Induction of Porphyromonas gingivalis GroEL signaling via binding to Toll-like receptors 2 and 4

BACKGROUND/AIMS

Heat shock protein 60 (HSP60) has been recognized as an important molecule in infectious and autoimmune diseases. Although Porphyromonas gingivalis GroEL, a homologue of HSP60, is a potent stimulator of inflammatory cytokines, its receptor and signaling mechanisms are not yet understood in detail. In this study, we investigated whether the Toll-like receptor (TLR) family plays a functional role as a P. gingivalis GroEL receptor.

METHODS

Human macrophage-like THP-1 cells were used and the nuclear factor-kappaB (NF-kappaB) activity of cells stimulated with a recombinant P. gingivalis GroEL was measured with a luciferase assay. Flow cytometry analysis was used to determine the binding to THP-1 cells of fluorescein isothiocyanate (FITC)-labeled GroEL. In addition, anti-human TLR (anti-hTLR)2 and anti-hTLR4 monoclonal antibodies were used to assess the functional role of TLR2 and TLR4 as the receptors for GroEL.

RESULTS

We observed by luciferase assay that the purified recombinant GroEL was able to stimulate NF-kappaB transcriptional activity in THP-1 cells. Flow cytometry analysis showed that the FITC-labeled GroEL bound to THP-1 cells in a dose-dependent fashion. Our binding competition analysis with FITC-labeled and unlabeled GroEL showed that it bound to the cells as a specific mode of action. On the other hand, GroEL-stimulated NF-kappaB transcriptional activity was significantly inhibited by anti-hTLR2 and anti-hTLR4 antibodies and was inhibited more strongly by a combination of both antibodies.

CONCLUSION

Our present study demonstrates that P. gingivalis GroEL induces its intracellular signaling cascade in THP-1 cells via TLR2 or TLR4 and via a combination of both receptors.

Ligand type-specific interactions of peroxisome proliferator-activated receptor gamma with transcriptional coactivators

The nuclear peroxisome proliferator-activated receptor gamma (PPARgamma) is a member of the nuclear receptor superfamily and acts as a ligand-dependent transcription factor mediating adipocyte differentiation, cell proliferation and inflammatory processes, and modulation of insulin sensitivity. Members of the 160-kDa protein (SRC-1/TIF2/AIB-1) family of coactivators, CBP/p300 and TRAP220/DRIP205, are shown to interact directly with PPARgamma and potentiate nuclear receptor transactivation function in a ligand-dependent fashion. Because PPARgamma ligands exert partially overlapping but distinct subsets of biological action through PPARgamma binding, we wished to examine whether interactions between PPARgamma and known coactivators were induced to the same extent by different classes of PPARgamma ligand. The natural ligand 15-deoxy-Delta12,14-prostaglandin J(2) induced PPARgamma interactions with all coactivators tested (SRC-1, TIF2, AIB-1, p300, TRAP220/DRIP205) in yeast and mammalian two-hybrid assays, as well as in a glutathione S-transferase pull-down assay. However, under the same conditions troglitazone, a synthetic PPARgamma ligand that acts as an antidiabetic agent, did not induce PPARgamma interactions with any of the coactivators. Our findings suggest that ligand binding may alter PPARgamma structure in a ligand type-specific way, resulting in distinct PPARgamma-coactivator interactions.

Molecular mechanism of a cross-talk between oestrogen and growth factor signalling pathways

Oestrogen (E2) plays significant roles in variety of biological events such as the development and maintenance of female reproductive organs, bone and lipid metabolisms. More recently, from study of knock-out mice deficient in oestrogen receptor (ER) alpha and ERbeta it turned out that normal spermatogenesis requires the E2 actions. Furthermore, this female steroid hormone is also well known to be deeply involved in many pathophysiological events such as osteoporosis and cancer development in female reproductive organs. It is particularly well known that most breast cancer is dependent on E2 in its development. Such E2 actions are thought to be mediated through two subtypes of ERs. Growth factors have been shown to synergize in this E2 signalling pathway, although the actual molecular mechanism largely remains unknown. Recently, we found that the MAP kinase activated by growth factors phosphorylates the Ser118 residue of the human ERalpha A/B domain and this phosphorylation potentiates the N-terminal transactivation function (AF-1) of human ERalpha, indicating the possible molecular mechanism of a novel cross-talk between E2 and growth factor signalling pathways. More recently, we have identified a coactivator associating with the hERalpha AF-1 in a MAPK-mediated phosphorylation-dependent manner. In this review, the molecular mechanism of this cross-talk is discussed in terms of the transactivation function of ERs, and their coactivators.

