Long Noncoding RNA RP11-357H14.17 Plays an Oncogene Role in Gastric Cancer by Activating ATF2 Signaling and Enhancing Treg Cells.

BACKGROUND: Gastric cancer (GC) is one of the most common malignant tumors in the world. The potential functions and mechanisms of long noncoding RNAs (lncRNAs) in GC development are still unclear. It is of great significance to explore the prognostic value of LncRNA signatures for GC. METHODS: LncRNAs differently expressed in GC and their prognostic value were studied based on The Cancer Genome Atlas (TCGA) database. The functional regulatory network and immune infiltration of RP11-357H14.17 were further studied using a variety of bioinformatics tools and databases. RESULTS: We found that the high expression of RP11-357H14.17 was closely associated with shortened overall survival (OS) and poor prognosis in gastric cancer patients. We also found that its expression was related to clinical features including tumor volume, metastasis, and differentiation. Functional enrichment analysis revealed that RP11-357H14.17 is closely related to enhanced DNA replication and metabolism; ssGSEA analysis implied the oncogenic roles of RP11-357H14.17 was related to ATF2 signaling and Treg cell differentiation. Furthermore, we verified such link by using real-time PCR and IHC staining in human GC samples. CONCLUSION: We demonstrate that RP11-357H14.17 may play a crucial role in the occurrence, development, and malignant biological behavior of gastric cancer as a potential prognostic marker for gastric cancer.

Sprouty2 Inhibits Migration and Invasion of Fibroblast-Like Synoviocytes in Rheumatoid Arthritis by Down-regulating ATF2 Expression and Phosphorylation.

Activating transcription factor 2(ATF2), a transcription factor belonging to the AP-1 family, plays an important role in inflammation. However, its biological functions and underlying molecular mechanisms in rheumatoid arthritis (RA) remain unclear. Western blot and immunohistochemistry were used to identify the expression of ATF2 and Sprouty2(SPRY2) in RA synovial tissues. SW982 cells were stimulated by TNF-α to establish an in vitro RA fibroblast-like synoviocyte (RA-FLS) model. Transwell and monolayer wound-healing were used to detect cell migration and invasion. RNA interference (si-ATF2) and adenovirus vector (Ad-SPRY2) methods were employed to manipulate ATF2 or SPRY2 expression in SW982 cells. The protein expression and phosphorylation levels in SW982 cells were evaluated by western blot. ATF2 expression and phosphorylation were upregulated in the RA synovial tissues. In RA-FLS model, ATF2 expression and phosphorylation were increased in a time-dependent manner. ATF2 knockdown inhibited the migration and invasion of RA-FLS model, reducing the inflammatory factors, which was consistent with the influence on cell behaviors caused by SPRY2 overexpression. Moreover, SPRY2 overexpression inhibited the TNF-α-induced phosphorylation of ERK and ATF2 in SW982 cells. The high expression and phosphorylation of ATF2 promoted migration and invasion of RA-FLSs. SPRY2 might inhibited the inflammatory responses of RA-FLSs via suppressing ERK-ATF2 pathway.

Inactivation of ATF-2 enhances epithelial-mesenchymal transition and gemcitabine sensitivity in human pancreatic cancer cells.

OBJECTIVE: This work aimed to study the activating transcription factor 2 or AMP-dependent transcription factor-2 (ATF-2) inhibition mediated gemcitabine sensitivity in human pancreatic cancer cells. METHODS: The protein and messenger RNA expressions of ATF-2 in 42 pancreatic cancer tissues and adjacent nontumorous tissues were detected. Kaplan-Meier survival analysis was performed based on the expression level of ATF-2 protein in tumor tissues. Then the pancreatic cancer cells were transduced with ATF-2-expressing lentivirus and small interfering RNAs (siRNAs) to investigate the effect of ATF-2 on pancreatic cancer cell invasion, epithelium to mesenchyme transition, apoptosis, and gemcitabine sensitivity. RESULTS: The expression of phosphorylated (p)-ATF-2 protein was upregulated in pancreatic cancer tissues compared with adjacent nontumorous tissues. Patients with relative higher p-ATF-2 level showed significantly lower survival time. Then we found that the transfection ATF-2 siRNA into BxPC3 cells inhibited cell proliferation, invasion, and epithelium to mesenchyme transition, but enhanced cell apoptosis. These changes could be enhanced by the additional administration of gemcitabine. In addition, we confirmed that the overexpression of ATF-2 in Panc-1 cells promoted cell invasion and epithelium to mesenchyme transition. CONCLUSION: We concluded that inhibition-promoted ATF-2 expression was responsible for epithelium to mesenchyme transition and invasion of pancreatic cancer cells, while the inhibition of ATF-2 confers to gemcitabine sensitivity in human pancreatic cancer cells in vitro.

