Rb2/p130 and protein phosphatase 2A: key mediators of ovarian carcinoma cell growth suppression by all-trans retinoic acid

Despite a number of attempts to improve treatment of ovarian cancer, it remains the most common cause of death from gynecological cancers. Thus, it is very important to identify more effective drugs for treatment and prevention of ovarian cancer. All-trans-retinoic acid (ATRA) has been shown to arrest the growth of ovarian carcinoma cells in G0/G1 and to significantly elevate levels of Rb2/p130 protein, a member of the retinoblastoma family of tumor suppressors. As ATRA treatment leads to a significant increase in the amount of Rb2/p130 protein but not mRNA, the elevated levels of Rb2/p130 protein is likely the result of increased stability. In studies to elucidate the mechanism by which ATRA alters Rb2/p130 stability in ovarian cancer cells, it was determined that PP2A, a serine/threonine phosphatase, binds and dephosphorylates Rb2/p130. Dephosphorylated Rb2/p130 exhibits decreased ubiquitination and thus is not degraded by the proteasome. The sites at which PP2A catalytic subunit (PP2Ac) interacts with Rb2/p130 have been localized to the NLS in the C-terminus of Rb2/p130. These sites are also involved in the interaction of Rb/p130 with importin beta and importin alpha, members of the nuclear transport machinery. It is known that importin alpha recognizes a NLS on a target protein and importin beta binds the nuclear pore complex. Moreover, it has been shown that the binding of importin alpha to NLS significantly decreases with phosphorylation of NLS. In ATRA-treated ovarian carcinoma cells, PP2A binds to Rb2/p130 and dephosphorylates the NLS of Rb2/p130 leading to the interaction of importin alpha with Rb2/p130. Importin beta then binds to the importin alpha-Rb2/p130 complex, leading to the translocation of the Rb2/p130 to the nucleus where it acts to arrest ovarian cancer cells in G1 and suppress proliferation.

Effect of all-trans retinoic acid on telomerase activity in ovarian cancer cells

Vitamin A is an essential nutrient important for growth, vision, embryonic development, immune response and reproduction. Various retinoids have been shown to be effective chemotherapeutic and chemopreventive agents for a number of human cancers. Telomeres are nucleoprotein structures found at the end of chromosomes. During cellular division, the telomeres in normal cells shorten progressively and thus, function as a "molecular clock". Telomerase is a ribonucleoprotein complex that extends and maintains telomeres. Activation of telomerase is required for cells to overcome proliferative crisis. Telomerase activation is observed in 90% of human cancers, but not in normal somatic cells. We examined the role of telomerase in mediating the growth suppression of ovarian carcinoma cells by all-trans-retinoic acid (ATRA). Using a number of cell lines with varying levels of growth sensitivity to ATRA, we found that cells that exhibit ATRA-dependant suppression of growth also contained significantly reduced telomerase activity. We also observed a reduction in expression of the telomerase components, hTERT and hTR in ATRA treated ovarian carcinoma cells. Our results suggest that one mechanism by which ATRA acid inhibits cancer cell growth is by suppressing telomerase activity, thereby pushing cells to proliferative crisis.

Cell cycle genes as targets of retinoid induced ovarian tumor cell growth suppression

We have examined the effect of all-trans-retinoic acid (RA) on cell cycle gene expression in RA sensitive CA-OV3 and RA resistant SK-OV3 ovarian carcinoma cell lines. Gene expression was analysed by multiprobe RNAse protection, Western blotting and in vitro kinase assays. No differences were observed between RA sensitive and RA resistant ovarian carcinoma cells in the levels of expression of many cell cycle genes including cyclin A, B and E, cdk 2,4 and 6, E2F-1, E2F-2, E2F-3, E2F-4, E2F-5, DP-1 and DP-2. However, RA sensitive CA-OV3 cells expressed higher levels of p53, p27, p21, and p16 compared to RA resistant SK-OV3 cells. In addition, RA treatment of CA-OV3 cells resulted in a significant decrease in hyperphosphorylated RB and RB-2/p130 and corresponding significant increases in the levels of hypophosphorylated and/or partially phosphorylated RB-2/p130 protein and hypophosphorylated RB. Also, RA treatment increased expression of the cdk inhibitor p27 and decreased activity of cdk 2, cdk 4 and cdk 6. Finally, amounts of p27-cyclin E and RB-2/p130-E2F4 complexes were found to increase in CA-OV3 cells growth arrested by RA. These results suggest that the pocket protein pathways are critical targets for retinoid suppression of ovarian carcinoma cell growth.

The synthetic retinoid AGN 193109 but not retinoic acid elevates CYP1A1 levels in mouse embryos and Hepa-1c1c7 cells

The synthetic retinoid AGN 193109 is a potent pan retinoic acid receptor (RAR) antagonist. Treatment of pregnant mice with a single oral 1 mg/kg dose of this antagonist on day 8 postcoitum results in severe craniofacial (median cleft face or frontonasal deficiency) and eye malformations in virtually all exposed fetuses. Using differential display analysis, we have determined that CYP1A1 mRNA levels are elevated in mouse embryos 6 h following treatment with AGN 193109. Similarly, an elevation in CYP1A1 mRNA levels, protein levels, and aryl hydrocarbon hydoxylase activity occurs in Hepa-1c1c7 cells, with the maximal elevation observed when the cells were treated with 10(-5) M AGN 193109 for 4 to 8 h. Elevation in CYP1A1 mRNA levels in mouse embryos and Hepa-1c1c7 cells does not occur upon treatment with the natural retinoid, all-trans-retinoic acid. Finally, elevation in CYP1A1 mRNA levels was not observed when mutant Hepa-1c1c7 cells, which are defective in either the aryl hydrocarbon receptor (AhR) or aryl hydrocarbon receptor nuclear translocator (ARNT), were treated with AGN 193109. This suggests that the AhR/ARNT pathway and not the RAR/RXR pathway is mediating the elevation of CYP1A1 mRNA levels by AGN 193109, at least in the Hepa-1c1c7 cells. This is the first example of a retinoid that displays the abililty to regulate both the RAR/RXR and AhR/ARNT transcriptional regulatory pathways.

beta-Amyloid protein induces the formation of purine dimers in cellular DNA

Experimental evidence implicates oxidative free radical reactions as central in the processes of neurodegenerative diseases. In particular, cellular interactions with the beta-amyloid protein have been linked to neuron cell death in Alzheimer's disease. Also, uncharacterized dimeric purine moieties have been detected in oxidized DNAs. It has been suggested that inadequate excision-repair of such products plays a functional role in the neurological degeneration observed in familial Alzheimer's disease, Down's syndrome, and xeroderma pigmentosum. Therefore, in order to obtain a reagent to monitor the presence of such products, the purine dimer 8-8-(2'-deoxyguanosyl)-2'-deoxyguanosine-5'-monophosphate was used as a hapten for elicitation of rabbit anti-purine dimer antiserum. This antiserum specifically recognizes various purified 8-8-bideoxyribonucleosides and 8-8-bideoxyribonucleotides. We found that DNA oxidized by the Fenton reaction is specifically recognized by this antiserum. This reagent can therefore be used to demonstrate formation and excision of DNA purine dimers. Moreover, incubation of cultured rat pheochromocytoma PC-12 cells with the beta-amyloid protein resulted in formation of these purine dimers in cellular DNA. These dimers were subsequently removed from cellular DNA. From these results we conclude that the free radicals generated by A beta cause oxidative DNA alterations including purine dimers. Deficient repair of this type of DNA damage might result in neural cell loss via apoptosis. Our findings suggest mechanisms for the roles of beta-amyloid and oxidative free radicals in neurodegenerative diseases and the role of DNA excision-repair in the prevention of lethal neurotoxicity.

