Oestrogen-mediated phosphorylation and stabilization of BRCA2 protein in breast

Disease-associated BRCA2 mutations typically result in protein truncations that delete the phosphorylation-regulated S3291 BRCA2 domain that interacts with Rad51. BRCA2 hereditary breast cancers are usually ER(+), differing from BRCA1 hereditary cancers, which are usually ER(-). We studied BRCA2 protein expression and S3291 phosphorylation in normal breast tissues and in sporadic breast cancers and observed that BRCA2 is expressed and phosphorylated in normal breast and 10 ER(+) breast cancers but not in 10 ER(-) breast cancers. In order to study this correlation between ER and BRCA2 expression, we studied ER(+) breast cancer cell lines. We found that a rapid increase in BRCA2 S3291 phosphorylation occurs following 17-beta-oestradiol (E2) treatment. This increase seen in BRCA2 total and phospho-S3291 protein levels was found to be unaffected with cycloheximide pre-treatment, but decreased following tamoxifen, ICI 182,780 or roscovitine treatment. This suggests a requirement for ER and cdk (cyclin-dependent kinase) in mediating the increased protein levels. MCF7 cell cycle distribution analysis following E2, in both the presence and absence of roscovitine (a cdk inhibitor), did not demonstrate any changes during an 8 h period, which further supports our hypothesis that mitogenic effects of E2 are not predominant at early time points. Studies with MG132 proteasome inhibitor and siRNA to skp2 support a model in which skp2-mediated proteasomal degradation of BRCA2 rapidly degrades BRCA2 protein in the absence of hormone treatment, which likely inhibits this pathway. E2 was shown to improve survival of MCF7 cells upon radiation treatment and roscovitine partially reversed this effect. We have demonstrated that BRCA2 protein is specifically expressed in ER(+) breast cancers and are investigating a pathway that may show a link between E2 action and BRCA2 protein function in breast cancer.

Gene therapy for breast and ovarian cancer with BRCA1

As an initial step toward gene therapy for ovarian cancer, we conducted a Phase 1 trial to assess the pharmacokinetics and toxicity of intraperitoneal BRCA1sv retroviral vector therapy. Gene transfer and expression were documented by PCR, southern blot, RT-PCR and nuclease protection assays. Pharmacokinetics were assessed by PCR and southern blots detecting vector DNA, and toxicity was evaluated by clinical exam and fluid analysis. Three of twelve patients developed an acute sterile peritonitis which spontaneously resolved within 48 hours. Plasma and peritoneal antibodies to the retroviral envelope protein were detected only in patients treated with the highest dose levels but not in others, despite repeat dosing for an interval of up to four months. Eight patients showed stable disease for 4 to 16 weeks. Three patients showed tumor reduction with diminished miliary tumor implants at reoperation (two patients) and radiographic shrinkage of measurable disease (one patient). Ovarian cancer may provide an imporant model for retroviral gene therapy studies due to vector stability, minimal antibody response, and access to tumor by intraperitoneal therapy.

Preclinical studies of a new generation retroviral vector for ovarian cancer BRCA1 gene therapy

OBJECTIVE

The aim of this study was to determine the preclinical stability, toxicity, and efficacy of a second-generation complement-resistant retroviral BRCA1 vector, MFG-BRCA1, for ovarian cancer gene therapy.

METHODS

MFG-BRCA1 was packaged in human 293 renal cells and manufactured and tested under cGMP conditions and is allowed for use in humans by the Food and Drug Administration. Vector stability studies were performed in mice and human serum by PCR analysis. Toxicity in the animals was assessed at necropsy, evaluating for histological signs of inflammation and organ damage. Tissue culture efficacy studies were performed on ovarian and breast cancer cells. Animal efficacy studies were conducted in female nu/nu mice. Mice were injected intraperitoneally with SKOV-3 ovarian cancer cells and tumors were allowed to grow for 4 weeks. Mice were treated intraperitoneally with MFG-BRCA1 or control vectors. Survival of animals was compared in the MFG-BRCA1 versus the control groups.

RESULTS

MFG-BRCA1 was more stable in human serum than LXSN-BRCA1sv. Toxicity as demonstrated by an inflammatory peritonitis was minimal. Significantly fewer clones were obtained using the MFG-BRCA1 versus the MFG vector alone in both cell lines. Efficacy studies in animals of MFG-BRCA1 demonstrated a near threefold increase in survival over control vector and twofold increase compared to the first generation LXSN-BRCA1sv vector.

CONCLUSION

The reengineered complement-resistant MFG-BRCA1 retroviral vector is more effective and more stable than the previous generation LXSN-BRCA1sv vector.

Ovarian cancer BRCA1 gene therapy: Phase I and II trial differences in immune response and vector stability

Gene therapy with viral vectors has shown some promise in nude mice models and in initial Phase I trials of patients with extensive metastatic cancer. A Phase I clinical trial (D. L. Tait et al., Clin. Cancer Res., 3: 1959-1968, 1997) of ovarian cancer patients treated with i.p. retroviral LXSN-BRCA1sv gene therapy reported stable vector, minimal antibody response, and tumor reduction. We initiated a Phase II trial on patients with less extensive disease to evaluate vector pharmacokinetics, immune response, toxicity, and efficacy. Patients received a surgically implanted peritoneal catheter to administer infusions of vector, as well as to retrieve daily samples of peritoneal fluid for analysis. Ovarian cancer patients received four daily i.p. injections of LXSN-BRCA1sv vector therapy for three cycles, 4 weeks apart. Patient peritoneal fluid and plasma were analyzed extensively by PCR, Western blot, complement level (CH50), and chemical and hematological tests. Phase II patients showed no response, no disease stabilization, and little or no vector stability. Because of vector instability and rapid antibody development, which differed dramatically from the Phase I trial data, the trial was terminated after treatment of six patients. Immune system status appears to have played a major role in whether gene therapy was effective. Comparison of Phase I and II patients showed significant differences in tumor burden, immune system status, and response to BRCA1 gene therapy.