MiR-451a suppressed cell migration and invasion in non-small cell lung cancer through targeting ATF2.

OBJECTIVE: To explore the role of miR-451a in the migration and invasion of non-small cell lung cancer (NSCLC) cells. MATERIALS AND METHODS: Quantitative Real time-polymerase chain reaction (qRT-PCR) and Western blot were performed to detect the levels of miR-451a and activating transcription factor 2 (ATF2) in NSCLC. Transwell assay was employed to analyze the migratory and invasive abilities in NSCLC cells. Dual-luciferase reporter assay was applied to confirm the binding condition of miR-451 and its target gene in NSCLC cells. RESULTS: MiR-451a was downregulated in NSCLC tissues and lung cancer cell lines A549 and NCI-H460, while ATF2 was upregulated. The mRNA level of miR-451a was negatively correlated to ATF2. Additionally, miR-451a regulated cell migration and invasion through targeting ATF2. Furthermore, ATF2 could reverse the inhibitory migration and invasion of A549 cells induced by miR-451a. CONCLUSIONS: MiR-451a inhibits the migratory and invasive abilities of NSCLC cells through ATF2 regulation. The newly identified miR-451a/ATF2 axis provides a novel insight into the pathogenesis ofNSCLC.

Noxin promotes proliferation of breast cancer cells via P38-ATF2 signaling pathway.

Noxin (also called chromosome 11 open reading frame 82 or DNA damage-induced apoptosis suppressor) is associated with anti-apoptosis and cell proliferation in response to stress signals. However, to our knowledge, the role of Noxin in regulating cell proliferation is still controversial and there are no reports of the function and clinicopathological association in breast cancer. In this study, immunohistochemistry results showed that Noxin expression was significantly correlated with advanced tumor-node-metastasis stage ( p = 0.027), positive regional lymph node metastasis ( p = 0.002), and poor overall survival ( p = 0.002). Proliferation assay results showed that Noxin obviously promoted the ability of proliferation of normal breast cells. Subsequent western blot results revealed that Cyclin D1 and Cyclin E1 were upregulated by overexpressing Noxin, whereas Cyclin D1 and Cyclin E1 were downregulated after depleting Noxin. The levels of phosphorylated P38 and activating transcription factor 2 were obviously increased after overexpressing Noxin, and their expression was downregulated accordingly by transfecting Noxin-small interfering RNA. Moreover, P38 inhibitor counteracted the elevating expression of phosphorylated activating transcription factor 2, Cyclin D1, and Cyclin E1 induced by Noxin overexpression and thereby reversed the effect of Noxin overexpression on facilitating cell growth. Taken together, our studies indicated that Noxin was overexpressed in breast cancer and its positive expression was significantly correlated with advance tumor-node-metastasis stage, positive lymph node metastasis, and poor prognosis. Noxin facilitated the expression of Cyclin D1 and Cyclin E1 through activating P38-activating transcription factor 2 signaling pathway, thus enhanced cell growth of breast cancer.

Alpha-1-antitrypsin suppresses oxidative stress in preeclampsia by inhibiting the p38MAPK signaling pathway: An in vivo and in vitro study.