Retinoids and ovarian cancer

Each year, an estimated 26,000 women in the United States are diagnosed with ovarian cancer. During any given year, approximately 14,500 women die from this disease. Ovarian cancer is the seventh most common cancer in women worldwide, after breast, cervix, colon/rectum, stomach, corpus uteri, and lung cancers. In the U.S., ovarian cancer is the second most common gynecologic cancer, and is the fourth leading cause of solid tumor cancer deaths among women. Currently, postoperative chemotherapy of ovarian cancer is still suboptimal. Drug resistance is a common problem resulting in only 20 approximately 30% overall 5-year survival rates. Clearly, continued development of alternative therapeutic strategies is essential for the management of this fatal disease. A number of recent studies have suggested that retinoids may play a potential role as an ovarian cancer chemotherapeutic agent. Retinoids, the natural and synthetic derivatives of vitamin A, have been shown to inhibit the growth of human ovarian cancer cells both in vivo and in culture. This review will initially summarize what is known about the pathological and molecular characteristics of ovarian carcinoma. It will then describe retinoid metabolism and the role of the cellular and nuclear retinoid binding proteins in mediating retinoid action. Following this general review of retinoids and their function, data supporting the role of retinoic acid as a suppresser of ovarian carcinoma cell growth will be presented. Particular attention will be paid to studies suggesting that members of the RB family of proteins and RB2/p130, in particular, are the molecular targets responsible for retinoid mediated inhibition of ovarian carcinoma cell growth. This review will then conclude with a brief discussion of two synthetic retinoids, 4 HPR R(fenretinide) and AHPN/CD437, which have been shown to induce apoptosis in ovarian tumor cells. It will be clear from the studies summarized in this review that retinoids represent a potentially powerful alternative to present chemotherapeutic approaches to the treatment of late stage ovarian cancer.

Induction of apoptosis in ovarian carcinoma cells by AHPN/CD437 is mediated by retinoic acid receptors

Retinoids have great promise in the area of cancer therapy and chemoprevention. These natural and synthetic derivatives of vitamin A have been shown to play an important role in regulating cell differentiation and proliferation. While all-trans-retinoic acid (ATRA) has been demonstrated to inhibit the growth of several ovarian tumor cell lines, other ovarian carcinoma cell lines have been found to be resistant to retinoid dependent growth suppression. Interestingly, a novel synthetic retinoid, CD437 or AHPN, has been demonstrated to inhibit the growth of both ATRA-sensitive (CA-OV3) and ATRA-resistant (SK-OV3) ovarian tumor cell lines as well as to induce apoptosis. The overall goal of this research was to understand the mechanism by which AHPN/CD437 induces apoptosis in ovarian tumor cell lines. Since a number of studies have demonstrated the importance of nuclear receptors (RARs and RXRs) in mediating cellular responses to retinoids, we wished to determine the role of RARs in mediating the AHPN/CD437 response. We modulated RAR level and function by overexpressing either wild type RAR-gamma or a pan dominant negative mutant of all RAR subtypes called RAR-beta (R269Q), or through the use of an RAR-gamma antagonist, MM11253. We found that inhibition of RAR function reduced but did not eliminate induction of apoptosis in both CA-OV3 and SK-OV3 cells by AHPN/CD437. Likewise, overexpression of wild type RAR-gamma was found to increase apoptosis after treatment with AHPN/CD437. Our results suggest that in ovarian carcinomas, AHPN/CD437 induced apoptosis is mediated at least in part via an RAR pathway.

Retinoids and ovarian cancer

Each year, an estimated 26,000 women in the United States are diagnosed with ovarian cancer. During any given year, approximately 14,500 women die from this disease. Ovarian cancer is the seventh most common cancer in women worldwide, after breast, cervix, colon/rectum, stomach, corpus uteri, and lung cancers. In the U.S., ovarian cancer is the second most common gynecologic cancer, and is the fourth leading cause of solid tumor cancer deaths among women. Currently, postoperative chemotherapy of ovarian cancer is still suboptimal. Drug resistance is a common problem resulting in only 20 approximately 30% overall 5-year survival rates. Clearly, continued development of alternative therapeutic strategies is essential for the management of this fatal disease. A number of recent studies have suggested that retinoids may play a potential role as an ovarian cancer chemotherapeutic agent. Retinoids, the natural and synthetic derivatives of vitamin A, have been shown to inhibit the growth of human ovarian cancer cells both in vivo and in culture. This review will initially summarize what is known about the pathological and molecular characteristics of ovarian carcinoma. It will then describe retinoid metabolism and the role of the cellular and nuclear retinoid binding proteins in mediating retinoid action. Following this general review of retinoids and their function, data supporting the role of retinoic acid as a suppresser of ovarian carcinoma cell growth will be presented. Particular attention will be paid to studies suggesting that members of the RB family of proteins and RB2/p130, in particular, are the molecular targets responsible for retinoid mediated inhibition of ovarian carcinoma cell growth. This review will then conclude with a brief discussion of two synthetic retinoids, 4 HPR R(fenretinide) and AHPN/CD437, which have been shown to induce apoptosis in ovarian tumor cells. It will be clear from the studies summarized in this review that retinoids represent a potentially powerful alternative to present chemotherapeutic approaches to the treatment of late stage ovarian cancer.

Role of Ser(289) in RARgamma and its homologous amino acid residue of RARalpha and RARbeta in the binding of retinoic acid

The biological actions of retinoic acid (RA) are mediated by retinoic acid receptors (RARalpha, -beta, and -gamma) and retinoid X receptors (RXRalpha, -beta, and -gamma). Although the ligand-binding domains of RARs and RXRs have been suggested to share the same novel folding pattern, the ligand-binding pockets of each of the retinoid receptors must have unique structural features since it has been possible to develop RAR subtype-selective and RXR-selective retinoids. We have previously demonstrated the importance for RA binding and RA-dependent transactivation of Arg(276) in RARalpha and Arg(278) in RARgamma; however, in RARbeta Arg(269) functions in conjunction with Lys(220). Here we have examined the role of the hydroxyl group of RARgamma Ser(289) and its homologous amino acid residues in RARalpha (Ser(287)) and RARbeta (Ser(280)) alone and in conjunction with their respective RARgamma Arg(278) homologs for RA binding and RA-dependent transactivation activity. The hydroxyl group of this Ser in all three RARs was found by itself not to be important for RA binding and RA-dependent transactivation activity. However, in RARalpha and RARgamma this Ser appears to play a small role in conjunction with Arg(276) and Arg(278), respectively, for these activities. Alternatively, strong synergism was observed in RARbeta between Ser(280) and Arg(269) for RA-binding and RA-dependent transactivation activity. This provides further evidence that the mechanism of interaction between the carboxylate group of retinoids and the amino acid residues in the ligand binding pocket of RARbeta is different from that of RARalpha and RARgamma.

Murine RARbeta4 displays reduced transactivation activity, lower affinity for retinoic acid, and no anti-AP1 activity

The biological actions of retinoic acid (RA) are mediated by retinoic acid receptors (RARalpha, beta, and gamma) and retinoid X receptors (RXR alpha, beta, and gamma). Each of the RARs is expressed as four to seven different isoforms. Four isoforms of RAR beta (beta1, beta2, beta3, and beta4), which differ only in their N-terminal sequence (A domain) have been described. These RARbeta isoforms display a specific pattern of expression in developing and adult animals and are highly evolutionarily conserved suggesting that they mediate distinct cellular effects of vitamin A. Experiments were performed to examine directly the RA-binding activity, transactivation activity, and anti-AP1 activity of each of these four RARbeta isoforms. The results demonstrate that RARbeta1, beta2, and beta3 bind RA with a similar K(d) value, have a similar EC(50) value in RA-dependent transactivation assays and inhibit AP1 activity to a similar level. By contrast, RARbeta4 has an elevated K(d) for RA, an increased EC(50) value in RA-dependent transactivation assays and does not display the ability to inhibit AP1 activity. This provides additional evidence that at least one RAR isoform, RARbeta4, may mediate distinct activities within a cell. Furthermore, these data suggest that the presence of an A domain in RARbeta is important for modulating these activities of RARs.

Modulation of retinoic acid receptor function alters the growth inhibitory response of oral SCC cells to retinoids

Retinoids have been shown to inhibit the growth of many human tumor cells including breast, ovarian and squamous cell carcinoma (SCC). While the exact mechanism of retinoid mediated growth suppression is not known, a role for the retinoic acid receptors (RARs) and retinoid X receptors (RXRs) has been established in both the breast and ovarian tumor cell models. We set out to determine if modulation of RAR/RXR function would alter the retinoid sensitivity of oral SCC cells. We found that the growth of SCC cells was significantly inhibited by treatment with either all-trans-retinoic acid (trans-RA) or the synthetic, conformationally restricted RARgamma selective retinoids MM11254 and MM11389. In order to demonstrate a role for RAR/RXR function in this process, stable oral SCC cell clones constitutively overexpressing the dominant negative mutant RARbeta2 (R269Q) were prepared and shown to exhibit reduced RAR/RXR transcriptional transactivation activity. We found that oral SCC cells exhibiting reduced RAR/RXR function became resistant to growth inhibition by all-trans-RA, MM11254 and MM11389. Likewise, treatment of oral SCC cells with the RARgamma antagonist MM11253 was found to block the ability of MM11254 and MM11389 to inhibit SCC cell growth. Thus, modulation of RAR function through the use of RAR-gamma selective agonists, an RAR-gamma selective antagonist or a pan-RAR dominant negative mutant significantly alters the growth inhibitory response of oral SCC cells to retinoids.

