Characterization of the PML-RAR alpha chimeric product of the acute promyelocytic leukemia-specific t(15;17) translocation

The acute promyelocytic leukemia 15;17 chromosomal translocation fuses the PML gene to the RAR alpha locus. The resulting chimeric gene encodes for a putative PML-RAR alpha fusion protein. PML is a putative transcriptional factor and RAR alpha is one of the nuclear retinoic acid receptors through which retinoic acid regulates gene expression. In this study, we investigated the retinoid binding and biochemical properties of the PML-RAR alpha protein by size exclusion high-performance liquid chromatography and immunoblot analysis and compared them with those of normal RAR alpha. The introduction of the expression vector PSG5/PML-RAR alpha into COS-1 cells led to high levels of expression of the PML-RAR alpha fusion protein. This protein was primarily localized in the nucleus and bound retinoids with the same affinity and specificity as the wild type RAR alpha receptor. The PML-RAR alpha fusion protein, but not the RAR alpha, was found in high molecular weight complexes with either itself or other nuclear factors. In the acute promyelocytic leukemia-derived cell line NB4, which contains the t(15;17) chromosomal marker, the PML-RAR alpha product was also found as a high molecular complex. The interaction of the PML-RAR alpha with itself or with other nuclear proteins may be important in understanding the role of the PML-RAR alpha fusion protein in promyelocytic leukemogenesis.

Heterodimeric receptor complexes determine 3,5,3'-triiodothyronine and retinoid signaling specificities

Thyroid hormone receptors (TRs) and retinoic acid receptors (RARs) have been shown to interact with nuclear auxiliary proteins resulting in heteromeric complexes that bind strongly to their responsive elements. Recently the retinoid X receptors (RXRs) have been identified as one class of these nuclear proteins. RXRs strongly increase binding of TRs and RARs to a synthetic thyroid hormone (and retinoic acid) responsive element. Here results show that the binding of the heteromeric complexes to various natural response elements is highly specific and dictated by the partner of RXR in the complex. TR alpha and TR beta formed complexes with RXR alpha that strongly and selectively bound to natural thyroid hormone responsive elements, i.e. those from the rat alpha-myosin heavy chain gene and the rat malic enzyme gene. RXR alpha complexes with RAR alpha, RAR beta, and RAR gamma bound selectively to retinoic acid responsive elements from the human RAR beta 2 gene (hRAR beta 2), the gene of the rat cellular retinol binding protein I and the human apolipoprotein A1 gene. Under the conditions used here RXR alpha by itself did not bind to any of the responsive elements tested. Although TRs and RARs formed heterodimers with RXR in solution, these complexes were strongly stabilized by specific, high affinity response elements, but not by low affinity response elements. Transfection analyses showed strong synergism between receptors that formed effective heterodimers in transcriptional activation on several but not all response elements. Overall, these data demonstrate that RARs and TRs are unlikely to function as monomers or homodimers on the response elements investigated here and require RXRs or comparable proteins for effective response element activation.

Binding characteristics of the thyroid hormone receptor homo- and heterodimers to consensus AGGTCA repeat motifs

Previous studies have shown that thyroid hormone receptors can form homo- and heterodimeric complexes when binding to response elements. We report here the binding characteristics of thyroid hormone receptor (TR) homo- and heterodimers binding to synthetic oligonucleotides with directly and palindromically repeated consensus motifs (AGGTCA). Binding assays showed that TR homodimer formation on DNA had a low specificity and cooperativity, and very fast off rates. In contrast, TRs and retinoic acid receptors readily formed heterodimers with higher specificity and affinity on direct repeats of the AGGTCA motif spaced by four or five nucleotides, although these heterodimer/DNA complexes were only moderately stable when compared to DNA-bound TR/retinoid X receptor heterodimers. Also, TR/retinoic acid receptor heteromeric binding to other elements, including the synthetic T3RE-pal element, was of low specificity. These biochemical results suggest that TRs are unlikely to regulate transcription as homodimers in vivo, and that TR heterodimers mediate the effects of thyroid hormone.