BRCA1 expression restores radiation resistance in BRCA1-defective cancer cells through enhancement of transcription-coupled DNA repair

The breast cancer predisposition genes, BRCA1 and BRCA2, are responsible for the vast majority of hereditary breast cancer. Although BRCA2 functions to help the cell repair double-stranded DNA breaks, the function of BRCA1 remains enigmatic. Here, we develop a human genetic system to study the role of BRCA1 in oxidative DNA damage. We show that human cancer cells containing mutated BRCA1 are hypersensitive to ionizing radiation. This hypersensitivity can be reversed by the expression of forms of BRCA1 that are not growth suppressing. Reversal of hypersensitivity requires the ring finger of BRCA1, its transactivation domain, and its BRCT domain. Lastly, we show that unlike BRCA2, BRCA1 does not function in the repair of double-stranded DNA breaks. Instead, it functions in transcription-coupled DNA repair (TCR). TCR ability correlated with radioresistance as cells containing BRCA1 showed both increased TCR and radioresistance, whereas cells without BRCA1 showed decreased TCR and radiosensitivity. These findings give physiologic significance to the interaction of BRCA1 with the basal transcription machinery.

Mitogen-activated protein kinase kinase 2 activation is essential for progression through the G2/M checkpoint arrest in cells exposed to ionizing radiation

An increasing body of evidence suggests that mitogen-induced activation of the RAF/ERK signaling pathway is functionally separate from the stress-induced activation of the SEK/JNK/p38 signaling pathway. In general, stress stimuli strongly activate the p38s and the JNKs while only weakly activating ERK1 and ERK2. However, a number of independent groups have now shown that the RAF/ERK signaling pathway is strongly activated by ionizing radiation. In this work, we examine this paradox. We show that both mitogen-activated protein (MAP) kinase kinase 1 (MEK1) and MAP kinase kinase 2 (MEK2) are activated by ionizing radiation. Blockage of this activation through the use of dominant negative MEK2 increases sensitivity of the cell to ionizing radiation and decreases the ability of a cell to recover from the G2/M cell cycle checkpoint arrest. Blocking MEK2 activation does not affect double-strand DNA break repair, however. Although MEK1 is activated to a lesser extent by ionizing radiation, expression of a dominant negative MEK1 does not affect radiation sensitivity of the cell, the G2/M checkpoint of the cell, or double-strand break repair. Because ionizing radiation leads to a different cell cycle arrest (G2/M arrest) than that typically seen with other stress stimuli, and because we have shown that MEK2 can affect G2/M checkpoint kinetics, these results provide an explanation for the observation that the MEKs can be strongly activated by ionizing radiation and only weakly activated by other stressful stimuli.

Double-strand break repair deficiency and radiation sensitivity in BRCA2 mutant cancer cells

BACKGROUND

The protein product of the BRCA2 gene mediates repair of double-strand breaks in DNA. Because a number of cancer therapies exert cytotoxic effects via the initiation of double-strand breaks, cancers comprised of cells carrying BRCA2 gene mutations may be more amenable to treatment with agents that cause such breaks.

METHODS

We identified a human pancreatic adenocarcinoma cell line lacking one copy of the BRCA2 gene and containing a mutation (6174delT) in the remaining copy. In vitro and in vivo experiments were conducted with this cell line and with other carcinoma cell lines matched for similar genetic mutations, similar differentiation status, and/or similar carcinoma type to examine double-strand break repair, sensitivity to drugs that induce double-strand breaks, and radiation sensitivity.

RESULTS

BRCA2-defective cells were unable to repair the double-strand DNA breaks induced by ionizing radiation. These cells were also markedly sensitive to mitoxantrone, amsacrine, and etoposide (drugs that induce double-strand breaks) (two-sided P = .002) and to ionizing radiation (two-sided P = .001). Introduction of antisense BRCA2 deoxyribonucleotides into cells possessing normal BRCA2 function led to increased sensitivity to mitoxantrone (two-sided P = .008). Tumors formed by injection of BRCA2-defective cells into nude mice were highly sensitive (>90% tumor size reduction, two-sided P = .002) to both ionizing radiation and mitoxantrone when compared with tumors exhibiting normal BRCA2 function. Histologic analysis of irradiated BRCA2-defective tumors showed a large degree of necrosis compared with that observed for control tumors possessing normal BRCA2 function.

CONCLUSION

BRCA2-defective cancer cells are highly sensitive to agents that cause double-strand breaks in DNA.

Ovarian cancer gene therapy

Retroviral-mediated delivery of BRCA1 gene therapy (LXN-BRCA1sv, a normal splice variant form of BRCA1) was tested extensively in mouse models. It was found to be effective in reducing tumor burden and to be minimally toxic. Twelve phase I clinical trial patients with recurrent or persistent epithelial ovarian cancer were treated with one to three cycles of intraperitoneal vector. There was minimal toxicity, four patients developed fevers (< 102.5 degrees F) and three had sterile peritonitis, which resolved within 48 hours. The vector was found to be fairly stable in some patients at 24 hours as well as transferred into and expressed in patient tissues. Stable disease was noticed in 8 of the 12 patients, suggesting that the peritoneal cavity may be an appropriate site for gene therapy.

Antisense c-myc retroviral vector suppresses established human prostate cancer

Prostate cancer eventually becomes androgen resistant, resumes growth, and kills the patient. Characterization of genetic events that lead to androgen refractory prostatic neoplasia has revealed the frequent overexpression of c-myc and uncontrolled prostate cancer proliferation. A novel strategy to combat advanced prostate cancer utilized a replication incompetent retrovirus that contained the mouse mammary tumor virus (MMTV) promoter within the retroviral vector to allow transcription of antisense c-myc gene within target prostate tumor cells. The transduction of cultured DU145 cells by XM6:MMTV-antisense c-myc RNA retrovirus did not affect cell proliferation in culture, yet a single direct injection of MMTV-antisense c-myc viral media into established DU145 tumors in nude mice produced a 94.5% reduction in tumor size compared to tumors treated with control virus MTMV sense fos and untreated tumor by 70 days. Two animals in the antisense c-myc-treated group had complete regression of their tumors. Histopathological examination of the tumors revealed that MMTV-antisense c-myc-transduced DU145 tumors had increased tumor cell differentiation, decreased invasion, and a marked stromal response. The mechanism for the antitumor effect of MMTV-antisense c-myc retrovirus appears to be suppression of c-myc mRNA and protein, and decreased bcl-2 protein. The in vivo transduction of prostate cancer cells with MMTV-antisense c-myc retroviruses reduced tumor growth by suppressing c-myc, resulting in the down-regulation of bcl-2 protein. Consequently, the MMTV-antisense c-myc retrovirus may be useful for gene therapy against advanced, hormone-refractory prostate cancer.

