The presence or absence of a vimentin-type intermediate filament network affects the shape of the nucleus in human SW-13 cells

Human SW-13 cells express the intermediate filament protein vimentin in a mosaic pattern (Hedberg, K. K. and Chen, L. B. (1986). Exp. Cell Res. 163, 509–517). We have isolated SW-13 clones that do (vim+) or do not (vim-) synthesize vimentin as analyzed using anti-intermediate filament immunofluorescence, electron microscopy and two-dimensional gel analysis of detergent-extracted preparations. Vimentin is the only cytoplasmic intermediate filament protein present in the vim+ cells, and the vim- cells do not contain any detectable cytoplasmic intermediate filament system. The presence or absence of intermediate filaments did not observably affect the distribution of mitochondria, endoplasmic reticulum, microtubules or actin stress fibers when these structures were visualized by fluorescence microscopy. However, electron microscopy and anti-lamin A/C immunofluorescence studies showed that nuclear morphology in vim- cells was frequently characterized by large folds or invaginations, while vim+ cells had a more regular or smooth nuclear shape. When vim- cells were transfected with a mouse vimentin expression plasmid, the synthesis of a mouse vimentin filament network restored the smooth nuclear morphology characteristic of vim+ cells. Conversely, when vim+ cells were transfected with a carboxy-terminally truncated mutant vimentin, expression of the mutant protein disrupted the organization of the endogenous vimentin filaments and resulted in nuclei with a prominently invaginated morphology. These results indicated that in SW-13 cells the vimentin filament system affects the shape of the nucleus.

Intermediate filaments and lipoprotein cholesterol

The ability of cells to utilize cholesterol derived from lipoprotein is important in plasma membrane biosynthesis, steroidogenesis and the regulation of sterol synthesis. While the endocytosis of lipoprotein-derived cholesterol has been well characterized, the subsequent events that mediate its post-lysosomal intracellular transport are not understood. Recent studies have suggested that vimentin-type intermediate filaments may have a role in cholesterol transport. The mechanism by which vimentin filaments affect this process is not known, but future studies promise to provide new insights into both the post-lysosomal transport of cholesterol and the intracellular functions of intermediate filaments.

RXR alpha mutant mice establish a genetic basis for vitamin A signaling in heart morphogenesis

We have established a targeted loss-of-function mutation in the RXR alpha gene in the mouse germ line that results in embryonic lethality between E13.5 and E16.5 when bred to homozygosity. The major defect responsible for lethality is hypoplastic development of the ventricular chambers of the heart, which is manifest as a grossly thinned ventricular wall with concurrent defects in ventricular septation. This phenotype is identical to a subset of the effects of embryonic vitamin A deficiency and, therefore, establishes RXR alpha as a genetic component of the vitamin A signaling pathway in cardiac morphogenesis. The cardiac outflow tracts and associated vessels, which are populated by derivatives of the neural crest and which are also sensitive to vitamin A deficiency, are normal in homozygous embryos, indicating the genetic independence of ventricular chamber development. Hepatic differentiation was dramatically but transiently retarded yet is histologically and morphologically normal. These results ascribe an essential function for the RXR alpha gene in embryonic development and provide the first evidence of a requirement for RXR in one of its predicted hormone response pathways.

Chromosomal localization of the human retinoid X receptors

The recently described retinoid X receptors (RXRs) respond to the novel retinoid 9-cis-retinoic acid and also serve as heterodimeric partners for the vitamin D, thyroid hormone, and retinoic acid receptors (VDR, TR, and RAR, respectively). In this work, we report high-resolution localization of the human RXR genes within cytogenetic bands and also within a standard reference map of cosmid DNA markers on human chromosomes. We have determined the location of the human RXR genes by pairwise hybridization of the RXR cosmids and reference markers, using fluorescence in situ hybridization. We localized (i) RXR alpha (RXRA) to chromosome 9 band q34.3; (ii) RXR beta (RXRB) to chromosome 6 band 21.3; and (iii) RXR gamma (RXRG) to chromosome 1 band q22-q23. Six retinoid-responsive transcription factors have been identified so far, including three retinoic acid receptors in addition to the three RXRs. Interestingly, each of these receptors in human and mouse is encoded by genes located at distinct chromosomal loci and on separate chromosomes. The proximity of RXR genes to loci known to be associated with genetic disorders suggests that their location may be useful in establishing a link between RXRs and certain human diseases.

