The long terminal repeat of VL30 retrotransposons contains sequences that determine retinoic acid-induced transcription in cultured keratinocytes

Retinoids as an Immunity-modulator in Dermatology Disorders

The skin is the largest epithelial surface protecting the body from invading microbes. Vitamin A plays vital roles in the host defence of the skin, including promoting epithelial cell integrity, proliferation, and differentiation and even mediating immune responses. Furthermore, vitamin A derivatives, retinoid drugs, are widely used to treat skin diseases, such as acne and psoriasis. However, the immunoregulatory mechanisms of retinoids in dermatology have not been systematically described. In this paper, we discuss the immunological functions of retinoids during disease treatment, especially in skin disorders caused by exogenous infections.

Reprogramming of the HepG2 genome by long interspersed nuclear element-1

Long Interspersed Nuclear Element-1 (LINE-1 or L1) is an autonomous, mobile element within the human genome that transposes via a "copy and paste" mechanism and relies upon L1-encoded endonuclease and reverse transcriptase (RT) activities to compromise genome integrity. L1 has been implicated in various forms of cancer, but its role in the regulation of the oncogenic phenotype is not understood. The present studies were conducted to evaluate mechanisms of genetic regulatory control in HepG2 cells by human L1, or a D702Y mutant deficient in RT activity, and their influence on cellular phenotype. Forced expression of synthetic L1 ORF1p and ORF2p was associated with formation of cytoplasmic foci and minor association with the nuclear compartment. While de novo L1 mobilizations were only identified in cells expressing wild type L1, and were absent in the D702Y mutant, changes in gene expression profiles involved RT dependent as well as RT independent mechanisms. Synthetic L1 altered the expression of 24 in silico predicted genetic targets; ten of which showed RT-dependence, ten RT-independence, and four reciprocal regulatory control by both wild type and RT mutant. Of five targets examined, only VCAM1 and PTPRB colocalized with newly retrotransposed wild type L1. Biological discretization to partition patterns of gene expression into unique frequencies identified adhesion, inflammation, and cellular metabolism as key processes targeted for molecular interference with disruption of epithelial-to-mesenchymal programming seen irrespective of the RT phenotype. These findings establish L1 as a key regulator of genome plasticity and EMT via mechanisms independent of RT activity.

Trim24-repressed VL30 retrotransposons regulate gene expression by producing noncoding RNA

Trim24 (Tif1α) and Trim33 (Tif1γ) interact to form a co-repressor complex that suppresses murine hepatocellular carcinoma. Here we show that Trim24 and Trim33 cooperatively repress retinoic acid receptor-dependent activity of VL30-class endogenous retroviruses (ERVs) in liver. In Trim24-knockout hepatocytes, VL30 derepression leads to accumulation of reverse-transcribed VL30 cDNA in the cytoplasm that correlates with activation of the viral-defense interferon responses mimicking the preneoplastic inflammatory state seen in human liver following exogenous viral infection. Furthermore, upon derepression, VL30 long terminal repeats (LTRs) act as promoter and enhancer elements deregulating expression of neighboring genes and generating enhancer RNAs that are required for LTR enhancer activity in hepatocytes in vivo. These data reinforce the role of the TRIM family of proteins in retroviral restriction and antiviral defense and provide an example of an ERV-derived oncogenic regulatory network.

A role for endogenous reverse transcriptase in tumorigenesis and as a target in differentiating cancer therapy

An unexpected result emerging from completion of the genome sequencing project is that a large portion of mammalian genomes is constituted by retrotransposons. A large body of published data supports the conclusion that retrotransposons are biologically active elements and indicates that retrotransposition is an ongoing process in mammalian genomes. Retroelements can act as insertional mutagens altering the coding integrity of genes and, recently, have been found to also affect the expression of cellular genes at the epigenetic level: in this light, they are a potential threat in that these events can trigger the onset of several pathologies including cancer. Retroelement genes, and particularly the gene coding for reverse transcriptase (RT), are typically expressed at high levels in transformed cells and tumors. In recent work, we have found that drug-mediated inhibition of the endogenous RT activity, or silencing of expression of active retrotransposons of the LINE-1 family by RNA interference, down-regulate cell growth and induce the activation of differentiating functions in several cancer cell lines. Moreover, the inhibition of endogenous RT activity in vivo antagonizes the growth of human tumors in animal models. In this review, we discuss newly emerging concepts on the role of retrotransposons and suggest that an abnormally high level of the RT activity that they encode may contribute to the loss of control in the proliferation and differentiation programs typical of transformed cells. In this light, RT-coding elements may be regarded as promising targets in the development of novel, differentiation-inducing approaches to cancer therapy.

Exposure of normal and transformed cells to nevirapine, a reverse transcriptase inhibitor, reduces cell growth and promotes differentiation

Endogenous, nontelomeric reverse transcriptase (RT) is encoded by two classes of repeated elements: retrotransposons and endogenous retroviruses. Expression of RT-coding genes is generally repressed in differentiated nonpathological tissues, yet is active in the mammalian germ line, embryonic tissues and tumor cells. Nevirapine is a non-nucleoside RT inhibitor with a well-characterized inhibitory activity on RT enzymes of retroviral origin. Here, we show that nevirapine is also an effective inhibitor of the endogenous RT in murine and human cell lines. In addition, progenitor and transformed cells undergo a significant reduction in the rate of cell growth upon exposure to nevirapine. This is accompanied by the onset of differentiation, as depicted in F9 and C2C7 progenitor cells cultures in which nevirapine triggers the expression of differentiation-specific markers. Consistent with this, an extensive reprogramming of cell cycle gene expression was depicted in nevirapine-treated F9 cultures. Furthermore, nevirapine exposure rescued the differentiation block present in acute myeloid leukemia (AML) cell lines and primary blasts from two AML patients, as indicated by morphological, functional and immunophenotypic assays. The finding that an RT inhibitor can modulate cell proliferation and differentiation suggests that RT may represent a novel target in the development of therapeutical approaches to neoplasia.