Arg278, but not Lys229 or Lys236, plays an important role in the binding of retinoic acid by retinoic acid receptor gamma

The diverse biological actions of retinoic acid (RA) are mediated by retinoic acid receptors (RARalpha, beta and gamma) and retinoid X receptors (RXR alpha, beta, and gamma). Although the ligand-binding domains of RARs share the same novel folding pattern, many RAR subtype-specific retinoids have been synthesized indicating that the ligand-binding pocket of each RAR subtype has unique features. Previously we have demonstrated the importance for RA binding and RA-dependent transactivation of Arg276 of RARalpha alone and in RARbeta Arg269 in conjunction with Lys220. In this study, we have examined the role of the homologous amino acid residues (Lys229 and Arg278) in RARgamma for these activities. Like RARalpha but dissimilar to RARbeta, Arg278 in RARgamma alone was found to play an important role in RA binding and RA-dependent transactivation. Since Lys236 in RARgamma was suggested from the crystal structure of holo-RARgamma to interact with RA, we also examined its role and that of its homologs in RARalpha and RARbeta. Despite the suggestion from the crystal structure, neither Lys236 nor its homologs in RARalpha and RARbeta play a role in the binding of RA or RA-dependent transactivation. It is likely that Lys236 in RARgamma and its homologs in RARalpha and RARbeta are solvent exposed rather than pointing into the RA-binding pocket.

Critical role of both retinoid nuclear receptors and retinoid-X-receptors in mediating growth inhibition of ovarian cancer cells by all-trans retinoic acid

Retinoids have been shown to inhibit the growth of a number of human tumor cells, including several ovarian adenocarcinoma cell lines. All-trans retinoic acid (RA) is an effective growth suppressor of CA-OV3 cells but not SK-OV3 cells. Since the effects of RA are known to be mediated via the nuclear receptors (RARs and RXRs), we initially compared levels of the various RARs and RXRs in the CA-OV3 and SK-OV3 cell lines. The RA resistant SK-OV3 cells expressed reduced levels of RAR-alpha and RXR-alpha. Furthermore, induction of RAR-alpha by RA was impaired in the RA resistant SK-OV3 cells as was RARE binding and RARE-dependent transcriptional activity. These results suggested that changes in the amounts and/or activity of RARs and/or RXR-alpha could determine the growth response of ovarian tumor cells to RA. This was confirmed by modulating the levels of RARs and RXR-alpha in the SK-OV3 cells using the LacSwitch inducible expression system. Stably transfected clones of RA resistant SK-OV3 cells exhibited a significant inhibition of growth by RA treatment when RAR-alpha was induced. Overexpression of both RAR-alpha and RXR-alpha resulted in a level of growth inhibition nearly equal to that exhibited by the RA sensitive CA-OV3 cell line. Similar results were obtained when a combination of RXR-alpha and either RAR-beta or RAR-gamma was overexpressed in SK-OV3 cells. Our results show that the nuclear receptors and RXR-alpha play a critical role in mediating growth suppression by RA in ovarian cancer cells.

Cellular distribution of retinoic acid receptor-alpha protein in serous adenocarcinomas of ovarian, tubal, and peritoneal origin: comparison with estrogen receptor status

Retinoids are effective growth modulators of human ovarian carcinoma cell lines. Their effects are mediated by nuclear retinoic acid receptors (RARs) and retinoid X receptors (RXRs), which are transcriptional factors and members of the steroid/thyroid receptor superfamily. To our knowledge, until now, the cellular distribution of RAR proteins in human ovarian tumor specimens is unknown. This study provides new data on the differential cellular localization of RAR alpha protein in 16 serous adenocarcinomas originating from the ovaries, fallopian tubes, and the peritoneum. Using an affinity-purified antiserum specific for RAR alpha and a monoclonal antibody recognizing the full-length estrogen receptor molecule (clone 6F11), we performed immunohistochemistry on frozen tissue sections and examined the relationship between RAR alpha and estrogen receptor protein expression by comparing the percentage of immunostained tumor cells for either receptor. Our findings indicate a strong linear relationship between the percentages of RAR alpha- and estrogen receptor-labeled tumor cells as determined by linear regression analysis (P < 0.005, r = 0.825). A modest inverse relationship was found between the percentage of RAR alpha-positive tumor cells and histological grade, attesting to a differentiation-dependent trend (P < 0.04). No significant relationship was found between RAR alpha-labeled cells and clinical stage (P = 0.139), site of tumor origin (ovaries versus fallopian tubes versus peritoneum) (P = 0.170), and primary versus metastatic lesion (P = 0.561). Thus, serous adenocarcinomas are capable of expressing RAR alpha and estrogen receptor despite high histological grade and advanced stage of neoplastic disease. Compared with the heterogeneous localization of RAR alpha in cancer cells, there was widespread RAR alpha immunoreactivity in tumor-infiltrating lymphocytes, vascular endothelial cells, and stromal fibroblasts, underscoring the value of immunohistochemistry in the accurate determination of RAR/(RXR) content in tumor specimens.

Effects of conformationally restricted synthetic retinoids on ovarian tumor cell growth

We have used conformationally restricted retinoids to investigate the role of individual RAR subtypes and RXR in mediating the growth response of ovarian tumor cells to retinoids. Our results show that treatment of all-trans-RA-sensitive CAOV-3 cells with retinoids that bind and activate a single RAR or RXR led to a partial inhibition of growth. Treatment of all-trans-RA- resistant SKOV-3 cells did not alter growth. Maximum inhibition of growth, comparable to that observed following treatment with natural retinoids such as all-trans-RA and 9-cis-RA, was obtained only following treatment with a combination of an RAR-selective compound and an RXR-selective one. These results suggest that activation of both RAR and RXR classes is required in order to obtain maximum inhibition of ovarian tumor cell growth by retinoids. In addition, one compound, AHPN, was found to inhibit both RA-sensitive CAOV-3 and RA-resistant SKOV-3 cells. Further study of the effects of this retinoid showed that AHPN acts through an apoptotic pathway. Taken together, our results suggest that retinoids may serve as effective anti-proliferative agents in the treatment of ovarian cancer.

The integrated activities of IRF-2 (HiNF-M), CDP/cut (HiNF-D) and H4TF-2 (HiNF-P) regulate transcription of a cell cycle controlled human histone H4 gene: mechanistic differences between distinct H4 genes

Maximal transcription of a prototypical cell cycle controlled histone H4 gene requires a proliferation-specific in vivo genomic protein/DNA interaction element, Site II. Three sequence-specific transcription factors interact with overlapping recognition motifs within Site II: interferon regulatory factor IRF-2 (HiNF-M), the putative H4 subtype-specific protein H4TF-2 (HiNF-P), and HiNF-D which represents a complex of the homeodomain protein CDP/cut, CDC2, cyclin A and pRB. However, natural sequence variation in the Site II sequences of different human H4 genes abolishes binding of specific trans-acting factors; the functional consequences of these variations have not been investigated. To address the precise contribution of H4 promoter factors to the level of H4 gene transcription, we performed a systematic mutational analysis of Site II transcriptional motifs. These mutants were tested for ability to bind each of the Site II cognate proteins, and subsequently evaluated for ability to confer H4 transcriptional activity using chimeric H4 promoter/CAT fusion constructs in different cell types. We also analyzed the effect of over-expressing IRF-2 on CAT reporter gene expression driven by mutant H4 promoters and assessed H4 transcriptional control in cells nullizygous for IRF-1 and IRF-2. Our results show that the recognition sequence for IRF-2 (HiNF-M) is the dominant component of Site II and modulates H4 gene transcription levels by 3 fold. However, the overlapping recognition sequences for IRF-2 (HiNF-M), H4TF-2 (HiNF-P) and CDP/cut (HiNF-D) together modulate H4 gene transcription levels by at least an order of magnitude. Thus, maximal activation of H4 gene transcription during the cell cycle in vivo requires the integrated activities of multiple transcription factors at Site II. We postulate that the composite organization of Site II supports responsiveness to multiple signalling pathways modulating the activities of H4 gene transcription factors during the cell cycle. Variations in Site II sequences among different H4 genes may accommodate differential regulation of H4 gene expression in cells and tissues with unique phenotypic properties.