Isoform-specific amino-terminal domains dictate DNA-binding properties of ROR alpha, a novel family of orphan hormone nuclear receptors

Three isoforms of a novel member of the steroid hormone nuclear receptor superfamily related to the retinoic acid receptors have been identified. The three isoforms, referred to as ROR alpha 1, ROR alpha 2, and ROR alpha 3, share common DNA- and putative ligand-binding domains but are characterized by distinct amino-terminal domains generated by alternative RNA processing. An exon encoding a functionally important subregion of the amino-terminal domain of the ROR alpha 2 isoform resides on the opposite strand of a cytochrome c-processed pseudogene. Binding site selection using in vitro-synthesized proteins reveals that the ROR alpha 1 and ROR alpha 2 isoforms bind DNA as monomers to hormone response elements composed of a 6-bp AT-rich sequence preceding a half-site core motif PuGGTCA (RORE). However, ROR alpha 1 and ROR alpha 2 display different binding specificities: ROR alpha 1 binds to and constitutively activates transcription from a large subset of ROREs, whereas ROR alpha 2 recognizes ROREs with strict specificity and displays weaker transcriptional activity. The differential DNA-binding activity of each isoform maps to their respective amino-terminal domains. Whereas truncation of the amino-terminal domain diminishes the ability of ROR alpha 1 to bind DNA, a similar deletion relaxes ROR alpha 2-binding specificity to that displayed by ROR alpha 1. Remarkably, transfer of the entire amino-terminal region of ROR alpha 1 or amino-terminal deletion of ROR alpha 2 confers RORE-binding specificities to heterologous receptors. These results demonstrate that the amino-terminal domain and the zinc finger region work in concert to confer high affinity and specific DNA-binding properties to the ROR isoforms and suggest a novel strategy to control DNA-binding activity of nuclear receptors.

A novel macromolecular structure is a target of the promyelocyte-retinoic acid receptor oncoprotein

Acute promyelocytic leukemia (APL) is associated with a t(15;17) translocation that creates the promyelocyte-retinoic acid receptor alpha (PML-RAR alpha) fusion gene. Immunohistochemistry demonstrates that PML is a part of a novel macromolecular organelle (including at least three other nuclear proteins) referred to as PML oncogenic domains (PODs). In APL cells, the POD is disrupted into a microparticulate pattern as a consequence of the expression of the PML-RAR oncoprotein. RA treatment of APL cells triggers a reorganization of PML to generate normal-appearing PODs. We propose that PML-RAR is a dominant negative oncoprotein that exerts its putative leukomogenic effect by inhibiting assembly of the POD. According to this proposal, not only is the POD a novel structure, but it can be ascribed an imputed function such that its disruption leads to altered myeloid maturation; this may represent a novel oncogenic target.

Functional ecdysone receptor is the product of EcR and Ultraspiracle genes

Although the biological activity of the insect moulting hormone ecdysone, is manifested through a hormonally regulated transcriptional cascade associated with chromosomal puffing, a direct association of the receptor with the puff has yet to be established. The cloned ecdysone receptor (EcR) is by itself incapable of high-affinity DNA binding or transcriptional activation. Rather, these activities are dependent on heterodimer formation with Ultraspiracle (USP) the insect homologue of vertebrate retinoid X receptor. Here we report that native EcR and USP are co-localized on ecdysone-responsive loci of polytene chromosomes. Moreover, we show that natural ecdysones selectively promote physical association between EcR and USP, and conversely, that high-affinity hormone binding requires both EcR and USP. Replacement of USP with retinoid X receptor produces heterodimers with distinct pharmacological and functional properties. These results redefine the ecdysone receptor as a dynamic complex whose activity may be altered by combinatorial interactions among subunits and ligand.