Retinoid receptors in vertebrate limb development

Although the precise role of retinoids in limb development remains obscure, the finding that retinoic acid can produce major alterations in limb patterning suggests that this ligand might be involved in the process of limb morphogenesis. Here we describe the patterns of expression of retinoic acid receptors and cytosolic retinoid binding proteins during the course of limb morphogenesis. Examining the distribution of these molecules in the limb and correlating their presence with important processes in limb development could help elucidate their possible functions.

RAR gamma 2 expression is regulated through a retinoic acid response element embedded in Sp1 sites

At the level of transcription, all signals of the vitamin A derivative retinoic acid (RA) are mediated by the RA receptors (RARs) as well as the retinoid X receptors (RXRs). The control of expression of the various receptor subtypes and their specific isoforms appears to be strictly regulated and can be assumed to play a pivotal role during development and in the adult tissue. It has previously been shown that the RAR beta 2 isoform can regulate its own synthesis through an RA response element (RARE) in its promoter. Recent evidence suggests that the expression of other RAR isoforms, including that of RAR gamma 2, are also regulated by RA. We present evidence that expression of the RAR gamma 2 isoform can be regulated through the RARE in its own promoter region. Similar to the beta 2 RARE, the gamma 2 RARE consists of a 6-bp direct repeat with a 5-nucleotide spacer, but it has different functional features, including receptor specificity, basal-level activity, and affinity for RAR. In agreement with recent observations, this response element is bound most effectively by RAR/RXR heterodimers. Single-base-pair mutations had different effects on the activity of this RARE. The gamma 2 RARE is surrounded by several binding sites for the transcription factor Sp1. Cotransfected Sp1 enhanced strongly the activity of gamma 2 promoter reporter constructs in Drosophila cells. Our data suggest an important role for RAR-containing heterodimers and Sp1 in the regulation of RAR gamma 2 expression.

Characterization of a retinoic acid responsive element isolated by whole genome PCR

We have used whole PCR in an attempt to isolate novel retinoic acid (RA) responsive genes. We cloned several small genomic fragments from total human DNA containing putative retinoic acid responsive elements (RAREs) selected by direct binding to the retinoic acid receptor alpha (RAR alpha). We report here that an oligonucleotide containing a sequence from one of the cloned human DNA fragments, and referred to as alpha 1, functions as an authentic RARE. It is shown that both RAR alpha and RAR beta produced in Cos cells as well as in vitro translated RAR alpha bind directly and sequence-specifically to the alpha 1RARE. By mutational analysis it is demonstrated that the alpha 1RARE consists of an imperfect direct repeat of the estrogen- and thyroid hormone-related AGGTCA half-site motif separated by a 5 bp spacer. The orientation and spacing of the half-site repeats are shown to play a critical role in RAR recognition. When cloned upstream of a TK-Luc reporter, the alpha 1RARE is shown to confer responsiveness to RA in an orientation-independent fashion in F9 and CV-1 cells. The magnitude of the RA response mediated by the alpha 1RARE differed in these cell lines.

Differential regulation of collagenase gene expression by retinoic acid receptors--alpha, beta and gamma

The mechanisms involved in retinoic acid (RA)-mediated regulation of the collagenase gene in a rabbit synovial fibroblast cell line (HIG82) were investigated. When HIG82 cells are cotransfected with expression vectors containing cDNAs for retinoic acid receptor (RAR) alpha 1, beta 2, or gamma 1 and collagenase promoter-driven CAT reporter constructs, only RAR-gamma 1 represses basal CAT expression upon RA treatment, while RAR-alpha 1, beta 2, and gamma 1 all suppress phorbol-induced CAT expression. Thus, transcriptional regulation of collagenase by RA is mediated by RARs in an RAR-type specific manner. Using mutational and deletional analysis, we find that interaction between elements within 182 bp collagenase promoter plays an important role in this process. In addition, cotreatment with RA results in a decrease of phorbol-induced mRNA levels of fos and jun, and binding of nuclear proteins to an AP-1 oligonucleotide. Furthermore, RA-induced nuclear protein(s) specifically bind to a 22 bp sequence (-182 to -161) of the collagenase promoter. We propose that RA-mediated regulation of the collagenase gene depends on the availability and interaction of specific RARs with multiple DNA elements within the promoter and with transcription factors, including AP-1 related proteins.