Reduction of both RAR and RXR levels is required to maximally alter sensitivity of CA-OV3 ovarian tumor cells to growth suppression by all-trans-retinoic acid

We wished to determine the effect of altering the levels or functional activity of retinoid receptors, in particular retinoic acid receptor-alpha (RAR-alpha) and retinoid X receptor-alpha (RXR-alpha) on the growth sensitivity of ovarian tumor cells to all-trans-retinoic acid (all-trans-RA). We found that CA-OV3 cells could be made resistant to all-trans-RA growth inhibition by overexpressing RAR-beta(R269Q), an efficient dominant negative mutant which inhibits the function of all RAR subtypes. Antisense technology was then used to prepare stable transfectants of the retinoid-sensitive ovarian carcinoma cell line CA-OV3 in which expression of RAR-alpha, RXR-alpha, or both RAR-alpha and RXR-alpha was reduced. The effect of all-trans-RA on ovarian tumor cell growth was determined by MTT assay, autoradiographic analysis of DNA synthesis, and anchorage-independent colony formation in soft agar. Our results show that cell lines expressing reduced levels of either RAR-alpha alone or RXR-alpha alone exhibited a small decrease in sensitivity to growth inhibition by all-trans-RA. However, maximum RA resistance was obtained in cell lines in which the levels of both RAR-alpha and RXR-alpha were reduced. These results demonstrate the importance of both retinoid nuclear receptors and retinoid-X receptors in general, and RAR-alpha and RXR-alpha in particular, as mediators of ovarian carcinoma cell growth inhibition by retinoids.

Differential expression of c-jun and junD in end-stage human cardiomyopathy

The proto-oncogenes c-jun and junD are closely related transcriptional factors with opposing actions on cell growth and division. Expression of c-jun rapidly increases as cells enter the cell cycle. Levels of c-jun are also increased in the early stages of experimental cardiac hypertrophy and failure but expression decreases with time. In contrast, junD accumulates in quiescent cells. Expression in end-stage cardiomyopathy has not been studied. Steady-state levels of c-jun and junD mRNA were determined in failing human myocardium (obtained at the time of cardiac transplantation) and in control myocardium from patients who died of noncardiac causes. Relative expression was normalized for glyceraldehyde-3-phosphate dehydrogenase expression. Levels of junD were almost four-fold depressed in myocardium from myopathic hearts (2.1 +/- 0.27, x +/- SE; n = 20) vs. the controls (7.7 +/- 1.1; n = 3). Levels of c-jun were similar in both myopathic and control hearts. Relative expression of beta-myosin heavy chain was the same in both myopathic and control hearts. Levels of junD were still found to be depressed in the myopathic hearts after normalization for myosin heavy chain gene expression. We conclude that c-jun and junD are differentially regulated in end-stage human cardiomyopathy with expression of junD being decreased while relative levels of c-jun mRNA remain unchanged. Further studies are needed to determine the role of junD down-regulation in the development and/or maintenance of the abnormalities present in end-stage heart disease.

All-trans-retinoic acid blocks cell cycle progression of human ovarian adenocarcinoma cells at late G1

We prepared single cell clones from two ovarian carcinoma cell lines, CA-OV3 and SK-OV3, and analyzed the effect of all-trans-RA treatment on cell division, DNA synthesis, and cell cycle stage distribution of these single cell clones. Our results show that despite the well-known heterogeneous nature of these cell lines, all single cell clones of SK-OV3 cells are resistant to the growth inhibitory effects of all-trans-RA. In contrast, all single cell clones of CA-OV3 cells were growth inhibited by all-trans-RA. However, the extent of growth inhibition did vary somewhat from clone to clone. Additional studies employing flow cytometry showed that all-trans-RA blocked CA-OV3 cell cycle progression in the G1 stage. Finally, all-trans-RA was able to inhibit G1 progression in growth-arrested CA-OV3 cells following stimulation with fetal bovine serum, insulin, IGF-1, or estrogen. Since each of these growth factors is known to act via distinct signal transduction pathways, our results suggest that all-trans-RA blocks G1 progression by targeting a downstream process or event which occurs at a point after the insulin/IGF-1, estrogen, and serum signal transduction pathways converge.

Differential role of homologous positively charged amino acid residues for ligand binding in retinoic acid receptor alpha compared with retinoic acid receptor beta

The diverse biological actions of retinoic acid (RA) are mediated by retinoic acid receptors (RARs) and retinoid X receptors. Although it has been suggested that the ligand binding domains (LBDs) of RARs share the same novel folding pattern, many RAR subtype-specific agonists and antagonists have been synthesized demonstrating that the LBD of each RAR subtype has unique features. We have examined the role of several positively charged amino acid residues located in the LBD of RARalpha in RA binding. These results are compared with previously published data for the homologous mutations in RARbeta. Lys227 of RARalpha does not appear to be important for RA binding or RA-dependent transactivation, whereas the homologous residue in RARbeta, Lys220, plays an important synergistic role with Arg269 in these two activities. In addition, Arg276 of RARalpha, like its homologous residue Arg269 of RARbeta, was found to play an important role in the binding of RA most likely by interacting with the carboxylate group of RA. However, the orientation of and electronic environment associated with Arg276 in RARalpha appears to be different from that of Arg269 in RARbeta, thus contributing to the uniqueness of the ligand binding pocket of each receptor.

Identification of sulfhydryl-modified cysteine residues in the ligand binding pocket of retinoic acid receptor beta

The diverse biological functions of retinoic acid (RA) are mediated through retinoic acid receptors (RARs) and retinoid X receptors. RARs contain a high affinity binding site for RA which is sensitive to treatment with sulfhydryl modification reagents. In an attempt to identify which Cys residues are important for this loss of binding, we created three site-specific RARbeta mutants: C228A, C258A, and C267A. The affinity for RA of all three mutant receptors was in the range of that of the wild type protein, suggesting that none of these Cys residues are critical for RA binding. Rather, these modified Cys residue(s) function to sterically hinder RA binding; however, the modified Cys residues critical for the inhibition of binding differ depending on the reagent employed. Only modification of Cys228 is necessary to inhibit RA binding when RARbeta is modified by reagents which transfer large bulky groups while both Cys228 and Cys267 must be modified when a small functional group is transferred. These data suggest that both Cys228 and Cys267 but not Cys258 lie in the ligand binding pocket of RARbeta. However, Cys228 lies closer to the opening of the RARbeta ligand binding pocket whereas Cys267 lies more deeply buried.

Overexpression of Mxi1 inhibits the induction of the human ornithine decarboxylase gene by the Myc/Max protein complex

We have previously shown that the Myc/Max protein complex plays a role in the growth-associated expression of the human ornithine decarboxylase gene. Mxi1 and Mad, novel Max-associated proteins have been identified and shown to form heterodimers with Max which bind efficiently to the Myc/Max consensus recognition sequence, CACGTG, in vitro. However, formation of Max/Mxi1 or Max/Mad heterodimers results in a reduction in Myc/Max dependent transcriptional activation of reporter plasmid constructs containing the consensus element. In light of the evidence that ODC is transcriptionally regulated in vitro and in vivo by the Myc/Max protein complex and the potential role of Mxi1 and Mad as antagonists of Myc transactivation activity, we set out to determine if one of these Max associated proteins, Mxi1, could affect the regulation of ODC expression by Myc/Max and if this regulation was correlated to growth status. Our results show that overexpression of Mxi1 does in fact inhibit ODC gene expression in a dose-dependent manner both in vivo and in vitro. In addition, evidence is presented which shows that levels of Mxi1 are up-regulated during long term quiescence and down-regulated following growth stimulation by serum. These results suggest that alterations in the levels of Max-associated proteins such as Mxi1 can modulate critical levels of functional Myc/Max protein complexes. This can alter transcriptional transactivation of Myc-regulated targets and as a consequence affect levels of genes essential for initiation and/or maintenance of growth.