Conserved, erythropoietin-responsive VL30 promoters isolated from erythroid progenitor cells

Virus-like 30S (VL30) elements are endogenous retro-elements of the mouse retrotransposon family. These elements are transcriptionally responsive in a temporal and tissue-specific manner due to the U3 promoter region of the elements' long terminal repeat (LTR). We have analyzed VL30 promoters from erythroid progenitor cell lines (MEL 585S and ELM-I-1) that contrasted in their response to erythropoietin (epo). Through RT-PCR-generated cDNAs, VL30 promoters were identified and showed homology to the third and fourth U3 subgroups, with GATA-1, Jak2/STAT5, and B10 RRE sites. One clone (ELM5) showed 97% homology to BVL-1, a putative epo-responsive VL30 element. In addition, a novel U3 promoter (MEL/ELM CONSTIT) showed complete sequence homology between both cell lines. Ribonuclease protection confirmed that epo-induced VL30 promoters were activated in ELM-I-1 cells, whereas the conserved VL30 MEL-ELM CONSTIT VL30 promoter showed no enhanced expression in the epo-unresponsive MEL cells. Identification of these U3 promoters suggests that VL30s are conserved and can be transcriptionally activated in an epo-specific manner.

Retinoids induce apoptosis in cultured keratinocytes

BACKGROUND

Apoptosis is a genetically controlled process linked to growth and differentiation, involving specific molecular and cellular events activated as a result of a variety of internal and external stimuli.

OBJECTIVES

To examine the ability of physiological and synthetic retinoids to induce apoptosis in the BALB/MK mouse keratinocyte cell line.

METHODS

Cell viability was assessed by flow cytometry, various staining techniques and the TUNEL method.

RESULTS

When keratinocytes were simultaneously exposed to retinoids and stimulated to differentiate at a high (1.5 mmol L(-1))extracellular Ca(2+) concentration over 48 h, apoptosis was induced. Of the retinoids tested, 3,4-didehydroretinoic acid and 3-methyl-tetrahydro-tetramethyl-naphthylenyl-propenyl benzoic acid were more potent than the others. In this system, the apoptosis induced by retinoids could not be correlated to the expression of tissue transglutaminase or epidermal transglutaminase. Furthermore, expression of antiapoptotic bcl-2 or proapoptotic Bax did not change significantly under the experimental conditions used, indicating that the regulation of apoptosis is complex and may be influenced by different factors.

CONCLUSIONS

The results suggest that retinoids activating either retinoic acid receptors or retinoid X receptors can induce apoptosis in cultured keratinocytes. Moreover, the well-established inhibitory effect of retinoids on keratinocyte differentiation implies that the apoptotic programme represents a distinct biological process.

Sequence analysis of Mus dunni endogenous virus reveals a hybrid VL30/gibbon ape leukemia virus-like structure and a distinct envelope

Mus dunni endogenous virus (MDEV) can be activated from M. dunni cells by exposing the cells to hydrocortisone or 5-iodo-2'-deoxyuridine. Interference analysis has revealed that MDEV uses a receptor for cell entry that is different from those used by other murine retroviruses. The entire genome has now been sequenced, revealing a long terminal repeat (LTR)-gag-pol-env-LTR structure typical of simple retroviruses of the murine leukemia virus genus, with no additional open reading frames between env and the 3' LTR. The LTRs and other noncoding regions of MDEV are most closely related to those of VL30 elements, while the majority of the coding sequences are most closely related to those of gibbon ape leukemia virus. MDEV represents the first example of a naturally occurring, replication-competent virus with sequences closely related to VL30 elements. The U3 region of MDEV contains six nearly perfect 80-bp repeats and the beginning of a seventh, and the region expected to contain the packaging sequence contains approximately four imperfect 33-bp repeats. The receptor specificity domains of the envelope are unique among retroviruses and show no apparent similarity to regions of known proteins.

Early genes induced in hepatic stellate cells during wound healing

Activation of mesenchymal cells is a central event in the wound healing response of most tissues. In liver, the mesenchymal element responsible for organ fibrosis is the hepatic stellate cell (HSC) (formerly known as lipocyte or Ito cell). The phenotypic cascade of stellate cell activation in liver fibrosis has been well documented and involves both marked morphologic changes and upregulation of several functional components including extracellular matrix, cytokine receptors, contractile filaments and metalloproteinases. However, the genetic regulation of stellate cell activation is poorly understood. In an attempt to clone genes that are involved in the regulation of HSC activation we have combined cDNA library amplification by PCR with subtraction hybridization/differential screening, and have successfully identified genes induced in vivo during early stellate cell activation in a rat model of liver fibrosis. The subtracted cDNA library comprised less than 100 unique sequences. Of these, 13 clones with sizes ranging from 322 to 745 were sequenced and characterized. Gene induction in HSCs was monitored by RNAse protection assay during early liver injury induced by the hepatotoxin CCl4. The sequenced cDNAs corresponding to the known genes included type II transforming growth factor beta receptor, glutathione peroxidase I, transferrin and several clones encoding cellular retrotransposons, whose expression was not previously identified in non-parenchymal liver cells. In addition, one partial cDNA predicted a zinc-finger motif, suggesting a possible role of a novel transcriptional regulator. Our approach represents a valuable strategy for clarifying in vivo regulatory mechanisms of mesenchymal cell activation in wound healing.