C-dependent mechanism.

Signal transduction by dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin) is mediated by the intracellular dioxin receptor which, in its dioxin-activated state, regulates transcription of target genes encoding drug-metabolizing enzymes, such as cytochrome P-450IA1 and glutathione S-transferase Ya. Exposure of the dioxin receptor to dioxin leads to an apparent translocation of the receptor to the nucleus in vivo and to a rapid conversion of the receptor from a latent, non-DNA-binding form to a species that binds to dioxin-responsive positive control elements in vitro. This DNA-binding form of receptor appears to be a heterodimeric complex with the helix-loop-helix factor Arnt. In this study, we show that activation of the cytochrome P-450IA1 gene and minimal dioxin-responsive reporter constructs by the dioxin receptor was inhibited following prolonged treatment of human keratinocytes with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate. Inhibition of the receptor-mediated activation response was also achieved by treatment of the cells with a number of protein kinase inhibitors, one of which, calphostin C, shows selectivity for protein kinase C. Taken together, these data suggest that protein kinase C-dependent phosphorylation may play an essential role in the dioxin signaling pathway. This hypothesis is supported by the observation that pretreatment of the cells with 12-O-tetradecanoylphorbol-13-acetate inhibited the DNA-binding activity of the dioxin receptor in vivo. In vivo, the dioxin receptor was found to be a phosphoprotein. In vitro, dephosphorylation of the ligand-activated, heteromeric dioxin receptor form or dephosphorylation of the individual ligand-binding and Arnt receptor subunits inhibited the xenobiotic response element-binding activity. Moreover, dephosphorylation experiments with the individual receptor subunits prior to assembly of the xenobiotic response element-binding receptor form indicated that phosphorylation seemed to be important for the DNA-binding activity per se of the receptor, whereas Arnt appeared to require phosphorylation to interact with the receptor. Finally, a protein kinase C inhibitor-sensitive cytosolic catalytic activity that could restore the DNA-binding activity of the dephosphorylated dioxin receptor form was identified.

The stability of dioxin-receptor ligands influences cytochrome P450IA1 expression in human keratinocytes.

Three dioxin-receptor ligands were analyzed for their effect on cytochrome P450IA1 mRNA expression in normal human keratinocytes. Although a 2 h pulsed treatment with the receptor agonists 2,3,7,8-tetrachlorodibenzofuran (TCDF) and beta-naphthoflavone (BNF) gave the same maximal induction response, the effect of BNF was transient compared to effect of TCDF. This was most likely due to metabolism of BNF as exemplified by the fact that a P450IA1 enzyme suicide-inhibitor, 1-ethynylpyrene, could prolong the induction response following a short BNF treatment. The TCDF induction of a reporter gene construct under the control of the -1140 to +2435 part of the CYPIA1 gene transiently transfected into HK was effectively inhibited by the dioxin-receptor antagonist alpha-naphthoflavone (ANF). In addition, ANF inhibited the accumulation of TCDF-activated nuclear receptors with capacity to bind to a xenobiotic response element. Interestingly, ANF could also suppress already maximally induced P450IA1 mRNA levels. The data demonstrate that the stability of the ligand influences the long-term effects on gene expression and that the effect of stable ligands may be masked due to receptor antagonist presence. In addition, the results support the hypothesis that a constant low level of activated nuclear receptors is required to maintain induced P450IA1 expression.

Allogeneic cultured keratinocytes in the treatment of leg ulcers. A pilot study.

Forty-two patients (10 males and 32 females) with 52 chronic leg ulcers were treated with sheets of cultured allogeneic keratinocytes. Sixty-five % of the ulcers healed completely and the healing rate differed between various diagnostic groups. The best results were obtained in patients with venous ulcers and wounds with mixed etiology, whereas less improvement was observed with ischaemic ulcers. Rheumatic ulcers also responded well in combination with oral corticosteroids. The overall impression was that the grafting procedure markedly enhanced wound healing.

Serum and extracellular calcium modulate induction of cytochrome P-450IA1 in human keratinocytes.

Culture conditions allowing for cytochrome P-450IA1 induction by 2,3,7,8-tetrachlorodibenzofuran (TCDF) in normal human keratinocytes (HK) were investigated. HK grown in serum-free low extracellular Ca2+ (0.1 mM) medium did not accumulate P-450IA1 mRNA in response to TCDF. If, however, the cultures were pretreated for more than 24 h with either serum or elevated extracellular Ca2+ (2.0 mM), induction of P-450IA1 was obtained by TCDF. Serum and elevated Ca2+ concentrations were found to be additive in this respect. When analyzing HK derived from five individuals, no apparent difference was found in the relative induction of P-450IA1 by increasing concentrations of TCDF, giving an EC50 of approximately 2 nM. The permissive effect of serum and elevated Ca2+ could be conferred to a reporter gene by the -1140 to +2435 part of the human CYPIA1 gene. Culture conditions allowing for P-450IA1 induction correlated with conditions that induced mRNA corresponding to the differentiation specific enzyme epidermal transglutaminase. This finding, together with the known differentiation promoting effects of serum and elevated Ca2+, suggest that terminal differentiation is necessary for P-450IA1 induction in HK by Ah receptor ligands.

Isolation and characterization of complementary DNA clones corresponding to genes induced in mouse epidermis in vivo by tumor promoters.

Complementary DNA clones representing genes in SENCAR mouse epidermis, the expression of which is induced 4 h after one topical application of the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) were isolated. Of 56 isolated complementary DNA clones, 32 were identified to be identical to either metallothioneins (MT-I and MT-II) or endogenous retroviral like (VL30) sequences. In situ hybridization and analysis of mRNA levels in cell fractions separated by density gradient centrifugation revealed that MT induction was restricted to keratinocytes in the basal cell layer. Immunohistochemistry and time-kinetic studies on mRNA levels in mouse epidermis showed that the increase in MT and VL30 RNAs coincide in time with a TPA-induced transient block in basal cell proliferation (3-12 h after TPA treatment). MT immunoreactivity and transcript levels had returned to control values at a time point (24 h after treatment) when epidermis is known to hyperproliferate. Treatment with other types of tumor promoters showed that MT-I and MT-II mRNAs were coordinately induced and indicated that sn-1,2-dioctanoylglycerol, 12-O-retinoylphorbol-13-acetate, and mezerein induced MT to a lesser degree than TPA. The calcium ionophore A23187 induced mRNA levels for MTs as well as VL30. VL30 and MT mRNA levels were not found to be elevated in epidermal tumors whereas the mRNA level corresponding to glyceraldehyde-3-phosphate dehydrogenase was elevated in tumors and induced by TPA with time-kinetics that correlate with a TPA-induced hyperproliferation. These complementary DNA clones provide useful tools in the study of the gene-regulating effects of TPA in a target tissue relevant for tumor promotion.