The erb-A family receptors for thyroid hormones, steroids, vitamin D and retinoic acid: characteristics and modulation

The stressful condition as a nutritionally dependent adaptive dichotomy

The injured body manifests a cascade of cytokine-induced metabolic events aimed at developing defense mechanisms and tissue repair. Rising concentrations of counterregulatory hormones work in concert with cytokines to generate overall insulin and insulin-like growth factor 1 (IGF-1), postreceptor resistance and energy requirements grounded on lipid dependency. Salient features are self-sustained hypercortisolemia persisting as long as cytokines are oversecreted and down-regulation of the hypothalamo-pituitary-thyroid axis stabilized at low basal levels. Inhibition of thyroxine 5'-deiodinating activity (5'-DA) accounts for the depressed T3 values associated with the sparing of both N and energy-consuming processes. Both the liver and damaged territories adapt to stressful signals along up-regulated pathways disconnected from the central and peripheral control systems. Cytokines stimulate liver 5'-DA and suppress the synthesis of transthyretin (TTR), causing the drop of retinol-binding protein (RBP) and the leakage of increased amounts of T4 and retinol in free form. TTR and RBP thus work as prohormonal reservoirs of precursor molecules which need to be converted into bioactive derivatives (T3 and retinoic acids) to reach transcriptional efficiency. The converting steps (5'-DA and cellular retinol-binding protein-I) are activated by T4 and retinol, themselves operating as limiting factors of positive feedback loops. Healthy adults with normal macrophage functioning and liver parenchymal integrity, who submitted to a stress of medium severity, are characterized by TTR-RBP plasma levels reduced by half and an estimated ten-fold increase in free ligand disposal to target cells during the days ensuing injury. This transient hyperthyroid and hyperretinoid climate creates a second defense line strengthening and fine-tuning the effects primarily initiated by cytokines. The suicidal behavior of thyroxine-binding globulin (TBG), corticosteroid-binding globulin (CBG), and IGFBP-3 allows the occurrence of peak endocrine and mitogenic influences at the site of inflammation. The production rate of TTR by the liver is the main determinant of both the hepatic release and blood transport of holoRBP, which explains why poor nutritional status concomitantly impairs thyroid- and retinoid-dependent acute-phase responses, hindering the stressed body to appropriately face the survival crisis. The prognostic significance of low TT4 blood levels may be assigned to the exhaustion of extrathyroidal hormonal pools normally stored in liver and plasma but markedly shrunken in protein-depleted states. These data offer new insights into the mechanisms whereby preexisting malnutrition and stressful complications are interrelated, emphasizing the pivotal role played by TTR in that context.

Fatty acids involved in signal cross-talk between cell membrane and nucleus

The results presented underline the fact that the nature and the concentration of the non-esterified fatty acids (NEFAs) liberated from membrane lipids, particularly the essential ones issued from lipid nutrition, clearly belong to a large group of factors (hormones, retinoids, growth factors, cytokines...) which control the shift between cell multiplication and differentiation. NEFAs act on this shift, per se or after being metabolized, by influencing, as second messengers or modulators, the intertwined mechanisms of action of growth factors and steroid hormones. These results may explain the molecular links which exist between endocrinology, oncology and nutrition.

Transcriptional regulators of expression of K#16, the disease-associated keratin

In most malignant and benign skin diseases, the normal pattern of keratin expression is altered. Among other phenotypic changes, the expression of hyperproliferation- and activation-associated keratins K#16 and K#6 is induced. Because the molecular mechanisms and the nuclear regulators involved in this induction are unknown, we have characterized the transcriptional regulators of expression of the keratin K#16 promoter. Our previous studies have shown that the transcription of K#16 is strongly and specifically induced in epidermal keratinocytes by epidermal growth factor (EGF), through the EGF-responsive element (RE). In the present work, using an electrophoretic mobility-shift assay, we have found several nuclear protein binding sites that have been identified as an Sp1 site, an AP2 site, the EGF-RE, and an enhancer element. The function of each site was assessed in transfection assays using specific deletions. Both the Sp1 and EGF-RE sites are essential for K#16 promoter activity. The site that functions as an independent enhancer, E, was found adjacent to and interacting with a sequence recognized by the AP2 transcription factor. This knowledge of the nuclear regulators of expression of the disease-associated K#16 keratin provides insight into the molecular parameters that might be important in skin diseases.