This present study was designed to investigate the effects of alpha-1-antitrypsin (AAT) on oxidative stress in preeclampsia (PE) by regulating p38 mitogen-activated protein kinase (p38MAPK) signaling pathway. HTR8/SVneo cells were randomly assigned into normal, hypoxia/reoxygenation (H/R), HR + AAT and HR + siRNA-AAT groups. Quantitative real-time polymerase chain reaction (qRT-PCR) and Western blotting were used to detect the mRNA and protein expressions of p-p38MAPK, AAT, signal transducer and activator of transcription 1 (STAT1) and activating transcription factor2 (ATF2). Flow cytometry, scratch test, cell counting kit-8 (CCK-8) assay and the 3-(4,5)-dimethylthiazol (-z-y1)-3,5-di- phenyltetrazolium bromide (MTT) assay were conducted to detect reactive oxygen species (ROS) and cell apoptosis, cell migration, proliferation and cytotoxicity, respectively. Mouse models in PE were established, which were divided into normal pregnancy (NP), PE and PE + AAT groups with blood pressure and urine protein measured. Chromatin immunoprecipitation (ChIP) and enzyme-linked immunosorbent assay (ELISA) were conducted to detect the activity of oxidative stress-related kinases and expressions of inflammatory cytokines and coagulation-related factors in cells and mice placenta. Immunohistochemistry, Western blotting and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) assay were performed to detect AAT and p38MAPK expressions, apoptosis-related protein expressions, and apoptosis rate in mice placenta. Compared with the normal group, the H/R group had decreased expression of AAT, activity of superoxide dismutase (SOD) and GSH-Px, cell proliferation and migration, but increased p38MAPK, STAT1, ATF2, MDA, H2O2, inflammatory cytokines, coagulation-related factors, cell cytotoxicity, ROS, apoptotic factors and apoptosis rate. Compared with the H/R group, the HR + ATT group had increased expressions of AAT, activity of SOD and GSH-Px, cell proliferation and migration but decreased p38MAPK, STAT1, ATF2, malonyldialdehyde (MDA), H2O2, inflammatory cytokines and coagulation-related factors, cell cytotoxicity, ROS, apoptotic factors and apoptosis rate, while opposite results were observed in the HR + siRNA-ATT group. Compared with the NP group, the PE group had decreased activity of SOD and GSH-Px but increased MDA, H2O2, AAT, p38MAPK, inflammatory cytokines, coagulation-related factors and apoptosis rate. The indexes in the PE + AAT group were between the NP and PE groups. Thus, we concluded that AAT suppressed oxidative stress in PE by inhibiting p38MAPK signaling pathway.

miRNA-204 suppresses human non-small cell lung cancer by targeting ATF2.

MicroRNAs (miRNAs) play a critical role in cancer development and progression. Deregulated expression of miR-204 has been reported in several cancers, but the mechanism through which miR-204 modulates human non-small cell lung cancer (NSCLC) is largely unknown. In this study, we investigate the expression and functional role of miR-204 in human NSCLC tissues and cell lines. RNA isolation, qRT-PCR, MTT, colony formation assay, cell cycle assay, cell apoptosis assay, cell migration assay, and Western blot were performed. Statistical analysis was performed using SPSS 18.0 software and statistical significance was accepted at p value <0.05. miR-204 level was significantly reduced in NSCLC tissues as compared to that of non-neoplastic tissues. Transient over-expression of miR-204 by transfecting with miR-204 mimics suppressed NSCLC cell proliferation, migration, and induced apoptosis and G1 arrest, whereas inhibition of miR-204 showed the converse effects. Additionally, activating transcription factor 2 (ATF2), an important transcription factor, was demonstrated as a potential target gene of miR-204. Subsequent investigations found a negative correlation between miR-204 level and ATF2 expression in NSCLC tissue samples. Moreover, we observed that miR-204 expression inversely affected endogenous ATF2 expression at both mRNA and protein levels in vitro. Taken together, miR-204 may act as a tumor suppressor by directly targeting ATF2 in NSCLC.

Activation of MAPK pathways and downstream transcription factors in 2-aminobiphenyl-induced apoptosis.

2-Aminobiphenyls (2-ABP) induces oxidative DNA damage and leads to apoptosis. The precise signaling pathways of inducing apoptosis in vitro are still unknown. This study provides insight into the relationship between 2-ABP-induced apoptosis and the activation of MAPK and downstream transcription factors using pharmacological inhibitors of ERK, p38, and JNK pathways. Results showed that 2-ABP induced the activation of ERK and JNK but not p38. The ERK/JNK pathways downstream transcription factors, c-Jun and ATF-2, were also activated by 2-ABP. The inhibitory effects of ERK inhibitor, U0126, on 2-ABP-induced caspase-3 activity were not detected. However, JNK inhibitor, SP600125, significantly attenuated the caspase-3 activity induced by 2-ABP. The expression of the transcription factors c-Jun and ATF-2 were decreased in 2-ABP treated cells in the presence of ERK/JNK inhibitors, suggesting that the expression of ERK/JNK pathways leads to the downstream activation of c-Jun and ATF-2. N-acetylcysteine, an ROS scavenger, inhibited 2-ABP-induced activation of ERK and JNK, the cell death and caspase-3 activity, which suggested that oxidative stress plays a crucial role in apoptosis through activation of caspase-3 in a ROS/JNK-dependent signaling cascade.