Breast cancer genes: therapeutic strategies

Although effective treatments for breast cancer predated the identification of causative molecular defects in humans, it is widely hoped that an understanding and/or manipulation of the key genetic events will lead to even more effective therapies or even cures. Powerful methods of positional cloning and gene identification have identified the breast cancer genes, BRCA1 and BRCA2, which together are responsible for the majority of cases of hereditary breast and ovarian cancer. Although the BRCA1 gene is rarely mutated in sporadic breast or ovarian cancer, levels of BRCA1 mRNA and protein are markedly decreased in the majority of sporadic cases of cancer. This suggests that hereditary and sporadic breast cancer share common genetic themes and that treatments aimed at increasing levels of BRCA1 or BRCA2 may be useful for both hereditary and sporadic cancers. We have demonstrated that gene transfer of wild-type BRCA1 inhibits the growth of sporadic breast and ovarian cancer cells and suppresses growth of established breast and ovarian tumor models in nude mice. Mutant BRCA1 genes do not inhibit growth or suppress tumor, providing additional evidence that BRCA1 is a tumor-suppressor gene. Strategies designed to increase BRCA1 expression or development of BRCA1-mimetic agents may be ultimately useful as therapeutic approaches.

Finkel-Biskis-Reilly osteosarcoma virus v-Fos inhibits adipogenesis and both the activity and expression of CCAAT/enhancer binding protein alpha, a key regulator of adipocyte differentiation

Finkel-Biskis-Reilly (FBR) osteosarcoma virus v-Fos causes tumors of mesenchymal origin, including osteosarcomas, rhabdomyosarcomas, chondrosarcomas, and liposarcomas. Because the cell of origin in all these tumors is a pluripotent mesenchymal cell, the variety of tumors seen in mice which express FBR v-Fos implies that FBR v-Fos inhibits multiple differentiation pathways. To study the mechanism of FBR v-Fos' inhibition of mesenchymal differentiation, we utilized an in vitro model of adipocyte differentiation. We show by both morphological and biochemical means that FBR v-Fos inhibits adipocyte differentiation in vitro. This inhibition is due to FBR v-Fos' inhibition of the growth arrest characteristic of terminal differentiation and FBR v-Fos' inhibition of the expression and activity of a key regulator of this growth arrest, C/EBPalpha. The in vitro inhibition of adipogenesis by FBR v-Fos has in vivo significance as immunostaining of FBR v-Fos-induced tumors shows no CCAAT/enhancer binding protein (EBP)-alpha expression. These data implicate C/EBPalpha as a protein involved in the generation of liposarcomas.

A phase I trial of retroviral BRCA1sv gene therapy in ovarian cancer

Gene transfer of BRCA1sv (a normal splice variant of BRCA1) into ovarian cancer cells produces growth inhibition in vitro and tumor suppression in nude mouse xenografts. As an initial step toward gene replacement therapy for ovarian cancer, we conducted a Phase I trial to assess the pharmacokinetics and toxicity of i.p. BRCA1sv retroviral vector therapy. Following placement of an indwelling Port-a-Cath in patients, a dose escalation study was performed of four daily i.p. infusions spanning doses from 3 to 300 ml (i.e., 10(10) viral particles) at half-log intervals (23 cycles in 12 patients). Gene transfer and expression were documented by PCR, Southern blot, reverse transcription-PCR, and nuclease protection assays. Pharmacokinetics were assessed by PCR and Southern blots detecting vector DNA, and toxicity was evaluated by clinical exam and fluid analysis. Three of 12 patients developed an acute sterile peritonitis, which spontaneously resolved within 48 h. Plasma and peritoneal antibodies to the retroviral envelope protein were detected only in patients treated with the highest dose levels but not in others, despite repeat dosing for an interval of up to 4 months. Eight patients showed stable disease for 4-16 weeks, and three patients showed tumor reduction with diminished miliary tumor implants at reoperation (two patients) and radiographic shrinkage of measurable disease (one patient). The vector-related complication of peritonitis was observed in three patients but resolved quickly as in preclinical mouse studies. Ovarian cancer may provide an important model for retroviral gene therapy studies due to vector stability, minimal antibody response, and access to tumor by i.p. therapy.

Finkel-Biskis-Reilly mouse osteosarcoma virus v-fos inhibits the cellular response to ionizing radiation in a myristoylation-dependent manner

DNA damage is recognized as a central component of carcinogenesis. DNA-damaging agents activate a number of signal transduction pathways that lead to repair of the DNA, apoptosis, or cell cycle arrest. It is reasoned that a cell deficient in DNA repair is more likely to acquire other cancer-promoting mutations. Despite the recent interest in the link between DNA damage and carcinogenesis, retroviral oncogenes have not yet been shown to affect the DNA damage-signaling pathway. In this report, we show that Finkel-Biskis-Reilly mouse osteosarcoma virus (FBR) v-fos, the retroviral homologue of the c-fos proto-oncogene, inhibits the cellular response to ionizing radiation. Cells that express FBR v-Fos show a decreased ability to repair DNA damage caused by ionizing radiation, and these cells show decreased survival in response to ionizing radiation. In addition, FBR v-Fos inhibits DNA-dependent protein kinase, a kinase specifically activated upon exposure to ionizing radiation. These effects were specific to ionizing radiation, as no effect of FBR v-Fos on the UV light signaling pathway was seen. Last, these effects were dependent on a lipid modification required for FBR v-Fos tumorigenesis, that of myristoylation of FBR v-Fos. A non-myristoylated mutant FBR v-Fos caused none of these effects. This study suggests that a retroviral oncogene can lead to an increased genomic instability, which can ultimately increase the carcinogenic potential of a cell.