Neuronal cell cultures: a tool for investigations in developmental neurobiology

The aim of this review is to describe environmental requirements for survival of neuronal cells in culture, and secondly to survey the complex interplay between hormones, neurotrophic factors, transport- and extracellular matrix- proteins, which characterize the developmental program of differentiating neurons. An overall reconsideration of the literature in this vast field is above the limits of the present paper; since progress and refinement in the techniques of neuronal cell cultures have paralleled the advancement in Developmental Neurobiology, we will run instead through the main steps which form the conceptual framework of neuronal cell cultures.

Identification of the retinoic acid and thyroid hormone receptor-responsive element in the human K14 keratin gene

The promoter of human K14 keratin gene, specific for the basal layer of stratified epithelia, is regulated by nuclear receptors for retinoic acid and thyroid hormone. However, the DNA sequences responsible for this regulation have not yet been identified. To identify the retinoic acid-responsive site, we have devised a simple site-specific mutagenesis method and introduced mutations into the K14 keratin gene promoter. These mutations identify the retinoic acid-responsive site. The site consists of a cluster of consensus palindrome half-sites in various orientations. As shown previously, retinoic acid and thyroid hormone receptors can recognize and bind common sequences in regulated genes. Here, we describe mutations that abolish regulation by both receptors. Interestingly, the hormone-dependent and -independent regulatory sites of the thyroid hormone nuclear receptor can be separated. Clusters of half-sites that share structural organization with the K14 regulatory site were found in the K5 and K10 keratin gene promoters. Similar clusters may be responsible for retinoic acid-mediated transcription regulation in epidermis.

Regulation of epidermal keratin expression by retinoic acid and thyroid hormone

In the epidermis, retinoic acid (RA) is known to regulate the gene expression of keratins, the intermediate filament proteins of epithelial cells. We have cloned the upstream regulatory regions of three human epidermal keratin genes, K5, K10, and K14, and engineered DNA constructs in which these regions drive expression of the CAT reporter gene. By co-transfecting the constructs into various epithelial cell types along with the vectors expressing the nuclear receptors for RA and thyroid hormone (T3), we have shown that RA and T3 directly regulate expression of these three keratin genes through the action of their nuclear receptors. In this paper, we review our previous results to stress that RA has a dual effect on keratin expression in epidermis: both direct and indirect. We also analyze the DNA sequences upstream from those three RA-regulated keratin genes and identify the clusters of degenerate consensus half-site motifs, which may comprise the putative retinoic acid recognition elements (RAREs). Furthermore, our recent results concerning the regulation of K5 and K14 expression by the RA receptor are also shown; these confirm our predictions regarding the location of the RAREs in epidermal keratin genes.

The dynamic properties of neuronal chromatin are modulated by triiodothyronine

The effect of triiodothyronine (T3) on the rate of synthesis of nuclear proteins was studied during terminal differentiation of rat cortical neurons cultured in a serum-free medium. To this aim total and acid soluble nuclear proteins were analyzed by different electrophoretic techniques. Our results show that: 1) during maturation in vitro, neuronal nuclei undergo a dramatic change in the rate at which different classes of histones and high mobility group (HMG) proteins are synthesized; the synthetic activity, measured as incorporation of radioactive precursors into nuclear proteins, slows indeed down with age: especially evident is the decrease in core histones synthesis; at day 15, on the other hand, HMG 14 and 17 and ubiquitinated H2A (A24) are synthesized at a high rate, especially in T3-treated neurons; 2) neurons treated with T3 show, at any age tested, a higher level of lysine incorporation into nuclear proteins; 3) even if during the first days of culture neurons synthesize core histones more actively in the presence of T3, there is no accumulation of these proteins at later stages, as compared with untreated cells. Possible implications of these data and relationship with the chromatin rearrangement which accompanies neuronal terminal differentiation are discussed.