p300 mediates functional synergism between AF-1 and AF-2 of estrogen receptor alpha and beta by interacting directly with the N-terminal A/B domains

Estrogen receptor (ER) alpha and beta mediate estrogen actions in target cells through transcriptional control of target gene expression. For 17beta-estradiol-induced transactivation, the N-terminal A/B domain (AF-1) and the C-terminal E/F domain (AF-2) of ERs are required. Ligand binding is considered to induce functional synergism between AF-1 and AF-2, but the molecular mechanism remains unknown. To clarify this synergism, we studied the role of reported AF-2 coactivators, p300/CREB binding protein, steroid receptor coactivator-1/transcriptional intermediary factor-2 (SRC-1/TIF2) family proteins and thyroid hormone receptor-associated protein-220/(vitamin D3 receptor-interacting protein- 205-(TRAP220/DRIP205) on the AF-1 activity in terms of synergism with the AF-2 function. We found that neither any of the SRC-1/TIF2 family coactivators nor TRAP220/DRIP205 is potent, whereas p300 potentiates the AF-1 function of both human ERalpha and human ERbeta. Direct interactions of p300 with the A/B domains of ERalpha and ERbeta were observed in an in vitro glutathione S-transferase pull-down assay in accordance with the interactions in yeast and mammalian two-hybrid assays. Furthermore, mutations in the p300 binding sites (56-72 amino acids in ERalpha and 62-72 amino acids in ERbeta) in the A/B domains caused a reduction in ligand-induced transactivation functions of both ERalpha and ERbeta. Thus, these findings indicate that ligand-induced functional synergism between AF-1 and AF-2 is mediated through p300 by its direct binding to the A/B regions of ERalpha and ERbeta.

p300/CBP acts as a coactivator of the cone-rod homeobox transcription factor

CRX (cone-rod homeobox) is shown to play an important role on the photoreceptor cell maturation and maintenance. However, little is known about the molecular mechanism how CRX potentiates transcription. Here, we investigated the effect of several coactivators (p300/CBP, SRC-1, TIF2, and AIB1) on the transactivation function of CRX. The transient expression assay with a luciferase reporter gene showed that only p300/CBP potentiates the transactivation function of CRX. Furthermore, by interaction studies in vivo (a mammalian two-hybrid assay) and in vitro (a pulldown assay), p300/CBP was found to directly bind CRX through their C-terminal domains. The C-terminal domain of CRX was mapped as a transactivation domain to associate with p300/CBP. Taken together, our results demonstrate that p300/CBP acts as one of the coactivators of CRX.

Purification and identification of p68 RNA helicase acting as a transcriptional coactivator specific for the activation function 1 of human estrogen receptor alpha

The estrogen receptor (ER) regulates the expression of target genes in a ligand-dependent manner. The ligand-dependent activation function AF-2 of the ER is located in the ligand binding domain (LBD), while the N-terminal A/B domain (AF-1) functions in a ligand-independent manner when isolated from the LBD. AF-1 and AF-2 exhibit cell type and promoter context specificity. Furthermore, the AF-1 activity of the human ERalpha (hERalpha) is enhanced through phosphorylation of the Ser(118) residue by mitogen-activated protein kinase (MAPK). From MCF-7 cells, we purified and cloned a 68-kDa protein (p68) which interacted with the A/B domain but not with the LBD of hERalpha. Phosphorylation of hERalpha Ser(118) potentiated the interaction with p68. We demonstrate that p68 enhanced the activity of AF-1 but not AF-2 and the estrogen-induced as well as the anti-estrogen-induced transcriptional activity of the full-length ERalpha in a cell-type-specific manner. However, it did not potentiate AF-1 or AF-2 of ERbeta, androgen receptor, retinoic acid receptor alpha, or mineralocorticoid receptor. We also show that the RNA helicase activity previously ascribed to p68 is dispensable for the ERalpha AF-1 coactivator activity and that p68 binds to CBP in vitro. Furthermore, the interaction region for p68 in the ERalpha A/B domain was essential for the full activity of hERalpha AF-1. Taken together, these findings show that p68 acts as a coactivator specific for the ERalpha AF-1 and strongly suggest that the interaction between p68 and the hERalpha A/B domain is regulated by MAPK-induced phosphorylation of Ser(118).