Activating transcription factor 2 in mesenchymal tumors.

Activating transcription factor 2 (ATF2) is a member of activator protein 1 superfamily, which can heterodimerize with other transcription factors regulating cell differentiation and survival. ATF2 assembles into a complex with the synovial sarcoma translocation, chromosome 18 (SS18)-synovial sarcoma, X breakpoint (SSX) fusion oncoprotein, and the transducin-like enhancer of split 1 (TLE1) corepressor, driving oncogenesis in synovial sarcoma. The fusion oncoproteins in many other translocation-associated sarcomas incorporate transcription factors from the ATF/cAMP response element binding or E26 families, which potentially form heterodimers with ATF2 to regulate transcription. ATF2 may therefore play an important role in the oncogenesis of many mesenchymal tumors, but as yet, little is known about its protein expression in patient specimens. Herein we perform immunohistochemical analyses using a validated specific antibody for ATF2 expression and intracellular localization on a cohort of 594 malignant and 207 benign mesenchymal tumors representing 47 diagnostic entities. Melanoma served as a positive control for nuclear and cytoplasmic staining. High nuclear ATF2 expression was mainly observed in translocation-associated and/or spindle cell sarcomas including synovial sarcoma, desmoplastic small round cell tumor, endometrial stromal sarcoma, gastrointestinal stromal tumor, malignant peripheral nerve sheath tumor, and solitary fibrous tumor. Cytoplasmic ATF2 expression was less frequently seen than nuclear expression in malignant mesenchymal tumors. Benign mesenchymal tumors mostly showed much lower nuclear and cytoplasmic ATF2 expression.

The expression of p-ATF2 involved in the chondeocytes apoptosis of an endemic osteoarthritis, Kashin-Beck disease.

BACKGROUND: The purpose of the study was to understand the function and expression of ATF2 by JNK and p38 signal pathways in the chondrocytes apoptosis of articular cartilage of the Kashin-Beck disease (KBD). METHODS: The changes of ATF2, JNK and p38 mRNAs and proteins were investigated between cartilage and chondrocyte as well as KBD and normal. JNK and p38 inhibitors were used as treatments to prevent apoptosis in chondrocytes from KBD patients. RESULTS: It was found that the protein levels of p-p38, p-JNK, ATF2 and p-ATF2 increased in KBD human cartilage which is in line with the higher mRNA levels of p38, JNK and ATF2 as compared both with normal cartilage and KBD chondrocytes. In addition, p-ATF2 was only detected in KBD cartilage. Furthermore, JNK inhibitor was more effective than p38 inhibitor in preventing chondrocyte apoptosis at equal concentrations of 10 µM. CONCLUSION: These findings indicated the expression of p-ATF2 by JNK and p38 signal pathways involved in the chondrocyte apoptosis in cartilage with KBD.

Exercise intensity-dependent regulation of peroxisome proliferator-activated receptor coactivator-1 mRNA abundance is associated with differential activation of upstream signalling kinases in human skeletal muscle.

Skeletal muscle contraction increases intracellular ATP turnover, calcium flux, and mechanical stress, initiating signal transduction pathways that modulate peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha)-dependent transcriptional programmes. The purpose of this study was to determine if the intensity of exercise regulates PGC-1alpha expression in human skeletal muscle, coincident with activation of signalling cascades known to regulate PGC-1alpha transcription. Eight sedentary males expended 400 kcal (1674 kj) during a single bout of cycle ergometer exercise on two separate occasions at either 40% (LO) or 80% (HI) of . Skeletal muscle biopsies from the m. vastus lateralis were taken at rest and at +0, +3 and +19 h after exercise. Energy expenditure during exercise was similar between trials, but the high intensity bout was shorter in duration (LO, 69.9 +/- 4.0 min; HI, 36.0 +/- 2.2 min, P < 0.05) and had a higher rate of glycogen utilization (P < 0.05). PGC-1alpha mRNA abundance increased in an intensity-dependent manner +3 h after exercise (LO, 3.8-fold; HI, 10.2-fold, P < 0.05). AMP-activated protein kinase (AMPK) (2.8-fold, P < 0.05) and calcium/calmodulin-dependent protein kinase II (CaMKII) phosphorylation (84%, P < 0.05) increased immediately after HI but not LO. p38 mitogen-activated protein kinase (MAPK) phosphorylation increased after both trials (2.0-fold, P < 0.05), but phosphorylation of the downstream transcription factor, activating transcription factor-2 (ATF-2), increased only after HI (2.4-fold, P < 0.05). Cyclic-AMP response element binding protein (CREB) phosphorylation was elevated at +3 h after both trials (80%, P < 0.05) and class IIa histone deacetylase (HDAC) phosphorylation increased only after HI (2.0-fold, P < 0.05). In conclusion, exercise intensity regulates PGC-1alpha mRNA abundance in human skeletal muscle in response to a single bout of exercise. This effect is mediated by differential activation of multiple signalling pathways, with ATF-2 and HDAC phosphorylation proposed as key intensity-dependent mediators.