Identification of a domain required for oncogenic activity and transcriptional suppression by v-erbA and thyroid-hormone receptor alpha

v-erbA, a mutated version of the chicken thyroid hormone (TH) receptor type alpha, can inhibit hormonal induction of target genes. In addition, v-erbA acts as a constitutive repressor of the basal promoter activity. In vivo, v-erbA can arrest the differentiation of erythroid precursor cells and suppresses transcription of erythrocyte-specific genes. We show that the v-erbA protein of the transformation-defective avian erythroblastosis virus mutant (AEVtd359) fails to suppress basal transcription level and exhibits impaired ability in antagonizing the TH and retinoic acid response. The inactivating mutation is a 1-nt change leading to a Pro-->Arg replacement in the "hinge region" of v-erbA protein. Introducing this mutation in the context of TH receptor alpha selectively inactivates the suppressor function, while hormone-binding and transcriptional-activation properties are unaffected. These data suggest that trans-repression rather than a dominant negative block of TH-receptor or retinoic acid-receptor activation may represent the primary molecular property underlying erbA oncogenesis.

Unexpected heterogeneity of PML/RAR alpha fused mRNA detected by nested polymerase chain reaction in acute promyelocytic leukemia

We have analyzed ten APL patients using reverse transcription polymerase chain reaction (RT-PCR) technique to detect PML/RAR alpha fused mRNA. All patients in this study had PML/RAR alpha fused mRNA (three cases of the short type and seven cases of the long type), although the chromosomal translocation t(15;17) was not detected in one patient. After ethidium bromide staining, two-thirds of the short type and all cases of the long type were found to have multiple PCR products (192 and 93 base pair (bp) bands in the short type and 666, 522, 263, and 164 bp in the long type). A total of six distinct fused mRNAs were sequenced (P1R1, P1R2, P3R1, P2R1, and P2R2). Southern hybridization analysis showed only one rearranged band in each of the patients. These results suggest that the longest mRNAs in each type are the authentic fused mRNAs and the other smaller mRNAs are generated through splicing events. In RAR alpha, a novel fusion point (R2) was identified within the fourth exon. This uncommon splicing may be caused by the instability of the splicing mechanism of the rearranged PML/RAR alpha gene. Among the ten APL patients, no correlation was observed between the type of fused mRNA and the clinical characteristics examined.

Determinants for selective RAR and TR recognition of direct repeat HREs

Recently, we have shown that receptors for vitamin D3 (VDR), thyroid hormone (TR), and retinoic acid (RAR) activate preferentially through direct repeats (DRs) spaced by 3, 4, and 5 nucleotides, respectively. In addition, the RAR can activate weakly through DRs spaced by 2 nucleotides. A common feature of RAR, TR, and VDR is their ability to heterodimerize with the retinoid X receptor (RXR) through their ligand-binding domains (LBDs) to form high-affinity DNA-binding complexes that are specific for appropriately spaced repeats. In this paper we demonstrate that selective binding of RAR-RXR and TR-RXR heterodimers to their cognate DRs is a consequence of a novel cooperative dimer interaction within the DNA-binding domains (DBDs). Accordingly, a region in the first zinc finger of the TR and RAR DBDs interacts with the second zinc finger in the RXR DBD to promote selective DNA-binding to DRs spaced by 4 and 5 nucleotides, respectively. The resulting polarity established by this interaction places RXR in the 5' position of the direct repeats. These data provide a mechanism for selective receptor recognition of a restricted set of target sequences in DR DNA and explains the structural basis for physiological specificity.

Structure of the retinoid X receptor alpha DNA binding domain: a helix required for homodimeric DNA binding

The three-dimensional solution structure of the DNA binding domain (DBD) of the retinoid X receptor alpha (RXR alpha) was determined by nuclear magnetic resonance spectroscopy. The two zinc fingers of the RXR DBD fold to form a single structural domain that consists of two perpendicularly oriented helices and that resembles the corresponding regions of the glucocorticoid and estrogen receptors (GR and ER, respectively). However, in contrast to the DBDs of the GR and ER, the RXR DBD contains an additional helix immediately after the second zinc finger. This third helix mediates both protein-protein and protein-DNA interactions required for cooperative, dimeric binding of the RXR DBD to DNA. Identification of the third helix in the RXR DBD thus defines a structural feature required for selective dimerization of the RXR on hormone response elements composed of half-sites (5'-AGGTCA-3') arranged as tandem repeats.