Convergence of transforming growth factor-beta and vitamin D signaling pathways on SMAD transcriptional coactivators

Cell proliferation and differentiation are regulated by growth regulatory factors such as transforming growth factor-beta (TGF-beta) and the liphophilic hormone vitamin D. TGF-beta causes activation of SMAD proteins acting as coactivators or transcription factors in the nucleus. Vitamin D controls transcription of target genes through the vitamin D receptor (VDR). Smad3, one of the SMAD proteins downstream in the TGF-beta signaling pathway, was found in mammalian cells to act as a coactivator specific for ligand-induced transactivation of VDR by forming a complex with a member of the steroid receptor coactivator-1 protein family in the nucleus. Thus, Smad3 may mediate cross-talk between vitamin D and TGF-beta signaling pathways.

Selective interaction of vitamin D receptor with transcriptional coactivators by a vitamin D analog

The nuclear vitamin D receptor (VDR) is a member of a nuclear receptor superfamily and acts as a ligand-dependent transcription factor. A family of cotranscriptional activators (SRC-1, TIF2, and AIB-1) interacts with and activates the transactivation function of nuclear receptors in a ligand-dependent way. We examined interaction of VDR with these coactivators that was induced by several vitamin D analogs, since they exert differential subsets of the biological action of vitamin D through unknown mechanisms. Unlike other vitamin D analogs tested, OCT (22-oxa-1alpha,25-dihydroxyvitamin D3) induced interaction of VDR with TIF2 but not with SRC-1 or AIB-1. Consistent with these interactions, only TIF2 was able to potentiate the transactivation function of VDR bound to OCT. Thus, the present findings suggest that the structure of VDR is altered in a vitamin D analog-specific way, resulting in selective interactions of VDR with coactivators. Such selective interaction of coactivators with VDR may specify the array of biological actions of a vitamin D analog like OCT, possibly through activating a particular set of target gene promoters.

Molecular mechanism of a cross-talk between estrogen and growth-factor signaling pathways

The actions of estrogen (E2) are considered to be mediated through its nuclear E2 receptor (ER). In cancer development, growth factors are shown to act synergistically with E2. Recently, we found that the mitogen-activated protein kinase, activated by growth factors, phosphorylates human ERalpha and this phosphorylation potentiates the transactivation function of human ERalpha demonstrating a novel cross-talk between E2 and growth factor-signaling pathways. In this review, the molecular mechanism of this cross-talk is discussed.

A putative tumor suppressor, TSG101, acts as a transcriptional suppressor through its coiled-coil domain

TSG101 is thought as a putative tumor suppressor gene, and mutations of this gene were recently found in 7 of 15 breast cancer patients, though the physiological function remains to be elucidated. In this report, we showed that TSG101 protein acts as a transcriptional suppressor for estrogen receptor (ER) as well as other members of the nuclear hormone receptor super-family, VP16, and on its own. The basal promoter activity was also inhibited by TSG101. The suppression of transcription by TSG101 protein required its coiled-coil domain, which is also shown to be required for the tumor suppressive function. Expressed TSG101 protein did not have any histone acetylase nor deacetylase activity, which certain transcriptional co-factors have. The requirement of the same domain in the TSG101 protein for transcriptional suppression and in the tumor suppression indicates a possibility that transcriptional suppression of TSG101 is related to its tumor suppression.

ERC-55, a binding protein for the papilloma virus E6 oncoprotein, specifically interacts with vitamin D receptor among nuclear receptors

VDR regulates gene expression in a ligand-dependent way by binding to cognate enhancer elements of target gene promoters. The ligand-dependent activation function, AF-2, of VDR is thought to require transcriptional co-activators/co-repressors together with basal transcriptional machinery. Using a yeast two hybrid system with VDR, we have isolated a mouse Ca(2+)-binding protein (designated as VAF1) specifically interacting in vivo and in vitro with VDR among nuclear receptors like RAR, RXR, ER and GR. VAF1 is a mouse homologue to human ERC-55, which has recently been shown to interact with human papillomavirus oncogenic protein, E6[1]. Unlike those of many previously identified co-activators, the VDR-VAF1 interaction was ligand-independent. Thus, VAF1 seems a putative VDR-specific cofactor modulating its function.