Neurodegenerative, with expression ATF-2 by p38 in cortical neurons.

DNA damage, as an important initiator of neuronal cell death, has been implicated in numerous neurodegenerative conditions. We previously delineated several pathways that control embryonic cortical neuronal cell death evoked by the DNA-damaging agent, camptothecin. The topisomerase-1 inhibitor, camptothecin, has been shown to induce cortical neuronal cell death in a reproducible and synchronistic manner. Primary embryonic neuronal cell culture cortical neurons were prepared. In the study, the survival % of neurons in camptothecin P38 group, after 6 hours (85%), 24 hours (64%) and 48 hours (50%), compared to camptothecin ATF-2 and P38 group after 4 hours (97 and 95%), have been significantly lower, and the expression % of neurons in camptothecin P38 group , after 6 hours (20%), 24 hours (40%) and 48 hours (55%), compared to camptothecin ATF-2 and P38 group after 4 hours (5 and 3%) have been significant lower (p<0.05). The expression % of neurons in camptothecin P38 group, after 24 hours (40%), compared to camptothecin ATF-2 group after 24hours (30%), have been significant lower (p<0.05). This study revealed that camptothecin induces P38 expression and P38 in embryonic cortical neurons to determine the importance of the P38 pathway in neuronal death following DNA damage, and P38 is induce phosphorylation of ATF-2 in embryonic cortical neurons following DNA damage.

Expression and prognostic value of activating transcription factor 2 (ATF2) and its phosphorylated form in mammary carcinomas.

BACKGROUND: The transcription factor ATF2 is overexpressed in various tumors, but its role in breast cancer is still not understood. MATERIALS AND METHODS: In a study of mammary carcinomas, the expression of ATF2 and its phosphorylated active forms was analyzed by Western blot analysis (WB; pThr69/pThr71-ATF2, n=134) and immunohistochemistry (IHC; p-ATF2-Thr6, n=110). Results were correlated with histological and clinical data, survival data, expression of ERK1/2 and two matrix metalloproteinases. RESULTS: Patients with high ATF2 expression as detected by WB had a significantly shorter overall survival (p = 0.038). This tendency was corroborated by IHC. In contrast, high p-ATF2 expression as found by WB correlated significantly with a well-differentiated phenotype, but not with prognosis. Immunohistochemically detected p-ATF2 overexpression was even associated with prolonged survival (p = 0.047). CONCLUSION: Although high ATF2 expression is associated with a poor prognosis, our data do not point to an oncogenic role of active p-ATF2 in mammary carcinomas.

Concordant overexpression of phosphorylated ATF2 and STAT3 in extramammary Paget's disease.