Overexpression of both RAR and RXR restores AP-1 repression in ovarian adenocarcinoma cells resistant to retinoic acid-dependent growth inhibition

Retinoids including retinoic acid (RA) have been demonstrated to be effective growth inhibitors of a number of human cancer cell lines including ovarian adenocarcinoma cells. To begin to determine the mechanism of action by which RA inhibits the growth of ovarian carcinoma cells, we have examined AP-1 activity in two representative cell lines: CaOV-3 a RA-sensitive cell line and SK-OV-3 a RA-resistant cell line. AP-1 activity was found to be inhibited by 50% upon RA treatment of the RA-sensitive cells while there was no change in AP-1 activity following RA treatment of the RA-resistant cells. Maximal inhibition of AP-1 activity could be achieved in the RA-resistant SK-OV-3 cells by overexpression of any one of the three retinoic acid receptor (RAR) subtypes in conjunction with retinoid X receptor (RXR) alpha. This inhibition of AP-1 activity was nearly comparable to that of the RA-sensitive cells. A similar change in AP-1 complex formation in vitro has also been observed. These results suggest that one mechanism by which RA inhibits growth of RA-sensitive ovarian carcinoma cells is by repressing AP-1 activity. Moreover, in the RA-resistant cells the RAR/RXR signalling pathway leading to inhibition of AP-1 activity is impaired however overexpression of one of the RAR subtypes along with RXR alpha is sufficient to restore this pathway.

Activation of a cell-cycle-regulated histone gene by the oncogenic transcription factor IRF-2

The human histone H4 gene FO108 is regulated during the cell cycle with a peak in transcription during early S phase. The cell-cycle element (CCE) required for H4 histone activation is a sequence of 11 base pairs that binds a protein factor in electrophoretic mobility shift assays that has been designated histone nuclear factor M (HiNF-M). Here we report the purification of HiNF-M, and show it to be a protein of relative molecular mass (M(r)) 48K that is identical to interferon (IFN) regulatory factor 2 (IRF-2), a negative transcriptional regulator of the IFN response. Recombinant IRF-2 (as well as the related protein IRF-1 (ref. 5)) binds the CCE specifically and activates transcription of this H4 histone gene. IRF-2 has been shown to have oncogenic potential, and our results demonstrate a link between IRF-2 and a gene that is functionally coupled to DNA replication and cell-cycle progression at the G1/S phase transition.

Endothelin-1 induces gene expression through stimulation of endothelin type A receptors in normal rat kidney cells

RNA blots of total cellular RNA isolated from quiescent and endothelin (ET-1)-stimulated normal rat kidney (NRK) cells demonstrated that ET-1 induced the expression of c-jun, jun B, and c-fos mRNA in a time-dependent manner with maximal expression of mRNA by 1 hr after the addition of ET-1. Five hundred picomolar ET-1 was sufficient to induce maximal mRNA expression. These data agreed with saturation experiments which demonstrated that maximal binding of [125I]ET-1 was achieved at concentrations greater than 100 pM. The Kd and Bmax values for [125I]ET-1 binding to NRK membranes were 20.5 pM and 22.2 fmol/mg protein, respectively. Competition experiments for the binding of [125I]ET-1 to NRK membranes demonstrated that ET-1 was a more potent inhibitor (Ki = 0.047 nM) than ET-3 (Ki = 10.8 nM). No specific binding of [125I]ET-3 (40 or 500 pM) to NRK membranes could be observed. The expression of c-jun, jun B, and c-fos mRNA was inhibited by the endothelin type A receptor (ET)-selective antagonist, BQ-123. Thus, these data demonstrate that ET-1 mediates the expression of immediate response gene mRNA in NRK cells via the ETA receptor. ET-1 stimulation of NRK cells also upregulated EGF receptors, providing a possible mechanism for ET-1 complementation of epidermal growth factor (EGF) mitogenicity in NRK cells.

Tyrosine protein kinase expression in long-term quiescent WI-38 cells following growth factor simulation

We have used the WI-38 cell long-term quiescent model system to study the regulation of cell cycle progression at the molecular level. By modulating the length of time that WI-38 cells are density arrested, it is possible to proportionately alter the length of the prereplicative or G-1 phase which the cell traverses after growth factor stimulation in preparation for entry into DNA synthesis. Stimulation of long- and short-term density arrested WI-38 cells with different growth factors or higher concentrations of individual growth factors does not alter the time required by long-term cells to enter S after stimulation. However, the time during the prereplicative period for which these growth factors are needed is different. Long-term quiescent WI-38 cells require EGF to traverse the G-0/G-1 border but do not need and apparently cannot respond to IGF-1 during the first 10 h after EGF stimulation, the length of the prolongation of the prereplicative phase. This suggests that EGF stimulation of long-term quiescent WI-38 cells initiates a series of molecular events which make these cells "competent" to respond to the "progression" growth factor, IGF-1. In light of the well-established role of protein tyrosine kinases in signal transduction, we set out to identify, clone, and analyze the expression of receptor and non-receptor tyrosine kinases which potentially could play a role during the prolongation of the prereplicative phase in making the long-term quiescent WI-38 cells competent to respond to IGF-1. We obtained 49 clones representing 11 different receptor and non-receptor type protein tyrosine kinases. Analysis of expression of these clones revealed a variety of different patterns of expression. However, the most striking pattern was exhibited by IGF-1 receptor. Our results suggest that induction of IGF-1 receptor mRNA by EGF may be an important event in the establishment of competence by EGF in long-term density arrested WI-38 cells.

Alteration in the retinoid specificity of retinoic acid receptor-beta by site-directed mutagenesis of Arg269 and Lys220

Retinoic acid receptor-beta (RAR-beta) specifically binds retinoic acid (RA) and functions as a RA-inducible transcriptional regulatory factor. Simultaneous mutation of Arg269 and Lys220 of RAR-beta to Ala results in a dramatic reduction in both transactivation and affinity for RA along with creating a RA concentration-dependent dominant negative mutant. In this report, we found that mutation of these two amino acid residues singly and simultaneously to Gln results in mutant RAR-beta s, each displaying a more dramatic reduction in transactivation and affinity for RA than their corresponding Ala mutant, with the R269Q more profoundly affected than K220Q. Furthermore, we examined both the Ala and Gln mutants for their ability to transactivate and bind two other retinoids with different functional end groups (all-trans-retinol and all-trans-retinal). Mutation of Lys220 to either an Ala or a Gln favors transactivation and binding of retinal, while mutation of either Lys220 or Arg269 to Gln favors retinol transactivation and binding. Taken together, these results suggest that Arg269 and Lys220 lie within the ligand binding pocket of RAR-beta and Lys220 lie within the ligand binding pocket of RAR-beta and that these two amino acid residues play an important role in determining retinoid specificity most likely by directly interacting with the carboxylate group of RA.

Modulation of limb bud chondrogenesis by retinoic acid and retinoic acid receptors

An excess of retinoic acid (RA) in the mouse embryo in utero produces hypochondrogenesis and severe limb bone deformities. Since one of the RA receptors--RAR-beta 2, is specifically induced in the limb bud cells upon treatment of embryos with teratogenic doses of RA, we investigated if this receptor played a role in teratogenesis by regulating the process of chondrogenesis. In micromass cultures of mouse limb bud mesenchymal cells, we found that a downregulation of RAR-beta 2 as well as several other RAR isoforms by supplementation of the culture medium with specific oligodeoxynucleotides stimulated chondrogenesis: cartilage nodule number, sulfated proteoglycans, and synthesis of collagen type IIB were all enhanced in a dose-dependent manner. However, only the antisense RAR-beta 2 probe efficiently prevented the strong inhibitory effects of exogenous RA on chondrogenesis in these cells. The data suggest that the RAR-RA complexes play a role in position-dependent patterning of the limb skeleton in normal development and that, in particular, RAR-beta 2 serves to prevent the mesenchymal cells from expressing their chondrogenic bias. Our results further strengthen the argument that RA-dependent elevation in RAR-beta 2 levels plays a unique role in RA-induced teratogenesis.

Effect of 9-cis-retinoic acid on growth and RXR expression in human breast cancer cells

A number of studies have demonstrated the ability of retinoic acid (RA) to inhibit the growth of estrogen receptor-positive (ER+) human breast cancer cell lines. The precise mechanism of growth inhibition is not known. However, the biological effects of RA in other model systems have been shown to be mediated via the nuclear retinoic acid receptors (RARs) and the retinoid X receptors (RXRs). While several laboratories have examined the expression of RARs in various breast cancer cell lines, no information is available concerning the role of the RXRs and 9-cis-RA, the natural ligand of RXRs, in the response of breast cancer cells to RA. Using a representative panel of breast cancer cell lines, we determined the effect of 9-cis-RA on growth and cell cycle stage distribution, analyzed steady-state mRNA levels of RXR-alpha, -beta, and -gamma, and determined the effect of all-trans-RA and 9-cis-RA on RXR expression. Our results show that: (1) the growth of ER+/RA-sensitive breast cancer cells is inhibited by treatment with 9-cis-RA by blocking entry into S phase; (2) both ER+/RA-sensitive and ER-/RA-resistant breast cancer cell lines express RXR-alpha and RXR-beta mRNAs but not RXR-gamma; however, levels of these transcripts did not correlate with the RA response; and (3) levels of RXR-alpha and RXR-beta mRNA were not significantly altered following treatment with either all-trans-RA or 9-cis-RA. These results suggest that the mechanism responsible for the retinoid sensitivity of breast cancer cells does not involve transcriptional modulation of the RXRs by RA.