Intron retention generates a novel isoform of the murine vitamin D receptor that acts in a dominant negative way on the vitamin D signaling pathway

We identified and characterized a novel rat vitamin D receptor isoform (rVDR1), which retains intron 8 of the canonical VDR (rVDR0) during alternative splicing. In this isoform protein directed by the stop codon in this newly identified exon, a part of the ligand binding domain (86 amino acids) is truncated at the C-terminal end but contains 19 extra amino acids. The rVDR1 transcript was expressed at a level 1/15 to 1/20 of that of rVDR0 in the kidney and intestine in adult rats but not in embryos. The recombinant rVDR1 protein showed no ligand binding activity. Homo- and heterodimers of the recombinant rVDR0 and rVDR1 proteins bound to a consensus vitamin D response element (VDRE) but not to consensus response elements for thyroid hormone and retinoic acid. However, unlike rVDR0, rVDR1 did not form a heterodimeric complex with RXR on the VDRE. A transient expression assay showed that this isoform acted as a dominant negative receptor against rVDR0 transactivation. Interestingly, the dominant negative activities of rVDR1 differed among VDREs. Thus, the present study indicates that this new VDR isoform negatively modulates the vitamin D signaling pathway, through a particular set of target genes.

Activation of the estrogen receptor through phosphorylation by mitogen-activated protein kinase

The phosphorylation of the human estrogen receptor (ER) serine residue at position 118 is required for full activity of the ER activation function 1 (AF-1). This Ser118 is phosphorylated by mitogen-activated protein kinase (MAPK) in vitro and in cells treated with epidermal growth factor (EGF) and insulin-like growth factor (IGF) in vivo. Overexpression of MAPK kinase (MAPKK) or of the guanine nucleotide binding protein Ras, both of which activate MAPK, enhanced estrogen-induced and antiestrogen (tamoxifen)-induced transcriptional activity of wild-type ER, but not that of a mutant ER with an alanine in place of Ser118. Thus, the activity of the amino-terminal AF-1 of the ER is modulated by the phosphorylation of Ser118 through the Ras-MAPK cascade of the growth factor signaling pathways.

Transcriptional activity of a fluorinated vitamin D analog on VDR-RXR-mediated gene expression

The transcriptional activity of the hexafluorinated derivative of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3], 26,26,26,27,27,27-hexafluoro-1,25-dihydroxyvitamin D3 [F6-1,25-(OH)2D3], was examined in cultured cells by a transient expression assay (CAT assay) using expression vectors for the rat nuclear vitamin D3 receptor (VDR) and the rat 9-cis-retinoic acid receptor (RXR beta), and a reporter plasmid containing a consensus vitamin D3 response element (VDRE) consisting of two directly repeated AGGTCA motifs spaced by 3 bp (DR3). At physiological concentrations, the transcriptional activity of F6-1,25-(OH)2D3 was 2-4 times more potent than that of 1,25-(OH)2D3 in both nontarget (HeLa) and target (UMR106) cells for 1,25-(OH)2D3. The transcriptional activity of F6-1,25-(OH)2D3 was also higher when the endogenous target gene (osteopontin), which has a VDRE related to the DR3 in its promoter, was induced. A gel-shift assay using DR3 as a probe and in vitro synthesized receptors showed that the ligand-induced DNA binding of VDR required RXR to form a heterodimer. Moreover, in this assay we found that F6-1,25-(OH)2D3 induced the receptor-DNA complex at a 10-fold lower concentration than 1,25-(OH)2D3 without influencing the dissociation kinetics. However, the binding affinity of F6-1,25-(OH)2D3 for VDR was slightly lower than that of 1,25-(OH)2D3. The increased DNA binding of ligand-bound VDR by introducing hexafluorines into 1,25-(OH)2D3 may potentiate the transcriptional activity. Thus, the higher biological activity of F6-1,25-(OH)2D3 may be exerted at least in part by enhanced transcriptional activity.