Functional inhibition of retinoic acid response by dominant negative retinoic acid receptor mutants

The diverse effects of retinoids on the development, growth, and homeostasis of vertebrate organisms are mediated in part by three distinct isoforms of retinoic acid receptors (RARs). These proteins, which are structurally and functionally closely related to thyroid hormone receptors and the oncogene product v-ErbA, regulate patterns of gene expression in target tissues. One approach to study the distinct effects of retinoic acid in cells is to subvert this activity of endogenous receptors by expression of dominant negative receptor derivatives. We demonstrate here that RAR alpha, RAR beta, and RAR gamma can be converted into potent negative transcriptional regulators that block wild-type RAR function. Furthermore, these mutant RARs, but not the wild-type receptors, actively repress the basal transcription level of target promoters. When expressed in transgenic mice, the most potent of these inhibitory receptor mutants is apparently able to disturb developmental processes by inducing a cleft palate in transgenic offspring.

The linear echogenic hilus in cervical lymphadenopathy--a sign of benignity or malignancy?

The linear echogenic hilus seen within lymph nodes on ultrasound examination has been proposed as a sign of benignity. The echogenic line is thought to represent the converging sinuses within the medulla of the lymph node. Forty-six cases with a linear echogenic hilus within a cervical lymph node are presented, where a cytological or histological diagnosis was obtained. In 27 cases (58.7%) the diagnosis was malignancy, in seven cases (15.2%) tuberculosis and the remaining 12 cases (26%) were benign. In our experience the linear echogenic hilus should not be regarded as a sole criterion for benignity. A cytological diagnosis, preferably by an ultrasound-guided fine needle aspiration, should always be sought.

Transactivation and synergistic properties of the mineralocorticoid receptor: relationship to the glucocorticoid receptor

The human mineralocorticoid (hMR) and glucocorticoid (hGR) receptors mediate biological responses to adrenal corticosteroids and synthetic ligands. In transient transfection studies, corticosteroid-responsive promoters were used to monitor the hormone-dependent transcriptional regulatory properties of both receptors. The hMR mediates a lower stimulation of the transcription rate than the hGR and does not show cooperative activity on promoters containing multiple palindromic glucocorticoid-responsive elements. The functional importance of the amino-terminus in this differential response was demonstrated by hMR/hGR hybrid receptors in which this region was exchanged or deleted. These experiments revealed that the hMR amino-terminus does not provide the strong transactivation function present in the equivalent hGR domain and, in contrast to the hGR amino-terminus, interferes with the synergistic activity mediated by the DNA- and ligand-binding domains of both receptors.

Normal development and growth of mice carrying a targeted disruption of the alpha 1 retinoic acid receptor gene

Three unlinked genes encode receptors for retinoic acid (RAR alpha, -beta, and -gamma). Each gene expresses two major protein isoforms differing in the amino terminal A domain by alternative promoter use, fused to common exons encoding most of the receptor protein. The two RAR alpha transcripts (RAR alpha 1 and -alpha 2) are differentially expressed and evolutionarily conserved, as are the RAR beta and -gamma transcripts, suggesting that each isoform may have specific functions in the development of animals. To address the biological function of the alpha 1 receptor, we have disrupted the portion of the RAR alpha gene encoding this isoform by homologous recombination in mouse embryonic stem cells. Surprisingly, offspring homozygous for this mutation were viable and showed no apparently altered phenotype. RNA analysis confirmed that the RAR alpha 1 transcript was absent in homozygous tissues, and no evidence for a compensatory increase of RAR alpha 2 or of another RAR gene was obtained to account for the vitality of the mutant animals. These results clearly demonstrate that loss of RAR alpha 1 function does not disrupt embryonic development and argue for combinatorial or overlapping functions among the RAR isoforms.