Myristylation of FBR v-fos dictates the differentiation pathways in malignant osteosarcoma

Myristylation of FBR v-fos, a c-fos retroviral homologue that causes osteosarcomas in mice, determines many of its transcriptional properties in vitro. To determine whether myristylation of FBR v-fos contributes to in vivo tumorigenicity, we examined its transforming capability in comparison to a nonmyristylated FBR v-fos (G2A-R). Retroviral infections with FBR v-fos and G2A-R transform BALB/c-3T3 cells. The number, size, and cellular morphology of foci generated by both FBR and G2A-R are indistinguishable. However, marked biological differences were found in transgenic mice expressing either the myristylated FBR v-fos or the nonmyristylated G2A-R. 11 of 26 FBR v-fos transgenic mice died as a result of gross tumor burden. None of the 28 G2A-R transgenic mice died from tumor burden, and only two of the G2A-R mice developed bone tumors. Histologic examination of the tumors reveals that the FBR v-fos bone tumors contain malignant cells with features of four cell lineages (osteocytes, chondrocytes, myocytes, and adipocytes) in an environment rich in extracellular matrix (ECM). However, the G2A-R tumors exist in an environment devoid of ECM and display malignant cells with features of adipocytes. Masson staining reveals that the ECM of the FBR tumors stains strongly for collagen. Immunohistochemical staining with collagen III antibody demonstrates an abundance of collagen III expression in this ECM. While NH2-terminal myristylation is not required for FBR immortalization and transformation, it is essential in determining the degree of differentiation and tumorigenicity of malignant cells.

Tissue-targeted antisense c-fos retroviral vector inhibits established breast cancer xenografts in nude mice

The c-fos proto-oncogene has been implicated as a regulator of estrogen-mediated cell proliferation. We have tested the tissue specificity and antitumor efficacy of a mouse mammary tumor virus-regulated antisense c-fos retroviral vector. Systemically administered vector could be detected in several tissues but was only expressed in breast epithelium, thus supporting targeting to mouse mammary tumor virus-regulated tissues. Ex vivo transduction of 30-70% of MCF-7 human breast cancer cells produced expression of antifos RNA, decreased expression of the c-fos target mRNA, induction of differentiation, and inhibition of s.c. tumor growth and invasiveness. In vivo transduction of established i.p. MCF-7 tumors with a single injection of XM6:antifos inhibited tumor growth in athymic mice with a corresponding inhibition of c-fos, transforming growth factor beta1 and transforming growth factor alpha expression. Four daily injections with the antifos RNA induced a much larger MCF-7 i.p. tumor inhibition, with a marked prolongation of survival in the absence of any host tissue toxicity. These results indicate that inhibition of key nuclear genes such as c-fos may lead to disruption of paracrine factors and an antitumor effect, providing a strategy for cancer gene therapy.

Growth retardation and tumour inhibition by BRCA1

Inherited mutations in BRCA1 predispose to breast and ovarian cancer, but the role of BRCA1 in sporadic breast and ovarian cancer has previously been elusive. Here, we show that retroviral transfer of the wild-type BRCA1 gene inhibits growth in vitro of all breast and ovarian cancer cell lines tested, but not colon or lung cancer cells or fibroblasts. Mutant BRCA1 has no effect on growth of breast cancer cells; ovarian cancer cell growth is not affected by BRCA1 mutations in the 5' portion of the gene, but is inhibited by 3' BRCA1 mutations. Development of MCF-7 tumours in nude mice is inhibited when MCF-7 cells are transfected with wild-type, but not mutant, BRCA1. Most importantly, among mice with established MCF-7 tumours, peritoneal treatment with a retroviral vector expressing wild-type BRCA1 significantly inhibits tumour growth and increased survival.

BRCA1 is secreted and exhibits properties of a granin

Germline mutations in BRCA1 are responsible for most cases of inherited breast and ovarian cancer. However, the function of the BRCA1 protein has remained elusive. We now show that BRCA1 encodes a 190-kD protein with sequence homology and biochemical analogy to the granin protein family. Interestingly, BRCA2 also includes a motif similar to the granin consensus at the C terminus of the protein. Both BRCA1 and the granins localize to secretory vesicles, are secreted by a regulated pathway, are post-translationally glycosylated and are responsive to hormones. As a regulated secretory protein, BRCA1 appears to function by a mechanism not previously described for tumour suppressor gene products.

Decreased expression of BRCA1 accelerates growth and is often present during sporadic breast cancer progression

We have characterized expression of the familial breast and ovarian cancer gene, BRCA1, in cases of non-hereditary (sporadic) breast cancer and analyzed the effect of antisense inhibition of BRCA1 on the proliferative rate of mammary epithelial cells. BRCA1 mRNA levels are markedly decreased during the transition from carcinoma in situ to invasive cancer. Experimental inhibition of BRCA1 expression with antisense oligonucleotides produced accelerated growth of normal and malignant mammary cells, but had no effect on non-mammary epithelial cells. These studies suggest that BRCA1 may normally serve as a negative regulator of mammary epithelial cell growth whose function is compromised in breast cancer either by direct mutation or alterations in gene expression.

Identification of genes expressed in premalignant breast disease by microscopy-directed cloning

Histopathologic study of human breast biopsy samples has identified specific lesions which are associated with a high risk of development of invasive breast cancer. Presumably, these lesions (collectively termed premalignant breast disease) represent the earliest recognizable morphologic expression of fundamental molecular events that lead to the development of invasive breast cancer. To study molecular events underlying premalignant breast disease, we have developed a method for isolating RNA from histologically identified lesions from frozen human breast tissue. This method specifically obtains mRNA from breast epithelial cells and has identified three genes which are differentially expressed in premalignant breast epithelial lesions. One gene identified by this method is overexpressed in four of five noncomedo ductal carcinoma in situ lesions and appears to be the human homologue of the gene encoding the M2 subunit of ribonucleotide reductase, an enzyme involved in DNA synthesis.

Myristylation-dependent transactivation by FBR v-fos is regulated by C/EBP

Viral oncogenes are generally believed to cause transformation through disregulated mimicry of their cellular homologues. However, here we show that FBR v-fos, unlike c-fos, transcriptionally activates unique genes in retrovirally induced chondro-osseous sarcomas. We show that FBR v-fos transactivates the collagen III and stromelysin promoters and that the unique transcriptional properties of transforming FBR depend upon its N-terminal myristylation and the differentiation state of the cell. Deletion or mutation of the myristylation site results in a loss of FBR v-fos transactivation in HeLa and undifferentiated 3T3-L1 preadipocyte cell lines. FBR v-fos transactivation of collagen III maps to a negative regulatory site which binds a key regulator of adipocyte differentiation, C/EBP alpha. Cotransfection of C/EBP alpha abolishes FBR v-fos transactivation of the alpha 1(III) collagen promoter. Furthermore, FBR v-fos cannot transactivate collagen III subsequent to adipocyte differentiation. We also demonstrate that collagen III transcription is reduced during adipocyte differentiation as the transcriptional activity of C/EBP alpha is concomitantly induced. Our results indicate that FBR v-fos transactivation depends upon its cotranslational myristylation and maps to a negative regulatory region which binds C/EBP alpha.