Retinoids as teratogens

Vitamin A is a necessary nutrient in the diet. However, excessive doses of retinoids by pregnant women result in teratogenesis. In this chapter, we initially discuss the occurrence and characteristics of fetal malformations associated with maternal ingestion of natural and synthetic retinoids in both experimental animals and humans. We then turn to an examination of the pharmacology of teratogenic retinoids, focusing on structure-function relationships and pharmacokinetics. Finally, we review the current literature on the molecular mechanism of action of teratogenic doses of retinoids and the role of the retinoic acid receptors and other target genes in this process.

Arginine 132 of cellular retinoic acid-binding protein (type II) is important for binding of retinoic acid

Cellular retinoic acid-binding protein type II (CRABP-II) is one of two small molecular weight, cytosolic proteins which specifically bind retinoic acid (RA). Crystallographic and site-directed mutagenesis studies of several related proteins have indicated that either one or two conserved amino acid residues, homologous to positions Arg111 and Arg132 of CRABP-II, are important for the binding of the hydrophobic ligand. In this report we have prepared site-directed mutations of these two positions of CRABP-II, Arg111 and Arg132, as well as Lys82 to determine the role of these residues in the binding of RA. Recombinant wild type and mutant CRABP-II proteins were expressed and purified, and the affinity for retinoids was determined by fluorometric titration and binding of 3H-labeled compounds. K82A displayed an identical Kd for all-trans-RA as wild type CRABP-II and the Kd for all-trans-RA of R111A was only slightly higher. On the other hand, the two Arg132 mutants, R132A and R132Q, of CRABP-II demonstrated undetectable binding of all-trans-RA. Taken together these data demonstrate that Arg132 is a critical amino acid residue for the binding of RA by CRABP-II.

Regulation of human ornithine decarboxylase expression following prolonged quiescence: role for the c-Myc/Max protein complex

WI-38 cells can remain quiescent for long periods of time and still be induced to reenter the cell cycle by the addition of fresh serum. However, the longer these cells remain growth arrested, the more time they require to enter S phase. This prolongation of the prereplicative phase has been localized to a point early in G1, after the induction of "immediate early" G1 genes such as c-fos and c-jun but before maximal expression of "early" G1 genes such as ornithine decarboxylase (ODC). Understanding the molecular basis for ODC mRNA induction can therefore provide information about the molecular events which regulate the progression of cells out of long-term quiescence into G1 and subsequently into DNA synthesis. Studies utilizing electrophoretic mobility shift assays (EMSA) of nuclear extracts from short- and long-term quiescent WI-38 cells identified a region of the human ODC promoter at -491 bp to -474 bp which exhibited a protein binding pattern that correlated with the temporal pattern of ODC mRNA expression. The presence of a CACGTG element within this fragment, studies with antibodies against c-Myc and Max, the use of purified recombinant c-Myc protein in the mobility shift assay, and antisense studies suggest that these proteins can specifically bind this portion of the human ODC promoter in a manner consistent with growth-associated modulation of the expression of ODC and other early G1 genes following prolonged quiescence. These studies suggest a role for the c-Myc/Max protein complex in regulating events involved in the progression of cells out of long-term quiescence into G1 and subsequently into S.

Transcriptional regulation of c-Jun expression during late G1/S in normal human cells is lost in human tumor cells

Previous results from our laboratory have identified a second peak in steady state levels of c-jun mRNA (in addition to the immediate early induction) which occurs at the G1/S border in WI-38 normal human diploid fibroblasts. The present studies were undertaken in an attempt to determine (1) the molecular mechanism responsible for the expression of c-jun in late G1/S, (2) the relationship between this second peak of c-jun mRNA expression and the induction of DNA synthesis and (3) whether this cell cycle specific c-jun expression is deregulated in transformed cells. Our results show that the second peak in steady state levels of c-jun mRNA is the result of new transcription during late G1 and not altered stability of the c-jun mRNA transcribed during G1. We also show that this second peak of expression still occurs even when DNA synthesis is inhibited by either hydroxyurea or aphidicolin. Thus, the second peak precedes and is independent of DNA synthesis. Finally, we find that while two other normal human fibroblast cell lines exhibit a second peak of c-jun mRNA during late G1/S, c-jun expression is not cell cycle-regulated but rather is constitutively expressed in a number of distinct transformed cell lines. Since events occurring throughout G1 are known to regulate cell growth, our results suggest that the extent of regulation of c-jun expression during G1 may affect molecular events which ultimately lead to altered growth control as a result of cellular transformation.

Induction of c-fos and c-jun mRNA at the M/G1 border is required for cell cycle progression

The proto-oncogenes c-fos and c-jun have been shown in numerous model systems to be induced within minutes of growth factor stimulation, during the G0/G1 transition. In this report we use the mitotic shake-off procedure to generate a population of highly synchronized Swiss 3T3 cells. We show that both of these immediate-early, competence genes are also induced during the M/G1 transition, immediately after completion of mitosis. While c-fos mRNA levels drop to undetectable levels within 2 hr after division, c-jun mRNA levels are maintained at a basal level which is approximately 30% maximum throughout the remainder of G1. In order to access the functional significance of these patterns of c-fos and c-jun expression, antisense oligodeoxynucleotides specific to c-fos or c-jun were added to either actively growing Swiss 3T3 cells or mitotically synchronized cells, and their ability to inhibit DNA synthesis and cell division determined. Our results show that treatment of Swiss 3T3 cells with either c-fos or c-jun antisense oligodeoxynucleotides, while actively growing, during mitosis, or in early G1, results in a reduction in ability to enter S and subsequently divide. This was also true if Swiss 3T3 cells were treated during mid-G1 with c-jun antisense oligodeoxynucleotides. These results demonstrate that the regulation of G1 progression following mitosis is dependent upon the expression and function of the immediate-early, competence proto-oncogenes c-fos and c-jun.

Arg269 and Lys220 of retinoic acid receptor-beta are important for the binding of retinoic acid

Retinoic acid receptors, RARs, are retinoic acid (RA)-inducible transcriptional regulatory proteins which transduce the RA signal at the level of gene expression via a retinoic acid response element. Three subtypes of RARs have been described termed RAR-alpha, RAR-beta, and RAR-gamma. RARs, like other members of the steroid/thyroid hormone receptor family, are composed of six structurally distinct domains, one of which is responsible for binding RA. No structural information is available concerning the nature of the amino acids which are responsible for binding of RA within the ligand binding domain of any RAR. In this report, the role of 2 positively charged amino acids of RAR-beta for binding of RA, Arg269 and Lys220, was examined using site-directed mutagenesis. When compared with wild type RAR-beta, mutation of either Arg269 or Lys220 singly to the small neutral amino acid Ala had only a small effect on both the EC50 value in all-trans-RA and 9-cis-RA transactivation assays and the apparent Kd for all-trans-RA. However, mutation of both of these positively charged amino acids simultaneously to Ala caused a 500- and 100-fold elevation in the EC50 for all-trans-RA and 9-cis-RA, respectively, compared with that of wild type RAR-beta. Similarly, the apparent Kd for all-trans-RA was increased 580-fold when that of the double mutant was compared with that of the wild type RAR-beta. Furthermore, this double mutant RAR-beta acted as a dominant negative mutant when transfected with wild type RAR-alpha, -beta, or -gamma in a RA concentration-dependent fashion. Taken together these data demonstrate the importance of both Arg269 and Lys220 of RAR-beta for the binding of RA, possibly by interacting with the negatively charged carboxyl group of RA.

Teratogenesis by retinoic acid analogs positively correlates with elevation of retinoic acid receptor-beta 2 mRNA levels in treated embryos

Retinoic acid (RA) plays an important role during normal embryogenesis, however high doses of RA are teratogenic. Retinoic acid receptor-beta 2 (RAR-beta 2) mRNA and protein levels were previously demonstrated to undergo rapid elevation in susceptible tissues after treatment with teratogenic doses of RA. In this report we compared the effects of a number of retinoids, which represent a wide variety of chemical structures and which differ in their teratogenic potencies, on RAR-beta 2 mRNA levels in mouse embryos 6 hr after treatment. Retinoid treatments which result in a high incidence of limb defects elevated RAR-beta 2 mRNA levels similarly (10-14 fold in the limb buds, 4-8 fold in the head, and 2-4 fold in the remainder of the body). On the other hand, retinoid treatments which cause a low or no incidence of limb defects resulted in minor changes in RAR-beta 2 mRNA levels in each embryonic region. Therefore, a strong positive correlation was found between the elevation of RAR-beta 2 mRNA levels and the retinoids which produce limb defects. This provides further evidence that an elevation of RAR-beta 2 mRNA levels, and subsequently protein levels, is an important event involved in mediating the effects of RA during dysmorphogenesis.