A PML/retinoic acid receptor alpha fusion transcript is constantly detected by RNA-based polymerase chain reaction in acute promyelocytic leukemia

The t(15;17) translocation is specifically observed in patients with promyelocytic leukemia (AML3). The chromosomal rearrangement juxtaposes the retinoic acid receptor alpha (RAR alpha) and PML genes, resulting in PML/RAR alpha fusion transcripts. Our previous studies have shown that a polymerase chain reaction (PCR) amplification product could be obtained from the cDNA of the NB4 promyelocytic cell line from which the chimaeric PML/RAR alpha was cloned. We report here that in all 14 AML3 patients tested, reverse transcriptase-PCR (RT-PCR) allows the detection of three specific fusion products. In eight patients, one amplification product was detected corresponding to the previously described abnormal fusion. Five patients displayed a different amplified fragment corresponding to a different fusion point. One other patient always showed a third different-sized product. The different types of fusion transcripts amplified were correlated to the size of the abnormal RAR alpha transcripts detected in these patients by Northern analysis, but did not prove determinant for either the phenotypic features or the retinoic acid responsiveness in AML3 cells in this group of patients. The consistent identification by RT-PCR of the fusion of the PML and RAR alpha genes in AML3 patients suggest that this method will provide a useful tool for the diagnosis and detection of minimal residual disease in these patients.

Induction of retinoic acid-binding protein in normal and malignant human myeloid cells by retinoic acid in acute promyelocytic leukemia patients

Retinoic acid has striking effects on development and cell differentiation. Its biological effect is a highly regulated process that is controlled by specific proteins. In the nucleus, different retinoic acid receptors have been identified and their genes cloned. In the cytosol, retinoid binding proteins, cellular retinoic acid-binding protein and cellular retinol-binding protein, have been correlated with normal and malignant tissue differentiation. Recently, differentiation therapy of acute promyelocytic leukemias (AML3 subtype) with all-trans-retinoic acid has been shown to be an efficient alternative to chemotherapy. The retinoic acid receptor alpha gene has been shown to be specifically rearranged in AML3 through the t(15;17) translocation. The molecular basis of the effect to reverse the leukemic phenotype of all-trans-retinoic acid is not yet elucidated. To further study retinoic acid efficacy in AML3 leukemia, retinoic acid-binding proteins were studied in the cytosol extracts of hematopoietic cells. No retinoic acid binding activity was detected in normal or malignant hematopoietic cells whether sensitive or not to retinoic acid. However, detectable binding to a cytosolic protein corresponding to cellular retinoic acid-binding protein (M(r) 15,000, Kd 3 nM) was observed in the bone marrow cells of AML3 patients undergoing all-trans-retinoic acid therapy. We suggest that both the induction and subsequent presence of cellular retinoic acid-binding protein may influence the therapeutic efficacy of retinoic acid and must be taken into account when studying its effect in acute promyelocytic patients.

Identification of DNA rearrangements at the retinoic acid receptor-alpha (RAR-alpha) locus in all patients with acute promyelocytic leukemia (APL) and mapping of APL breakpoints within the RAR-alpha second intron. Italian Cooperative Study Group "GIMEMA"

Seventy patients with acute promyelocytic leukemia (APL) were characterized at the DNA level using genomic retinoic acid receptor-alpha (RAR-alpha) probes on Southern blot experiments. Sixty-two cases were defined as M3 according to the French-American-British (FAB) criteria, and eight had a diagnosis of microgranular or variant (M3v) APL. The use of two restriction enzymes and three probes exploring the second intron of the RAR-alpha gene allowed us to detect specific abnormal DNA fragments in every case, with clustering of rearrangements within the 20-kb intronic region between RAR-alpha exons II and III. A more detailed mapping of APL breakpoints was performed in 52 cases in which three EcoRI subregions of the RAR-alpha second intron were analyzed with corresponding probes. Comparison of clinical and hematological features in the three subgroups of patients with distinct RAR-alpha breakpoints did not show significant differences regarding age, peripheral blood (PB) counts, presence of coagulopathy, or FAB classification (M3 v M3v). Interestingly, a significant difference was observed in the M/F ratio of the three subgroups, with a higher incidence of rearrangements at the 5' end of the RAR-alpha second intron in female patients, and more frequent 3' breakpoints in males. The results of this study indicate that a unique genomic alteration consistently occurs on the 17q- derivative of the APL specific t(15;17) aberration. Moreover, the clinical relevance of RAR-alpha gene analysis both at diagnosis and in follow-up studies is further emphasized.