Gene transplantation: combined antisense inhibition and gene replacement strategies

Optimal gene replacement protocols would include both inhibition of the endogenous gene and overexpression of the preferred (or mutant) gene. We have developed a novel gene transfer method to test whether antisense-resistant genes (designed by deletion of antisense RNA target sequences) can replace the function of endogenous genes. Immunoprecipitation studies demonstrated that inducible anti-fos RNA (antisense directed against the c-fos gene) reduces endogenous c-fos expression by 90%, but did not affect the transfected antisense-resistant mutant c-fos genes. Cell growth studies demonstrated that full-length and minimally truncated c-fos expression vectors could restore serum-induced DNA synthesis but that C-terminally truncated Fos mutants including FBR v-fos could not. Transcriptional studies demonstrate that the endogenous c-fos protein contributes to AP-1 activity and normally suppresses regulated SRE (serum response element) activity. This "gene transplant" method for inhibition of endogenous genes and replacement with preferred genes has implications for gene therapy of hereditary hematologic disorders and for the correction or "repair" of oncogenes or tumor suppressor genes in leukemias and lymphomas.

Antisense rescue defines specialized and generalized functional domains for c-Fos protein

Serum induces the expression of a number of proteins with similar transcriptional properties, including those encoded by the proto-oncogenes c-fos and c-jun. This study employs a novel antisense rescue method to determine whether antisense-resistant genes (constructed by deletion of antisense RNA target sequences) can replace c-fos expression during serum-induced DNA synthesis. Immunoprecipitation studies and nuclease protection assays demonstrated that anti-fos RNA inhibited endogenous c-fos expression but did not inhibit expression of transfected antisense-resistant mutant c-fos genes. The results of nuclear-labelling and cellular-proliferation studies indicated that C terminally truncated Fos mutants, including FBR v-fos, could not rescue endogenous Fos, although full-length and minimally truncated c-fos expression vectors could restore serum-induced DNA synthesis in cells expressing anti-fos RNA. Overexpression of c-Jun protein (Jun) could not restore serum-induced DNA synthesis to cells expressing inducible anti-fos RNA despite equivalent transactivation of an AP-1 target gene. Thus, the antisense rescue method defines a specialized function for c-Fos protein which is distinct from the function(s) of Jun and/or transforming FBR v-Fos proteins.

In vivo regional delivery of retrovirally mediated foreign genes to rat liver cells: need for partial hepatectomy for successful foreign gene expression

A novel surgical technique was developed to deliver retroviral gene vectors directly to a rat liver lobe in vivo. It was observed that viral infection efficiency was enhanced by inducing hepatocyte DNA synthesis by prior partial hepatectomy. Two retroviral vectors were used to integrate specific bacterial genes: an amphotropic virus expressing the hph gene for hygromycin B phosphotransferase and an ecotropic virus expressing the lac-Z gene for beta-galactosidase. The vectors were directed to the liver by in situ selective perfusion of the posterior liver lobes with a viral suspension with inflow and outflow catheters. Male Sprague-Dawley rats were divided into three groups. Animals in the first group underwent 70% partial hepatectomy and the remnant liver lobes were allowed to regenerate for 20 hours before perfusion with the viral supernatant. Group 2 rats were perfused with viral supernatant and 2 hours later underwent 70% partial hepatectomy. Animals in the third group were perfused with the viral supernatant without partial hepatectomy. Viral transduction of hepatocytes was assessed 4 or 6 days after treatment. Hygromycin B-resistant hepatocytes were isolated from the liver remnants of rats in group 1 (21.6%) and group 2 (26.9%). No resistant hepatocytes could be detected in hepatocytes from either control rats perfused with medium alone or those from rats that did not undergo hepatectomy (group 3). In animals that received the ecotropic virus, only those that underwent hepatectomy before virus exposure (group 1) showed a small number of hepatocytes expressing beta-galactosidase in liver sections.

Antisense promoter mapping. Inhibitory methods of transcriptional analysis

We have employed antisense methods to study the transcriptional functions of c-fos protein (Fos). Clones expressing inducible anti-fos RNA have been employed to inhibit c-fos expression, resulting in activation of c-fos transcription by inhibiting its normal repressor function. The sites of negative regulation by Fos have been mapped using this antisense mapping method which demonstrates that the serum response element represents the major site of repression by endogenous c-fos protein. A similar strategy (antisense cloning) has been employed to clone four target genes that are Fos dependent. These cDNAs encode mRNAs that are rapidly induced by serum (although this induction is blocked by cycloheximide) but are blocked by induction of anti-fos RNA. These inhibitory methods of studying transcription factor function are extremely useful for transcription factors (like Fos) that require cooperation with other factors to modulate gene transcription.

Platelet-derived growth factor regulation of fos stability correlates with growth induction

A brief exposure of quiescent, density-arrested Balb/c 3T3 fibroblasts to platelet-derived growth factor (PDGF) results in expression of the proto-oncogene c-fos; furthermore, the translation product of c-fos, p55fos, was shown to have increased stability in cells upon continued exposure to PDGF. The induction of competence or growth initiation requires a longer exposure to PDGF than that necessary for the induction of the immediate-early, growth-related genes. The need for the continued presence of PDGF for growth initiation beyond the time required for the induction of immediate-early gene expression may be due, in part, to PDGF-dependent post-translational stabilizations of gene products. We speculate that a PDGF-mediated event increases p55fos stability, resulting in a continued elevated level of Fos protein, which in turn allows a continued Fos-mediated activity required by Balb/c 3T3 cells to become competent to enter the cell cycle.