Pyomyositis and staphylococcal scalded skin syndrome

Pyomyositis is a rare musculoskeletal infection in non-tropical countries. We report a child who had pyomyositis complicated by staphylococcal scalded skin syndrome. This complication has not previously been described in patients with pyomyositis. Early diagnosis of pyomyositis was made by ultrasound examination, and percutaneous needle drainage under ultrasound guidance was performed. Pus aspirated and blood cultures grew Staphylococcus aureus. No open surgical drainage was required and resolution of the abscess was documented by serial ultrasound examinations.

Role of ultrasound in the management of thyroid nodules

One hundred twenty patients undergoing thyroid surgery for thyroid nodules or goiter were examined by preoperative ultrasound and fine needle aspiration (FNA) cytology. In the determination of whether a lesion was malignant, FNA had sensitivity, specificity, and positive predictive values of 86%, 85%, and 58%, respectively. Ultrasound had sensitivity, specificity, and positive predictive values of 74%, 83%, and 51%, respectively. The different types of thyroid pathology showed different ultrasonic features in most cases, although no single feature was pathognomonic. Malignant lesions tended to be solid and hypoechoic without a halo, but there was a cystic element in 26% of the lesions and calcification in 37%. Ultrasound was superior to FNA in diagnosing nodular goiter with sensitivity, specificity, and positive predictive values of 70%, 93%, and 92%, respectively, compared with 55%, 86%, and 83%, respectively. The two modalities are complementary.

Ultrasound, computed tomography and magnetic resonance in the investigation of iliopsoas compartment disease

The authors reviewed the results of imaging studies performed in 30 patients with diseases involving the iliopsoas compartment (IPC) to assess the role of Ultrasound (US), Computed Tomography (CT) and Magnetic Resonance (MR) in the investigation of these deep seated and often clinically undetectable lesions. 16 (53%) patients had histologically proven benign or malignant IPC tumours with bacterial infection in 12 (40%) and haemorrhage into the IPC in 2 (7%). US correctly detected iliopsoas pathology in only 9 of 17 cases (53%) and was satisfactory only in limited situations, such as abscess formation in children and haemorrhage in haemophiliacs. CT was much more accurate (16/17, 94%) than US and correctly diagnosed 7 cases overlooked on US, while also offering greater information in 5 cases where both studies were abnormal. CT was particularly helpful for the detection of both abscess formation and calcification within mass lesion. MR was accurate in all 9 cases imaged, and was very helpful in the assessment of associated vertebral, disc or spinal canal involvement by tumour or infection. A review of the literature, and the results of this study, suggest that CT and MR often have complementary roles where IPC disease is known or suspected. The accuracy of US in IPC disease is limited, preferably being followed by CT or MR imaging.

Drosophila ultraspiracle modulates ecdysone receptor function via heterodimer formation

The vertebrate retinoid X receptor (RXR) has been implicated in the regulation of multiple hormonal signaling pathways through the formation of heteromeric receptor complexes that bind DNA with high affinity. We now demonstrate that ultraspiracle (usp), a Drosophila RXR homolog, can substitute for RXR in stimulating the DNA binding of receptors for retinoic acid, T3, vitamin D, and peroxisome proliferator activators. These observations led to the search and ultimate identification of the ecdysone receptor (EcR) as a Drosophila partner of usp. Together, usp and EcR bind DNA in a highly cooperative fashion. Cotransfection of both EcR and usp expression vectors is required to render cultured mammalian cells ecdysone responsive. These results implicate usp as an integral component of the functional EcR. By demonstrating that receptor heterodimer formation precedes the divergence of vertebrate and invertebrate lineages, these data underscore a central role for RXR and its homolog usp in the evolution and control of the nuclear receptor-based endocrine system.