[Retinoic acid receptor alpha gene in t (15; 17) APL]

A t(15; 17) (q22; q21) translocation is identified in most patients with acute promyelocytic leukemia (APL). This translocation constructs fusion genes between retinoic acid receptor alpha (RARA) at 17q21 and PML at 15q22. These rearrangements can be detected in the majority of patients with APL but not in other types of leukemias, by Southern blotting. Breakpoints cluster in limited regions of RARA and PML, and PML/RARA and RARA/PML transcripts can also be detected by RT-PCR. Although PML/RARA and RARA/PML fusion products are transcribed in APL, PML/RARA may play an important role in the etiology of APL. Clinically, most APL patients achieve remission by oral administration of high dose all-trans retinoic acid.

Occurrence of distinct PML-RAR-alpha fusion gene isoforms in patients with acute promyelocytic leukemia detected by reverse transcriptase/polymerase chain reaction

A specific 'nested' reverse transcriptase/polymerase chain reaction (RT/PCR) procedure was used to characterize the expression patterns of PML-RAR-alpha chimeric mRNAs in 32 patients with acute promyelocytic leukemia (APL). The sensitivity of the technique was such that the fusion gene transcript could be detected from as little as 2.5 pg of total leukemic cell RNA against a background of 1 microgram of cellular RNA lacking the PML-RAR-alpha fusion gene transcript(s). In 19 cases the PML-RAR-alpha isoform referred to here as long was identified. A short isoform, which in comparison with the long form lacks three PML exons, was detected in 11 other cases. A third PML-RAR-alpha mRNA isoform, in which the most 3' PML exon present in the long-type isoform was truncated in its sequences lying immediately upstream of RAR-alpha B region, was found and characterized in a single patient. In one APL patient with a variant translocation t(11;17), the PCR product corresponding to PML-RAR-alpha chimeric mRNAs could not be amplified despite the presence of RAR-alpha gene rearrangement. Genomic and PCR analysis showed that the different PML-RAR-alpha isoforms found in APL patients arise as a result of distinct translocation breakpoints. In each case the exons encoding the B-F regions of RAR-alpha are expressed and are spliced downstream from variable PML gene exons. The 'nested' RT/PCR analysis of the PML-RAR-alpha fusion gene proved to be a rapid and sensitive tool for the diagnosis of the APL and for monitoring the residual APL chimeric mRNA expression during complete remission.

The Drosophila retinoid X receptor homolog ultraspiracle functions in both female reproduction and eye morphogenesis

Ultraspiracle (usp) encodes the Drosophila cognate of RXR, the human retinoid X receptor. To examine how RXR subfamily members function in development, we have undertaken a phenotypic analysis of usp mutants. usp is required at multiple stages of development for functions that occur in a wide variety of tissues. usp is required in the eye-antennal imaginal disc for normal eye morphogenesis and in the somatic and germline tissues of adult females for fertilization, eggshell morphogenesis and embryonic development. An unusual sunken eye phenotype with marked ventral-dorsal polarity appears to be caused by a lack of usp function in the imaginal disc cells that reside between the eye and antennal anlage. The usp functions include cell autonomous and non-cell autonomous components, suggesting that usp controls the production of factors important for both cell-cell communication and cellular differentiation. These usp signalling pathways have mechanistic parallels to steroid and retinoid action in developing vertebrate tissues.

Retinoic acid-mediated repression of human papillomavirus 18 transcription and different ligand regulation of the retinoic acid receptor beta gene in non-tumorigenic and tumorigenic HeLa hybrid cells