Transforming growth factor beta 1 regulation of c-myc expression, pRB phosphorylation, and cell cycle progression in keratinocytes

Transforming growth factor beta 1 (TGF-beta 1) is a potent inhibitor of cellular proliferation in a variety of cell types, including skin keratinocytes. TGF-beta 1 suppression of c-myc transcription has been implicated in the mechanism of TGF-beta 1 inhibition of keratinocytes, and evidence suggests that the protein product of the retinoblastoma gene (pRB) is a necessary component in this pathway. Following growth factor stimulation of quiescent keratinocytes, TGF-beta 1 can inhibit cell cycle progression into S phase at any point prior to the G1-S transition but does not inhibit progression through the S phase of the cell cycle. Since pRB is hypophosphorylated during G1 and hyperphosphorylated during S and G2, the G1-S-specific phosphorylation of pRB becomes an attractive target for the growth-inhibitory activities of TGF-beta 1. However, in TGF-beta 1-treated primary human keratinocytes and in a series of human papilloma virus and SV40 immortalized human keratinocyte cell lines, the phosphorylation status of pRB strictly correlated with cell growth. No evidence was found for a direct effect of TGF-beta 1 on the phosphorylation state of pRB in these cells. It was further demonstrated that synthesis of c-myc protein can be rapidly inhibited by TGF-beta 1 addition throughout G1 and S phases, indicating that the phosphorylation state of pRB, at least as it varies during the cell cycle, does not alter the ability of TGF-beta 1 to suppress c-myc expression.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibitory effect of myristylation on transrepression by FBR (Gag-Fos) protein

The myristylated v-fos product, FBR murine sarcoma virus (Gag-Fos) protein, exhibits a lower level of transrepression of the serum response element (SRE) than does c-fos protein (Fos). Mutation of the N-terminal myristylation site in FBR protein restored SRE transrepression. Replacement of N-terminal viral Gag sequences with the Fos N terminus also restored this activity, providing additional evidence that myristylation inhibits transrepression by FBR protein. However, the myristylated Gag domain did not inhibit SRE transrepression when fused to Fos, indicating that myristylation of a fos protein is not by itself sufficient to prevent SRE transrepression and that C-terminal mutation is necessary to inhibit transrepression by N myristylation. Comparison of transfection results with Fos C-terminal deletion mutants and the Fos/FBR chimeric mutant revealed that the FBR C terminus retained the potential for transrepression despite deletion of the normal Fos C terminus, whereas similar Fos deletion mutants did not. These results indicate that both N- and C-terminal mutations are required to inhibit transrepression by FBR protein and that multiple structural mutations accompanied by posttranslational protein modification alter gene regulation by FBR protein.

Differential induction of the c-fos promoter through distinct PDGF receptor-mediated signaling pathways

The multiple isoforms of PDGF induce fibroblastic mitogenesis through two distinct PDGF receptors, alpha and beta. The molecular mechanisms by which these alpha and beta PDGF receptors regulate gene expression are poorly understood. We present data which indicates that differential induction of c-fos gene expression by PDGF isoforms occurs through distinct PDGF alpha and beta receptor-mediated signaling pathways. Comparison of PDGF-AA with PDGF-BB stimulation showed that PDGF-BB induced prolonged expression of the c-fos gene in BALB/c-3T3 cells, but that PDGF-AA induced more potent activation of the serum response element (SRE) in transient transfection assays. PDGF-AA, which binds alpha but not beta PDGF receptors, could only induce the SRE through a protein kinase C (PKC)-dependent pathway, whereas PDGF-BB, which binds both alpha and beta PDGF receptors, could also induce the SRE through a PKC-independent pathway. These results suggest that PDGF alpha receptors activate the PKC-dependent signaling pathway while PDGF beta receptors also activate a PKC-independent pathway. In addition, we found that PDGF-BB could induce another c-fos promoter element within the -90 to +10 region, suggesting that the more potent mitogenic effect and prolonged c-fos gene expression induced by PDGF-BB may result from cooperativity between more than one c-fos promoter elements.

Antisense inhibition of c-myc expression reveals common and distinct mechanisms of growth inhibition by TGF beta and TNF alpha

Downregulation of the c-myc gene in HL-60 cells is associated with growth inhibition and induction of differentiation. Previous studies have reported that the growth inhibitors TGF beta and TNF alpha downregulate c-myc mRNA levels, suggesting the possibility that these agents may exert some of their phenotypic effects via c-myc downregulation. Our study demonstrates that although both growth inhibitors produce a similar decrease in c-myc protein synthesis, TNF alpha produces a greater growth inhibition and differentiation induction in HL-60 cells. Combined addition of anti-myc oligomer with either growth inhibitor produces no additive effect. In fact, 4 microM anti-myc oligomer produces the same growth and differentiation effects as does 10 ng/ml TGF beta 1. We conclude that downregulation of c-myc expression represents a common mechanism of growth inhibition by TGF beta and TNF alpha, but that TNF alpha possesses an additional effect that is independent of c-myc expression.

Myristylation alters DNA-binding activity and transactivation of FBR (gag-fos) protein

FBR murine sarcoma virus (gag-fos) protein, a virally transduced Fos protein, exhibits decreased gene transactivation in comparison with the cellular Fos protein. Biochemical analysis suggests that myristylation of the virally encoded N-terminal gag region results in decreased DNA binding and transcriptional activation without affecting heterodimerization with Jun protein. These findings demonstrate that protein myristylation can modulate gene regulation by a DNA-binding protein.

Cutting the chain of command: specific inhibitors of transcription

Cell growth and differentiation are regulated (at least in part) by changes in gene transcription. The cloning and characterization of transcription factors has revealed that these factors coordinately regulate the transcription of specific genetic programs; for example, a number of phorbol ester-induced genes are activated by binding of the transcription factors Fos and Jun to specific DNA sequences. Clearly, inhibition of either the production or function of specific transcription factors would alter complete genetic programs, changing the expression of a great number of genes (analogous to cutting the chain of military command and affecting an entire brigade or division). Our laboratory and others have employed genetic methods to specifically inhibit transcription by two distinct methods: (1) antisense inhibition of the production of transcription factors; and (2) introduction of target DNA sequences to "soak up"or quench transcription factors. In this report, we present data showing that serum-stimulated induction of the c-fos gene may be reduced more than 90% by introduction of target DNA sequences containing the serum response element (SRE); identical amounts of mutant SRE sequences have no effect on gene induction. These studies demonstrate that specific inhibitors of transcription can have significant effects on cellular gene expression. The challenge is to modulate transcriptional programs without deleterious effects on normal cells.