An unusual expression of a squamous cell marker, small proline-rich protein gene, in tracheobronchial epithelium: differential regulation and gene mapping

An unusual expression of a putative squamous cell marker, small proline-rich protein (spr1), in mucociliary epithelial cells of conducting airways was demonstrated in a serum-free culture system. A cDNA clone was isolated from the cDNA library of monkey tracheobronchial epithelial (TBE) cells by differential hybridization. This cDNA clone, MT5, exhibited 98% homology to a DNA sequence obtained from human keratinocytes treated with either UV light or phorbol esters (T. Kartasova et al., 1988, Mol. Cell. Biol. 8:2195-2230). The predicted peptide of MT5 is unusual for its high content of proline (29%), glutamine (18%), and cysteine (9%) and its repeated PKVPEPC units. The level of spr1 mRNA in cultured cells was inhibited more than 90% by vitamin A. In contrast, phorbol 12-myristate 13-acetate (PMA) stimulated the level of spr1 mRNA by 3- to 8-fold. This differential regulation coincided with the effects of these chemicals on the cornification of cultured TBE cells. Using MT5 as a probe, we have localized the tracheal spr1 gene on the human chromosome 1 by a Southern blot analysis using a panel of human-rodent somatic cell hybrid DNAs. The gene was further sublocalized to bands q22-23 by in situ hybridization.

Accumulation of different c-erbA transcripts during rat brain development and in cortical neurons cultured in a synthetic medium

1. Accumulation of different c-erbA transcripts was studied, during rat brain maturation and in cortical neurons differentiating in a serum-free medium, by quantitative Northern blot hybridization. 2. The alpha and beta forms of c-erbA mRNAs exhibit different patterns of accumulation, with a precocious increase in the alpha forms compared with the beta forms both in vivo and in culture. 3. erbA alpha 2 mRNA (2.6 kb) is by far the predominant form, with a maximum at birth (PO). 4. The accumulation patterns of both alpha and beta forms show discrete differences in isolated neurons compared to brain cortices; in particular the pattern of alpha 2 mRNA accumulation in culture suggests its predominant localization to neurons. 5. The presence of T3 in the culture medium does not have significant effects on the level of any of erbA mRNAs. 6. Possible implications and relationships with neuronal terminal differentiation are discussed.

Regulation of keratin gene expression: the role of the nuclear receptors for retinoic acid, thyroid hormone, and vitamin D3

Keratinization, the orderly process of differentiation of epidermal keratinocytes from stratum basale to stratum corneum, is influenced by hormones and vitamins. We have used expression of epidermal keratins as a paradigm of keratinization processes and analyzed the effects of retinoic acid, thyroid hormone, and vitamin D3 on keratin gene expression. DNA constructs in which keratin gene promoters drive expression of reporter genes were co-transfected with vectors expressing nuclear receptors for the above molecules into various cell types. The keratin promoters studied included K3, K5, K10, K14, and K16. The recipient cell types were HeLa and primary cultures of rabbit corneal and esophageal epithelial cells and of human epidermal keratinocytes. We found that retinoic acid, via its nuclear receptor, suppresses expression of all the above-listed keratin genes. Thyroid hormone and its receptor similarly suppressed those genes. The site of interaction between these two receptors and the promoter sequences of K10 and K14 genes has been identified. Surprisingly, vitamin D3 and its receptor had no direct effect on keratin promoters. Our results suggest that a retinoic acid has a twofold effect on keratin gene expression: by regulating keratinocyte differentiation it determines which keratins are expressed, basal cell specific or differentiation specific; by direct interaction between its receptor and keratin genes, retinoic acid determines the total amount of keratin protein within the cell. Vitamin D3, on the other hand, also regulates keratinocyte differentiation, but does not directly interact with the keratin genes.

Nuclear receptors for retinoic acid and thyroid hormone regulate transcription of keratin genes

In the epidermis, retinoids regulate the expression of keratins, the intermediate filament proteins of epithelial cells. We have cloned the 5' regulatory regions of four human epidermal keratin genes, K#5, K#6, K#10, and K#14, and engineered constructs in which these regions drive the expression of the CAT reporter gene. By co-transfecting the constructs into epithelial cells along with the vectors expressing nuclear receptors for retinoic acid (RA) and thyroid hormone, we have demonstrated that the receptors can suppress the promoters of keratin genes. The suppression is ligand dependent; it is evident both in established cell lines and in primary cultures of epithelial cells. The three RA receptors have similar effects on keratin gene transcription. Our data indicate that the nuclear receptors for RA and thyroid hormone regulate keratin synthesis by binding to negative recognition elements in the upstream DNA sequences of the keratin genes. RA thus has a twofold effect on epidermal keratin expression: qualitatively, it regulates the regulators that effect the switch from basal cell-specific keratins to differentiation-specific ones; and quantitatively, it determines the level of keratin synthesis within the cell by direct interaction of its receptors with the keratin gene promoters.