Human papillomavirus type 18 (HPV18) belongs to the group of genital papillomaviruses involved in the development of cervical carcinomas. Since retinoic acid (RA) is a key regulator of epithelial cell differentiation and a growth inhibitor in vitro of HPV18-positive HeLa cervical carcinoma cells, we have used HeLa and HeLa hybrid cells in order to analyse the effects of RA on expression of the HPV18 E6 and E7 oncogenes and of the cellular RA receptor genes RAR-beta and -gamma. We show here that RA down-regulates HPV18 mRNA levels apparently due to transcriptional repression. Transient cotransfection assays indicated that RARs negatively regulate the HPV18 upstream regulatory region and that the central enhancer can confer RA-dependent repression on a heterologous promoter. RA treatment resulted in induction of RAR-beta mRNA levels in non-tumorigenic HeLa hybrid cells, but not in tumorigenic hybrid segregants nor in HeLa cells. No alterations of the RAR-beta gene or of the HeLa RAR-beta promoter could be revealed by Southern and DNA sequence analysis, respectively. As determined by transient transfection assays, however, the RAR-beta control region was activated by RA more strongly in non-tumorigenic hybrid cells than in HeLa cells, thus indicating differences in trans-acting regulatory factors. Our data suggest that the RARs are potential negative regulators of HPV18 E6 and E7 gene expression, and that dysregulation of the RAR-beta gene either causatively contributes to or is an indicator of tumorigenicity in HeLa and HeLa hybrid cells.

[Induction of differentiation of human leukemia cells]

Human myeloid leukemia cells do not differentiate into functional end-cells but remain in the proliferation pool. Human cell lines can serve as model for hematopoietic cells arrested at different stages of myeloid differentiation and helps to dissect the cellular and molecular events involved in leukemogenesis. Furthermore, several agents have been identified as inducers of differentiation of leukemia cells. Exciting new clinical observation have shown that patients with APL respond well to the treatment with all-trans retinoic acid. RAR-alpha gene was proved to translocated from chromosome 17 to a locus PML on chromosome 15. This new chimeric gene, PML-RAR alpha is extremely important to understand the leukemogenesis of APL.

Retinoic acid receptors and binding proteins in human skin

Nuclear retinoic acid receptors (RAR) are likely to mediate many of the pleiotypic cutaneous actions of retinoids by acting as ligand-dependent enhancer factors. The presence of nuclear RAR in skin was confirmed by identification of a 45-kDa nuclear RA binding activity by fast protein liquid chromatography (FPLC). Analysis of RNA extracted from skin specimens demonstrated expression of RAR-alpha and RAR-gamma transcripts, as well as expression of the homologous low-affinity receptor, RXR-alpha. Both isoforms of RAR-gamma RAR-gamma 1 and RAR-gamma 2 were detectable, with RAR-gamma 1 being the more strongly expressed. FPLC analysis also demonstrated a 15-kDa peak of specific RA binding activity, consistent with the presence of cellular retinoic acid binding protein (CRABP). Of the two known forms of CRABP, CRABP-II was much more strongly expressed than CRABP-I at the level of steady-state mRNA. CRABP-II was also expressed in keratinocytes and fibroblasts in vitro. CRABP-II was up-regulated by agents that induce keratinocyte differentiation, and inhibited by prolonged exposure to high concentrations of RA. In contrast, CRABP-II was consistently induced by RA in dermal, but not in lung fibroblasts. CRABP-I was expressed at low to undetectable levels under all these conditions. The presence of tissue-specific and differentiation-related regulation of CRABP-II suggests that it may be an important regulator of RA action in human skin.

Expression pattern of the RAR alpha-PML fusion gene in acute promyelocytic leukemia

Two chimeric genes, PML-RAR alpha and RAR alpha-PML, are formed as a consequence of the acute promyelocytic leukemia (APL)-specific reciprocal translocation of chromosomes 15 and 17 [t(15;17)]. PML-RAR alpha is expressed as a fusion protein. We investigated the organization and expression pattern of the RAR alpha-PML gene in a series of APL patients representative of the molecular heterogeneity of the t(15;17) and found (i) two types of RAR alpha-PML mRNA junctions (RAR alpha exon 2/PML exon 4 or RAR alpha exon 2/PML exon 7) that maintain the RAR alpha and PML longest open reading frames aligned and are the result of chromosome 15 breaking at two different sites; and (ii) 10 different RAR alpha-PML fusion transcripts that differ for the assembly of their PML coding exons. A RAR alpha-PML transcript was present in most, but not all, APL patients.