WI-38 cell long-term quiescence model system: a valuable tool to study molecular events that regulate growth

A number of cell culture model systems have been used to study the regulation of cell cycle progression at the molecular level. In this paper we describe the WI-38 cell long-term quiescence model system. By modulating the length of time that WI-38 cells are density arrested, it is possible to proportionately alter the length of the prereplicative or G-1 phase which the cell traverses after growth factor stimulation in preparation for entry into DNA synthesis. Through studies aimed at understanding the cause and molecular nature of the prolongation of the prereplicative phase, we have determined that gene expression plays an important role in establishing growth factor "competence" and that once the cell becomes "competent" there is a defined order to the molecular events that follow during the remainder of G-1. More specifically, we have determined that the prolongation represents a delay in the ability of long term quiescent cells to become fully "competent" to respond to growth factors which regulate progression through G-1 into S. This prolongation appears to occur as a result of changes during long term quiescence in the ability of immediate early G-1 specific genes (such as c-myc) to activate the expression of early G-1 specific genes (such as ornithine decarboxylase). While ODC is the first and thus far only growth associated gene identified as a target of c-myc (and the Myc/Max protein complex), it is likely that further studies in this model system will reveal other early G-1 growth regulatory genes. We anticipate that future follow-up studies in this model system will provide additional valuable information about the function of growth-regulatory genes in controlling growth factor responsiveness and cell cycle progression.

A sustained elevation in retinoic acid receptor-beta 2 mRNA and protein occurs during retinoic acid-induced fetal dysmorphogenesis

We have previously shown that oral treatment of pregnant mice with all-trans retinoic acid (RA) at doses which cause 100% fetal dysmorphogenesis results in a rapid elevation in the mRNA of one specific isoform of the RA receptor-beta, RAR-beta 2, in susceptible embryonic regions. To further investigate the involvement of RAR-beta 2 mRNA in teratogenesis, we have examined its expression in mouse embryos exposed to marginal/nonteratogenic and teratogenic dosing regimens of both 13-cis RA and all-trans RA. We have found that the mere elevation in embryonic RAR-beta 2 mRNA levels and free retinoid levels is not sufficient to result in dysmorphogenesis. Rather, retinoid-induced dysmorphogenesis of embryos appears to occur only when RAR-beta 2 mRNA and unbound retinoid levels remain elevated for at least 6-9 h following retinoid treatment resulting in a significant and prolonged elevation in RAR-beta protein levels.

Regulation of human ornithine decarboxylase expression by the c-Myc.Max protein complex

The presence of a CACGTG element within a region of the human ornithine decarboxylase (ODC) promoter located at -491 to -474 base pairs 5' to the start site of transcription suggested that the c-Myc.Max protein complex may play a role in the regulation of ODC expression during growth. Electrophoretic mobility shift assays and methylation interference analysis showed that the nuclei of WI-38 cells expressing ODC contained proteins that bound to this region of the ODC gene in a manner that correlated with growth-associated ODC expression. Also, use of antibodies against c-Myc and Max and purified recombinant c-Myc and Max protein in the electrophoretic mobility shift assay confirmed that these proteins can specifically bind this portion of the human ODC promoter. Transient transfection studies showed that increase in the level of c-Myc and/or Max led to a significant enhancement of expression of a human ODC promoter-CAT reporter construct. Moreover, treatment of actively growing WI-38 cells with an antisense oligomer to c-Myc reduced the amount of endogenous protein complex formed and the amount of endogenous ODC mRNA expressed. These studies show that the c-Myc.Max protein complex plays a role in the transcriptional regulation of human ODC in vivo.

Induction of RAR-beta 2 gene expression in embryos and RAR-beta 2 transactivation by the synthetic retinoid Ro 13-6307 correlates with its high teratogenic potency

Vitamin A (retinol), its metabolite all-trans retinoic acid (RA), and many synthetic analogs (retinoids) express variable potencies as teratogens. Although biological activities of retinoids are mediated by nuclear RA receptors (RARs) and retinoid X receptors (RXRs), it is not known if any of these receptors mediate teratogenicity, and if the potency also depends on the nature of the ligand-receptor interactions. Previous evidence has implicated that one specific isoform, RAR-beta 2, does play a role in mediating retinoid teratogenicity. Here, we employed an aromatic retinoid with a triene side chain, Ro 13-6307, to study its interactions with RAR-beta 2 since its teratogenicity is much higher and its accessibility to the embryo is much lower than RA. A fully teratogenic dose of Ro 13-6307 (10 mg-kg) given to pregnant mice preferentially elevated the level of RAR-beta 2 mRNA in susceptible embryonic regions (maximal induction, 10- to 12-fold above control in limb buds) in a manner comparable to a fully teratogenic dose of all-trans RA (100 mg-kg). Using the RAR-beta 2 promoter linked to a reporter gene in cotransfection experiments, the efficacy of Ro 13-6307 and RAR-beta 2 in transcription transactivation was found to be 30-40 times greater than all-trans RA. Since the teratogenic potency of Ro 13-6307 is estimated from a previous study to be 44-fold greater than all-trans RA, we suggest that the teratogenicity of this synthetic retinoid is generally proportional to its ability to enhance receptor function.

Insulin binding and degradation by rat liver Kupffer and endothelial cells

The liver is the major site of insulin metabolism. Previous studies have suggested that hepatocytes were chiefly responsible for this activity, while contributions of Kupffer and other nonparenchymal liver cells remained controversial. In this study, we compared 125I-insulin binding and degradation by rat hepatocytes with insulin binding and degradation by sinusoidal Kupffer and endothelial cells. Kupffer cells were separated from endothelial cells by centrifugal elutriation. Hepatocytes had approximately 3.5 times more insulin binding sites than Kupffer cells and approximately eight times more binding sites than endothelial cells. In addition, wheat germ agglutinin (WGA)-purified solubilized receptors from all three cell types bound insulin in proportions similar to whole cells. Moreover, all three cell types were shown with a ribonuclease (RNase) protection assay to express insulin receptor mRNA. Hepatocytes degraded approximately four times more insulin than Kupffer cells, while endothelial cells degraded only negligible amounts of insulin. Based on morphometric data available in the literature, we estimated that nonparenchymal cells could account for approximately 10% to 15% of hepatic insulin degradation. We concluded that rat hepatocytes, Kupffer cells, and endothelial cells all have specific insulin receptors, and that nonparenchymal cells play a small but significant role in insulin degradation.

Epidermal growth factor receptors lose ligand binding ability as WI-38 cells progress from short-term to long-term quiescence

WI-38 cells, density arrested for short periods of time, can be stimulated to re-enter the cell cycle by epidermal growth factor (EGF) alone. However, cells density arrested for longer periods have a prolonged prereplicative phase when serum stimulated and cannot be stimulated by EGF alone. Radio-ligand binding studies performed on WI-38 cells showed that actively growing cells bind [125I]EG at relatively low levels that increase to a maximum as the cells become contact inhibited. As the cells enter a state of deeper quiescence, EGF binding falls to one-third to one-fifth the short-term growth arrested levels, remaining constant thereafter. The EGF-receptor complexes internalize more slowly in long-term growth arrested cells, and the rate of ligand association to the receptor is lower than short-term growth arrested cells. The amount of EGF receptor protein in lysates of equal numbers of both short- and long-term quiescent cells remains the same. These results suggest that the failure of long-term growth arrested cells to respond to EGF is not due to dramatic changes in the amount of receptor protein during prolonged quiescence but more likely to an alteration in the ability of these receptors to bind ligand and/or activate the EGF signal transduction pathway.