A functional role for vimentin intermediate filaments in the metabolism of lipoprotein-derived cholesterol in human SW-13 cells

Numerous studies have indicated that cytoplasmic intermediate filaments (cIFs) can associate with cellular lipids. To determine if these interactions might have functional consequences, we have studied the lipid metabolism of human SW-13 adrenal tumor cell lines that either contain vimentin-type cIFs (vim+) or lack any detectable cIF network (vim-). Although there were no significant differences in phospholipid or glyceride synthesis, vim- cell lines had elevated levels of cholesterol synthesis and decreased cholesterol esterification, compared with vim+ cells. These differences in cholesterol synthesis and esterification were found to be due to an impaired ability of vim- cells to utilize low density lipoprotein (LDL)-derived cholesterol, although receptor-mediated endocytosis of LDL and the capacity of these cells to esterify endogenously produced cholesterol were not affected. Expression of a mouse vimentin cDNA in stably transfected cell lines, derived from vim- cells, restored the capacity of these cells to utilize LDL cholesterol. The uptake and metabolism of [3H]cholesterol linoleate-loaded LDL showed that the impaired ability of vim- cells to esterify LDL cholesterol was not associated with an accumulation of cellular free cholesterol but rather an increase in the appearance of [3H]cholesterol in the culture medium. These studies indicate that in SW-13 cells, the intracellular movement of LDL-derived cholesterol from the lysosome to the site of esterification is a vimentin-dependent process.

Modulation of glucocorticoid receptor function by protein kinase A

Protein kinase A (PKA) has been shown to modulate the pattern of gene expression via transcription factors such as cAMP response element binding protein. However, in F9 embryonal carcinoma cells which lack endogenous functional cAMP response element binding protein, we have found that PKA is still able to control gene transcription through the glucocorticoid receptor (GR) by up-regulating its hormone-dependent trans-activation. Dose-response analysis indicates that PKA does not alter the ligand binding affinity of GR. PKA seems to act through the DNA binding domain of GR, since GR mutants which lack either the amino-terminal or the ligand binding domain are still able to be up-regulated by PKA. In support of this proposal, we demonstrate that PKA can enhance the DNA binding activity of GR. Our results suggest a novel mechanism by which PKA modulates the steroid sensitivity of a target cell by enhancing the DNA binding activity of GR for its cognate hormone response elements.

Convergence of 9-cis retinoic acid and peroxisome proliferator signalling pathways through heterodimer formation of their receptors

Peroxisomes are cytoplasmic organelles which are important in mammals in modulation of lipid homeostasis, including the metabolism of long-chain fatty acids and conversion of cholesterol to bile salts (reviewed in refs 1 and 2). Amphipathic carboxylates such as clofibric acid have been used in man as hypolipidaemic agents and in rodents they stimulate the proliferation of peroxisomes. These agents, termed peroxisome proliferators, and all-trans retinoic acid activate genes involved in peroxisomal-mediated beta-oxidation of fatty acids. Here we show that the receptor activated by peroxisome proliferators and the retinoid X receptor-alpha (ref. 6) form a heterodimer that activates acyl-CoA oxidase gene expression in response to either clofibric acid or the retinoid X receptor-alpha ligand, 9-cis retinoic acid, an all-trans retinoic acid metabolite; simultaneous exposure to both activators results in a synergistic induction of gene expression. These data demonstrate the coupling of the peroxisome proliferator and retinoid signalling pathways and provide evidence for a physiological role for 9-cis retinoic acid in modulating lipid metabolism.

Retinoic acid receptors initiate induction of the cytomegalovirus enhancer in embryonal cells

Reactivation of latent virus is believed to result from a signal transduction event that induces immediate-early (IE) gene transcription. Evidence is presented that the major IE promoter (MIEP) of human cytomegalovirus (hCMV) is activated by physiological levels of retinoic acid (RA) in human embryonal carcinoma cells. Mutagenesis experiments localized in the MIEP enhancer, a retinoic acid-responsive element composed of a direct repeat separated by five nucleotides. Protein-DNA binding experiments revealed that this element functions as a specific target site for the direct interaction of nuclear receptor proteins for RA. These findings implicate the biologically active derivative of vitamin A (RA) as a potential modulator of hCMV pathogenesis in infants and immunocompromised adults.