Cellular localization of retinoic acid receptor-gamma expression in normal and neoplastic skin

Retinoids profoundly affect the normal growth and differentiation of epithelial tissues. Retinoic acid receptor-gamma (RAR-gamma) is a member of a family of retinoid receptors, and has been shown to be expressed almost exclusively in skin. However, little is known about the cellular localization of this receptor in human skin. The authors studied the expression of RAR-gamma in normal skin and human skin tumors by Northern blot analysis and in situ hybridization. RAR-gamma mRNA was detected in normal skin as well as in cultures of neonatal keratinocytes. Using an oligonucleotide specific for the RAR-gamma cDNA isoform 1 (RAR-gamma 1), RAR-gamma 1 mRNA was localized to all layers of the epidermis, the outer root sheath of hair follicles, follicular hair bulbs, eccrine and sebaceous glands. Basal cell carcinoma constitutively expressed gamma-1 mRNA and one of seven squamous cell carcinomas showed loss of gamma-1 mRNA expression, relative to adjacent epithelium. By contrast, normal melanocytic nevi and tumor-associated lymphocytes expressed little or no RAR-gamma mRNA. These results suggest that RAR-gamma 1 may play an important role in the maintenance and differentiation of normal epidermis and skin appendages.

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.

Transgenic mice expressing a constitutively active retinoic acid receptor in the lens exhibit ocular defects

Retinoic acid receptors (RARs) modulate gene expression following association with retinoic acid (RA). In transient transfection, an RAR alpha-beta-galactosidase fusion protein (RAR-LacZ) was able to transactivate expression in the absence of RA. When expressed in the ocular lens of transgenic mice, this constitutively active RAR-LacZ fusion gene resulted in founder and progeny animals that exhibited cataracts and microphthalmia, both being characteristics of retinoid-induced teratogenesis. The transgenic phenotypes indicate that retinoid teratogenesis can be mimicked by expression of a constitutively active RAR-LacZ fusion protein in retinoid-sensitive tissues.

Cellular retinoic acid binding protein type II was preferentially localized in medium and posterior parts of the progress zone of the chick limb bud

The expression and distribution of cellular retinoic acid binding protein II (CRABP II) was examined in chick limb buds. CRABP II was detected in the limb buds at Hamburger and Hamilton (1) stage 21 and the amount of CRABP II was gradually increased during stages 21-27 and thereafter decreased. CRABP II was mainly located in the progress zone, and the dorsal and ventral premuscular mass in the proximal region of the limb buds at stage 23. CRABP II was preferentially localized in the medium and posterior parts rather than the anterior part of the progress zone; The content of CRABP II in the medium and posterior parts was 8-9 times more than that in the anterior part.

Reverse transcription polymerase chain reaction for the rearranged retinoic acid receptor alpha clarifies diagnosis and detects minimal residual disease in acute promyelocytic leukemia

The characteristic t(15;17) of acute promyelocytic leukemia (APL) fuses the retinoic acid receptor alpha (RAR-alpha) gene on chromosome 17 to a gene on chromosome 15 called PML, a putative transcription factor. This distinct translocation results in a fusion mRNA detected by Northern analysis. Two cDNAs have been isolated that differ in the extent of 3' PML nucleic acid sequence contained. This study describes a reverse transcription polymerase chain reaction (RT-PCR) assay for the PML/RAR-alpha fusion transcript, which amplifies PML/RAR-alpha mRNA from APL cells with either reported breakpoint. DNA sequencing of the predominant RT-PCR products from 6 patients showed identical RAR-alpha exonic breakpoints and two PML breakpoints. This RT-PCR assay was positive in leukemic cells from 30/30 APL patients with the molecular rearrangement confirmed by cytogenetics or Northern analysis. In leukemic cells of patients with a morphologic diagnosis of APL lacking the t(15;17) by routine cytogenetics, a positive RT-PCR assay predicted clinical response to all-trans-retinoic acid (RA) therapy. Dilutional studies with leukemic cells that express (NB4) or do not express (HL-60) a PML/RAR-alpha fusion mRNA reveal that this RT-PCR assay detects the transcript from as little as 50 pg of total RNA. In APL cells from 5/6 patients treated with RA alone, a complete response by clinical and cytogenetic criteria accompanied a persistently positive RT-PCR assay. This preceded relapse by 1-6 months. RT-PCR for PML/RAR-alpha mRNA provides a more-sensitive test for the t(15;17) than routine cytogenetics or Northern analysis. This molecular rearrangement detected by RT-PCR best defines this RA-responsive malignancy. The RT-PCR assay for the PML/RAR-alpha transcript yields important diagnostic and prognostic information in the management of APL patients.