Correlations of RAR isoforms and cellular retinoid-binding proteins mRNA levels with retinoid-induced teratogenesis

Retinoic acid (RA) plays an important role in normal embryogenesis; however, excessive doses are teratogenic. At present, the molecular mechanisms responsible for these effects of RA are not well understood. The action of retinoids are believed to be mediated by two classes of proteins, nuclear receptors (retinoic acid receptors [RARs] and retinoid X receptors [RXRs]) and small cellular retinol-binding and retinoic acid-binding proteins (CRBP-I, CRBP-II, CRABP-I and CRABP-II). Teratogenic doses of RA increase the level of RAR-beta 2 mRNA, RAR-alpha 2 mRNA, CRBP-I mRNA and CRABP-II mRNA in mouse conceptuses and embryos. The elevation in the level of only RAR-beta 2 mRNA correlates with the target tissues, as well as developmental stages that are sensitive to the teratogenic effects of RA. In addition, we have screened a few other natural and synthetic retinoids with similar results. These results are consistent with the possibility that RAR-beta 2 may mediate at least some of the effects of retinoids during abnormal development.

Cycloheximide protection against actinomycin D cytotoxicity

Pretreatment plus concomitant treatment with 10 micrograms/ml cycloheximide protected Chinese hamster ovary cells and Swiss 3T3 cells against the cytotoxicity of actinomycin D. The cycloheximide treatment reduced the intracellular concentration of actinomycin D by reducing the level of actinomycin D bound to the acid precipitable fraction of the cell. Levels of unbound actinomycin D were unaffected by cycloheximide, indicating that the plasma membrane permeability to AD was not reduced. Actinomycin D inhibited total transcription but did not reduce cytoplasmic levels of rRNA nor of most tested mRNA; however, cytoplasmic levels of c-myc mRNA were reduced below detectability. Cycloheximide treatment further inhibited total transcription and had no effect on cytoplasmic levels of rRNA nor of most tested mRNA. Cytoplasmic levels of c-myc were elevated by cycloheximide and remained so even in the presence of actinomycin D. These data suggested that a reduction in cytoplasmic levels of short lived, essential mRNA, such as c-myc mRNA, was one lethal lesion of actinomycin D. Furthermore, cycloheximide's protection may result, in part, from its ability to stabilize and/or elevate cytoplasmic levels of these mRNA, thus counteracting their depletion by actinomycin D. Protection may also result from the cycloheximide-induced reduction of actinomycin D bound to the acid precipitable fraction of the cells.

Use of antisense oligomers to study the role of c-jun in G1 progression

In actively proliferating Swiss 3T3 fibroblasts, expression of the protooncogene c-jun is maximally induced early in G1, immediately after completion of mitosis. Within 2 hours, c-jun mRNA levels drop to a basal amount that is approximately 30% of the maximum. This is maintained throughout the remainder of G1. To access the functional implications of this pattern of c-jun expression, antisense oligomers specific to c-jun were added to either actively proliferating or synchronized Swiss 3T3 cells, and their ability to inhibit DNA synthesis and division was determined. Our results show that if Swiss 3T3 cells are treated with anti-c-jun while actively growing or at any time during G1 after completion of mitosis, they exhibit a reduced ability to enter S-phase and subsequently divide. These results demonstrate that the regulation of G1 progression following mitosis depends on the expression and function of the protooncogene c-jun.

The Jun family members, c-Jun and JunD, transactivate the human c-myb promoter via an Ap1-like element

The c-myb protooncogene, which is preferentially expressed in hematopoietic cells at the G1/S boundary of the cell cycle, encodes a transcriptional activator that functions via DNA binding. The regulatory mechanisms governing this specific pattern of expression are not fully understood, although human c-myb expression appears to be positively autoregulated via myb-binding sites in the 5'-flanking region of the c-myb gene (Nicolaides, N. C., Gualdi, R., Casadevall, C., Manzella, L., and Calabretta, B. (1991) Mol. Cell. Biol. 11, 6166-6176). To determine the contribution of other transcription regulators such as JUN family members in the control of c-myb expression, transient expression assays were carried out which revealed a 6- to a 15-fold enhancement by c-Jun and JunD, but not JunB, in chloramphenicol acetyltransferase reporter gene expression driven by different segments of the human c-myb 5'-flanking region. An Ap1-like element located at nucleotide -149 from the c-myb initiation site appears to be required for this transactivation upon binding to a nuclear protein complex containing c-Jun and JunD, since site-directed mutations of this Ap1-like element abolished c-Jun and JunD binding and transactivation. Exposure of phytohemagglutinin-stimulated peripheral blood mononuclear cells to c-jun and junD antisense oligodeoxynucleotides resulted in a 46 and 43% inhibition of T-lymphocyte proliferation that was accompanied by a decrease in c-myb mRNA levels as compared with sense-treated cultures. Because T-lymphocytes induced to proliferate express c-jun and junD before c-myb, these data suggest a mechanism whereby c-Jun and JunD contribute to the transcriptional activation of c-myb that, in turn, is maintained at the G1/S transition and during S phase by positive autoregulation.

Alpha-1 antitrypsin response of stimulated alveolar macrophages

Alpha-1 antitrypsin messenger RNA (A1AT mRNA) was determined in alveolar macrophages and in peripheral blood monocytes of healthy individuals using a sensitive RNase protection assay. Determinations were made of bacterial lipopolysaccharide (LPS) stimulated and unstimulated cells. We found that the amount of A1AT mRNA increased 7.3 and 14 times after 4 h of incubation with LPS for monocytes and macrophages, respectively (relative to total RNA). The increase was 12.3 and 14.8 times, respectively, when expressed as increase per cell. In both cell types there was wide interindividual variation in LPS response: 2-36 and 5-12 times for monocytes and macrophages, respectively. The possible significance of A1AT production of monocytes and macrophages may be the local control of granulocytic proteases such as elastase and cathepsin G.

Mouse conceptuses have a limited capacity to elevate the mRNA level of cellular retinoid binding proteins in response to teratogenic doses of retinoic acid

In these studies, we wished to determine the effect of teratogenic doses of retinoic acid on the expression of cellular retinoic acid binding protein I (CRABP-I) mRNA, cellular retinoic acid binding protein II (CRABP-II) mRNA, cellular retinol binding protein I (CRBP-I) mRNA, and cellular retinol binding protein II (CRBP-II) mRNA in mouse conceptuses. Levels of CRABP-II mRNA and CRBP-I mRNA were modestly elevated (2.5-fold and 1.5-fold, respectively) in 9-day gestation conceptuses following treatment of dams with 100 mg/kg b.w. of retinoic acid. These levels were elevated by 6 hr following treatment and remained elevated until 48 and 24 hr, respectively. Two other retinoids, etretinate and retinoyl beta-glucuronide, also moderately elevated CRABP-II mRNA and CRBP-I mRNA levels in conceptuses. In contrast, the levels of CRABP-I mRNA in the conceptuses remained unaffected by treatment with any of these three retinoids. These results demonstrate that conceptuses have a limited capacity to elevate the cellular retinoid binding proteins mRNA levels and presumably the synthesis of their respective proteins in response to high, teratogenic doses of retinoic acid. As a result, an excess of free retinoic acid becomes available to the nuclear retinoic acid receptors, which may lead to inappropriate gene expression and eventual maldevelopment.

Retinoic acid receptor beta 2 mRNA is elevated by retinoic acid in vivo in susceptible regions of mid-gestation mouse embryos

Many of the biological effects of retinoic acid are mediated by its nuclear receptors (RAR-alpha, RAR-beta, and RAR-gamma), and each of these three receptors exist in multiple isoforms. As a first step to identify if any of the receptor isoforms are involved in dysmorphogenesis which is induced in mouse embryos after treatment with retinoic acid (RA), we examined the levels of mRNA of several isoforms of each RAR in the limb buds and other embryonic regions of normal and RA-treated embryos. Within 3 to 6 hr after treatment of mice on day 11 of gestation with RA, RAR-beta 2 mRNA levels in the whole embryo increased 7-fold while both RAR-alpha 2 and RAR-gamma 1 mRNA levels were elevated only 2-fold. Since RA treatment of day 11 embryos especially produces limb defects in virtually every embryo, we next examined individual embryonic regions separately. Limb buds showed the highest elevations in RAR-beta 2 mRNA levels (12-fold) compared to a moderate elevation in the head/craniofacial region (8-fold) and a small elevation in the remainder of the body (4-fold). In contrast, RAR-alpha 2 and RAR-gamma 1 mRNA levels were elevated in all these tissues to a similar extent, which amounted to only about a 2-fold increase. Retinol, the precursor of RA in the embryo, was also capable of elevating RAR-beta 2 mRNA levels in the limb bud, but the increase was delayed, apparently indicating that metabolic conversion of retinol to RA preceded the effect on mRNA levels.(ABSTRACT TRUNCATED AT 250 WORDS)