Antisense mapping of the c-fos promoter: role of the serum response element

Using an antisense RNA approach to eliminate endogenous expression of the c-fos protein, we have verified by nuclear run-on and transient expression assays that the Fos protein is a negative regulator of its own transcription in vivo. The negative autoregulation of the c-fos promoter by Fos was further confirmed by overexpression of an antisense-resistant c-fos expressing vector. Antisense mapping of the c-fos promoter demonstrated that the serum responsive element (SRE) represents the major site for c-fos suppression only during the first hour, but that additional adjacent DNA sequences are required for suppression at later times. We propose that antisense inhibition of transcriptional repressors provides a useful method for analyzing the significance and mechanism of transcriptional repression in vivo.

Development of mammary hyperplasia and neoplasia in MMTV-TGF alpha transgenic mice

To study the role of transforming growth factor alpha (TGF alpha) in normal mammary development and mammary neoplasia in vivo, we have generated transgenic mice in which a human TGF alpha cDNA is expressed under the control of the MMTV enhancer/promoter. Overexpression of TGF alpha in the mammary epithelium, as confirmed by in situ hybridization and immunohistochemistry, is associated with hyperplasia of alveoli and terminal ducts in virgin female and pregnant transgenic mice. A range of morphologic abnormalities including lobular hyperplasia, cystic hyperplasia, adenoma, and adenocarcinoma is seen in mammary tissue of transgenic females. In contrast, no morphologic abnormalities are seen in transgenic males in spite of TGF alpha overexpression in salivary glands and reproductive organs. TGF alpha can therefore act as an oncogene in vivo and appears to predispose mammary epithelium to neoplasia and carcinoma.

Transforming growth factor beta 1 suppression of c-myc gene transcription: role in inhibition of keratinocyte proliferation

Transforming growth factor beta 1 (TGF-beta 1) is a potent growth inhibitor for many cell types, including most epithelial cells. However, the mechanism of growth inhibition is unknown. In skin keratinocytes, TGF-beta 1 has been shown to inhibit growth and to rapidly reduce c-myc expression. It has been demonstrated that protein synthesis is required for TGF-beta 1 regulation of c-myc in keratinocytes. Here we present evidence that treatment of mouse BALB/MK keratinocyte cells with either antisense c-myc oligonucleotides or TGF-beta 1 inhibited cell entry into S phase. These results suggest that TGF-beta inhibition of c-myc expression may be essential for growth inhibition by TGF-beta 1. The block in c-myc expression by TGF-beta 1 occurred at the level of transcriptional initiation. Studies with a series of 5' deletion c-myc/chloramphenicol acetyltransferase constructs indicated that a cis regulatory element(s), which resides between positions -100 and +71 relative to P1 transcription start site, is responsible for the TGF-beta 1 responsiveness. Based on these data, it is proposed that the mechanism of TGF-beta 1 growth inhibition involves synthesis or modification of a protein that may interact with a specific element(s) in the 5' regulatory region of the c-myc gene, resulting in inhibition of transcriptional initiation.

Activation of the second promoter of the transforming growth factor-beta 1 gene by transforming growth factor-beta 1 and phorbol ester occurs through the same target sequences

Two distinct regions of the transforming growth factor-beta 1 (TGF-beta 1) promoter are responsive to autoregulation and activation by phorbol ester (12-O-tetradecanoylphorbol-13-acetate): sequences located between nucleotides -454 to -323 (first promoter) and between the two transcriptional start sites. We have now characterized in detail the induction of the second promoter (sequences between nucleotides + 1 to +271) of the TGF-beta 1 gene by both TGF-beta 1 and phorbol ester. By assaying progressively deleted mutations in the second promoter, we have found two regions responsible for the induction; each contains a phorbol ester-responsive element. In vitro transcription of the second promoter-chloramphenicol acetyltransferase chimeric genes using nuclear extracts of A-549 cells showed that deletion of the putative phorbol ester-responsive elements results in a 70-80% decrease in activity. DNase I footprinting and gel mobility shift assays showed that binding to an Sp1 site and the putative TRE elements are required for maximal expression of the second promoter region of the TGF-beta 1 gene. These results suggest that AP-1, which is capable of conferring phorbol ester or TGF-beta 1 responsiveness, is the major transcription factor involved in the second promoter-derived transcription of the TGF-beta 1 gene.

Growth factors regulate transin gene expression by c-fos-dependent and c-fos-independent pathways

The rapid induction of the proto-oncogene c-fos by growth factors and other bioactive agents, and the recent evidence that the c-fos protein (Fos) is associated with transcriptional complexes, suggests that Fos may represent an integral part of an intracellular messenger pathway that triggers changes in gene expression and ultimately phenotypic alterations. This report examines the role of c-fos in growth factor stimulation of transin, a matrix-degrading secreted metalloproteinase. Platelet-derived growth factor (PDGF) stimulation of transin RNA was blocked by a selective reduction in Fos synthesis with antisense c-fos mRNA, whereas epidermal growth factor (EGF) stimulation of transin occurred despite an equivalent inhibition of Fos levels. The stimulatory effect of both PDGF and EGF on transin transcription involved factors recognizing the sequence TGAGTCA, which is found in the transin promoter and is reported to be a binding site for the transcriptional factor Jun/AP-1 and for associated Fos and Fos-related complexes. Thus both Fos-dependent and Fos-independent pathways exist for growth factor regulation of gene expression, and both effects may be mediated through the same cis-acting transcription element.

Specific inhibition of c-myc protein biosynthesis using an antisense synthetic deoxy-oligonucleotide in human T lymphocytes

C-myc protein expression in human T cells was specifically inhibited by a 15-mer deoxy-oligonucleotide complementary to the 5' end of the human c-myc gene second exon. The oligonucleotide penetrates the cells without any treatment, with a plateau of cell association reached in 2 h. The oligonucleotide specifically blocked the de novo synthesis of c-myc protein, induced by PHA in human resting peripheral T cells, without impairing the overall synthesis of other proteins, as shown by two-dimensional analysis of [35S]methionine pulse-labeled proteins. The specific inhibition of c-myc protein synthesis prevented the entry into S phase of resting T cells, induced to proliferate by PHA, or IL-2-dependent T cells induced by IL-2, as shown by [3H]thymidine incorporation. The inhibition of proliferation was specific since it was not observed with the corresponding sense-oligonucleotide and was reversed by preincubation of the cells with an excess of sense oligonucleotide. These results clearly support a role for c-myc protein in the proliferation process and show that inducible protein expression can be blocked by means of synthetic oligonucleotides complementary to a coding exon.