BACKGROUND: Activating transcription factor 2 (ATF2) and signal transducer and activator of transcription 3 (STAT3) play important roles in the pathogenesis of various tumors, but ATF2 expression/activation and the relationship with STAT3 activation have not yet been investigated in extramammary Paget's disease (EMPD). OBJECTIVE: To investigate potential contributions of ATF2 and STAT3 pathways to the pathogenesis of EMPD. METHOD: Paraffin-embedded 45 EMPD specimens (43 primary EMPD and 2 nodal metastases) were subjected to immunohistochemical staining for ATF2, phosphorylated (p)-ATF2 and p-STAT3. RESULTS: P-ATF2 expression in advanced EMPD, non-invasive EMPD and normal skin (NS) controls were 97.9 +/- 1.8%, 82.0 +/- 23.4% and 45.8 +/- 3.2%, respectively, and p-STAT3 expression in advanced EMPD, non-invasive EMPD and NS were 97.0 +/- 2.9%, 83.2 +/- 23.3% and 50.1 +/- 6.7%, respectively. P-ATF2 and p-STAT3 expressions in EMPD were significantly higher than those in NS, indicating a possible contribution of these pathways to the tumor development. P-ATF2 and p-STAT3 expressions in advanced EMPD were significantly higher than those in non-invasive EMPD, possibly indicating that these pathways might also contribute to the tumor invasion and/or metastasis. We also found an exceptionally high positive correlation between p-ATF2 and p-STAT3 expressions in EMPD. CONCLUSIONS: P-ATF2 and p-STAT3 are concordantly overexpressed in EMPD and their expressions may possibly be associated with the tumor stage.

Overexpression of phosphorylated-ATF2 and STAT3 in cutaneous angiosarcoma and pyogenic granuloma.

BACKGROUND: Activating transcription factor-2/Activator protein-1 (AP-1), Signal transducer and activator of transcription-3 and p53 are important regulators of cellular proliferation, apoptosis, differentiation in the pathogenesis of many human tumors, but the expression of phosphorylated (p)-activating transcription factor-2 (p-ATF2), phosphorylated (p)-signal transducer and activator of transcription-3 (p-STAT3) and p53 family (p63 and p73) has not been investigated in cutaneous angiosarcoma (CAS) and pyogenic granuloma (PG) so far. OBJECTIVES: To investigate the expression of p-ATF2, p-STAT3 and p53 and its family in cutaneous vascular tumors (CAS and PG). METHODS: Paraffin-embedded specimens of 14 CAS and 19 PG were subjected to immunohistochemical staining for p-ATF2, p-STAT3, p53, p63 and p73. RESULTS: P-ATF2 was expressed in 13 out of 14 CAS and in all of 19 PG. P-STAT3 was expressed in all of 14 CAS and 19 PG. P53 was expressed in all of 14 CAS and 19 PG, while both p63 and p73 were negative in CAS and PG. The p-ATF2-, p-STAT3- and p53 expression (% positive cells) in CAS and PG were significantly higher than in normal dermal vessels, but none of these transcription factors distinguished malignant (CAS)- from benign (PG) vascular tumor. CONCLUSIONS: The present study suggests that overexpression of p-ATF2, p-STAT3 and possibly p53, but not p63 or p73, may contribute to the tumorigenesis of cutaneous vascular tumors.

The requirement for and changing composition of the activating protein-1 transcription factor during differentiation of human leukemia HL60 cells induced by 1,25-dihydroxyvitamin D3.

The activating protein-1 (AP-1) transcription factor complex is a heterogeneous entity, composed in mammalian cells of dimers chosen from a group of at least eight proteins belonging to three families: jun, fos, and activating transcription factor (ATF). The AP-1 complexes participate in diverse biological processes that include cell proliferation, survival, and differentiation. These seemingly contrasting functions have been attributed to the intensity and duration of the signals provided by AP-1, but the biological consequences of changing composition of the AP-1 complex have not been fully explored. Here, we show that functional AP-1 is required for 1,25-dihydroxyvitamin D3 (1,25D)-induced monocytic differentiation, and that the composition of the AP-1 protein complex that binds TRE, its cognate DNA element, changes as cells differentiate. In HL60 cells in an early stage of differentiation, the principal AP-1 components detected by gel shift analysis include c-jun, ATF-2, fos-B, fra-1, and fra-2. In cells with a more established monocytic phenotype, the demonstrable AP-1 components are c-jun, ATF-2, jun-B, and fos-B. Following the addition of 1 nmol/L of 1,25D, the cellular content of each of these four proteins markedly increased in a sustained manner, whereas the increases in c-fos, fra-1, fra-2, and jun-D were minimal, if any. Small increases in mRNA levels encoding all AP-1 component proteins, except c-fos, were also noted. These findings provide a basis for the previously found participation of the c-Jun N-terminal kinase pathway in 1,25D-induced differentiation of myeloid leukemia cells, and direct attention to jun-B and fos-B as new cellular therapeutic targets, that may promote replicative quiescence associated with differentiation of malignant cells.