The lungs and vitamin A

Evidence is reviewed supporting the view that vitamin A (retinol) and its metabolite, retinoic acid, called natural retinoids, are major factors involved in differentiation and in maturation of the lungs. This conclusion is based on morphological observation that lack of this dietary micronutrient causes keratinizing squamous metaplasia of the bronchopulmonary tree that can be reversed by refeeding the animal with retinol. In addition to these observations suggesting an indirect participation of retinol and/or retinoic acid in the differentiation of this organ, more direct evidence is presented that this vitamin is involved in pulmonary gene expression. Adult as well as fetal lungs accumulate retinyl esters, the storage form of vitamin A, contain specific retinol- and retinoic acid-binding proteins and, more importantly, express several isoforms of nuclear retinoic acid receptors. These proteins are involved in activation and repression of specific genes regulated by retinoic acid. That retinol and/or retinoic acid may be involved in lung maturation is suggested by experimental results showing depletion of lung retinyl esters at birth, by significant alterations in the levels of the cellular retinoic acid-binding protein during lung maturation and, more importantly, by reduction in the morbidity of prematurely born human neonates who are given vitamin A because they are susceptible to bronchopulmonary dysplasia.

Retinoic acid receptor expression in human skin keratinocytes and dermal fibroblasts in vitro

Retinoic acid is essential for the normal differentiation of epithelia but its cellular function is obscure. The expression patterns of retinoic acid receptors (RARs) in skin cell types may give an insight into the role of retinoic acid in skin. We have compared the patterns of RAR expression in human keratinocytes and dermal fibroblasts in vitro, and studied the effects of retinoic acid on RAR expression. RAR-alpha and RAR-gamma were expressed in keratinocytes and fibroblasts: RAR-gamma was expressed at similar levels in both cell types but RAR-alpha was more abundant in fibroblasts. There were no differences in expression of either RAR-alpha or RAR-gamma between stratifying (high-calcium medium) and proliferating (low-calcium medium) keratinocytes and expression of these RARs was unaffected by retinoic acid. RAR-beta was undetectable in keratinocytes. In the majority of fibroblast cell lines, RAR-beta transcripts were either undetectable or expressed at a low level. Retinoic acid at low concentrations (10(−10) to 10(−9) M) rapidly induced the expression of RAR-beta. Cyclic adenosine monophosphate (cAMP) analogues inhibit RAR-beta induction in teratocarcinoma cells. However, dibutyryl-cAMP did not affect RAR-beta induction in fibroblasts. Forskolin, an adenylate cyclase activator, and the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) decreased constitutive RAR-beta mRNA levels but did not block induction of RAR-beta by retinoic acid. Since intracellular cAMP levels were only increased detectably in response to forskolin, the reduction in constitutive levels of RAR-beta mRNA may be mediated by mechanisms other than via cAMP.

Inhibition of differentiation by leukemia inhibitory factor distinguishes two induction pathways in P19 embryonal carcinoma cells

The ability of leukemia inhibitory factor (LIF) to block differentiation of P19 embryonal carcinoma (EC) cells under a variety of induction conditions was determined. LIF inhibits differentiation under several conditions which lead to endodermal and mesodermal cell lineages including skeletal and cardiac muscle. In contrast, LIF does not block differentiation when cells are induced under conditions which lead to neuro-ectodermal cell types including neurons and astroglial cells. These studies demonstrate that P19 EC cell differentiation can be divided into LIF sensitive and insensitive pathways which correlate with differentiation of endodermal/mesodermal and neuro-ectodermal cell types, respectively. The effect of LIF on mRNA levels for several genes which have previously been implicated in mediating differentiation in P19 EC cells was determined. LIF has no effect on the mRNA levels for retinoic acid receptor (RAR) alpha, RAR beta, RAR gamma, jun A, jun D, c-fos, or fra-1. In contrast LIF stimulates jun B mRNA expression by a factor of four to six under all induction conditions.