Specific inhibition of c-myc protein biosynthesis using an antisense synthetic deoxy-oligonucleotide in human T lymphocytes

C-myc protein expression in human T cells was specifically inhibited by a 15-mer deoxy-oligonucleotide complementary to the 5' end of the human c-myc gene second exon. The oligonucleotide penetrates the cells without any treatment, with a plateau of cell association reached in 2 h. The oligonucleotide specifically blocked the de novo synthesis of c-myc protein, induced by PHA in human resting peripheral T cells, without impairing the overall synthesis of other proteins, as shown by two-dimensional analysis of [35S]methionine pulse-labeled proteins. The specific inhibition of c-myc protein synthesis prevented the entry into S phase of resting T cells, induced to proliferate by PHA, or IL-2-dependent T cells induced by IL-2, as shown by [3H]thymidine incorporation. The inhibition of proliferation was specific since it was not observed with the corresponding sense-oligonucleotide and was reversed by preincubation of the cells with an excess of sense oligonucleotide. These results clearly support a role for c-myc protein in the proliferation process and show that inducible protein expression can be blocked by means of synthetic oligonucleotides complementary to a coding exon.

An oligomer complementary to c-myc mRNA inhibits proliferation of HL-60 promyelocytic cells and induces differentiation

To study the role of a nuclear proto-oncogene in the regulation of cell growth and differentiation, we inhibited HL-60 c-myc expression with a complementary antisense oligomer. This oligomer was stable in culture and entered cells, forming an intracellular duplex. Incubation of cells with the anti-myc oligomer decreased the steady-state levels of c-myc protein by 50 to 80%, whereas a control oligomer did not significantly affect the c-myc protein concentration. Direct inhibition of c-myc expression with the anti-myc oligomer was associated with a decreased cell growth rate and an induction of myeloid differentiation. Related antisense oligomers with 2- to 12-base-pair mismatches with c-myc mRNA did not influence HL-60 cells. Thus, the effects of the antisense oligomer exhibited sequence specificity, and furthermore, these effects could be reversed by hybridization competition with another complementary oligomer. Antisense inhibition of a nuclear proto-oncogene apparently bypasses cell surface events in affecting cell proliferation and differentiation.

Inducible production of c-fos antisense RNA inhibits 3T3 cell proliferation

Antisense RNA complementary to c-fos mRNA was produced in mouse 3T3 cells by gene transfer techniques. Transcriptional units were constructed consisting of a steroid-inducible mouse mammary tumor virus (MMTV) promoter, mouse or human 5' c-fos gene fragments in either the sense (5' to 3') or antisense (3' to 5') orientation, and splice and poly(A) signals from the human beta-globin gene. A gene that confers neomycin resistance was included in the vectors to allow isolation of stable transformants. Dexamethasone caused a marked induction of hybrid MMTV-fos-globin RNA. Induction of the hybrid transcript containing antisense c-fos RNA decreased colony formation following DNA transfer and inhibited the proliferation of cells into which the antisense transcriptional unit had been integrated. In contrast, colony formation and cell proliferation were not inhibited by induction of hybrid RNA containing c-fos RNA sequences in the sense orientation. These results indicate that the strategy of generating antisense RNA to inhibit gene expression may be useful in delineating the function of protooncogenes. The c-fos gene product appears to have a required role in normal cell division.

Expression of the human c-fms proto-oncogene product (colony-stimulating factor-1 receptor) on peripheral blood mononuclear cells and choriocarcinoma cell lines

The c-fms gene product is related, and possibly identical, to the receptor for the mononuclear phagocyte colony stimulating factor, CSF-1. Using antisera to a recombinant v-fms--coded polypeptide, glycoproteins encoded by the human c-fms locus were detected in mononuclear cells from normal peripheral blood and in promyelocytic HL-60 cells 24 h after induction of monocytic differentiation with phorbol ester. The 150-kD human c-fms--coded glycoprotein was expressed at the cell surface, was active as a tyrosine-specific protein kinase in vitro, and shared primary structural features with the product of the feline retroviral v-fms oncogene. A biochemically indistinguishable glycoprotein was detected in human choriocarcinoma cell lines. Like peripheral blood mononuclear cells and phorbol ester-treated HL-60 cells, the choriocarcinoma cells expressed high affinity binding sites for human CSF-1. In addition to serving as a lineage specific growth factor in hematopoiesis, CSF-1 may play a role in normal trophoblast development.

Glutamate dehydrogenase in Reye's syndrome. Evidence for the presence of an altered enzyme in serum with increased susceptibility to inhibition by GTP

The glutamate dehydrogenase activity found in the serum of patients with Reye's syndrome is shown to be inhibited about 1000-fold more potently by GTP than is the normal human enzyme. 1 mM ADP, which with the normal enzyme effectively reverses GTP inhibition, has no effect in the GTP inhibition of the Reye's syndrome serum activity.

Masking by enzyme inhibitor of raised serum glutamate dehydrogenase activity in Reye's syndrome

Serum glutamate dehydrogenase (GDH) activity was greatly raised (up to 830 times the upper limit of normal) in 16 patients with Reye's syndrome. The serum activity was masked by an inhibitor, and the rises were observed only after dialysis or sample dilution. Serum GDH values from 38 paediatric patients, including 10 with hyperammonaemia due to other causes, showed no such rise after dialysis. Only 1 of 13 adult patients with liver disease had high GDH activity, but this level was not increased after dialysis. Serum ornithine carbamyl transferase activity was also raised in patients with Reye's syndrome, but levels were not increased after dialysis. The ratio of dialysed/undialysed GDH activity clearly distinguished all Reye's patients from controls. The inhibition of a mitochondrial enzyme which regulates ammonia metabolism may contribute to the hyperammonaemia of Reye's syndrome. Serum GDH levels before and after dialysis would seem to be a useful diagnostic aid in Reye's disease.