Signal transduction in fibroblasts stably transformed by [Val12]Ras--the activities of extracellular-signal-regulated kinase and Jun N-terminal kinase are only moderately increased, and the activity of the delta-inhibitor of c-Jun is not alleviated

Radiation-induced EGFR-signaling and control of DNA-damage repair

PURPOSE

Over the last decade evidence has accumulated indicating that cell membrane-bound growth factor receptor of the erbB family and especially the epidermal growth factor receptor EGFR (erbB1) mediates resistance of tumor cells to both chemo- and radiotherapy when mutated or overexpressed. More recently a novel link between EGFR signaling pathways and DNA repair mechanisms, especially non-homologous end joining (NHEJ) repair could be demonstrated. The following review summarizes the current knowledge on the role of EGFR and its downstream signaling pathways in the regulation of cellular radiation response and DNA repair.

CONCLUSION

The novel findings on radiation-induced EGFR-signaling and its involvement in regulating DNA-double strand break repair need further investigations of the detailed mechanisms involved. The results to be obtained may not only improve our knowledge on basic mechanisms of radiation sensitivity/resistance but also will promote translational approaches to test new strategies for clinically applicable molecular targeting.

Cell fate determination factor DACH1 inhibits c-Jun-induced contact-independent growth

The cell fate determination factor DACH1 plays a key role in cellular differentiation in metazoans. DACH1 is engaged in multiple context-dependent complexes that activate or repress transcription. DACH1 can be recruited to DNA via the Six1/Eya bipartite transcription (DNA binding/coactivator) complex. c-Jun is a critical component of the activator protein (AP)-1 transcription factor complex and can promote contact-independent growth. Herein, DACH1 inhibited c-Jun-induced DNA synthesis and cellular proliferation. Excision of c-Jun with Cre recombinase, in c-jun(f1/f1) 3T3 cells, abrogated DACH1-mediated inhibition of DNA synthesis. c-Jun expression rescued DACH1-mediated inhibition of cellular proliferation. DACH1 inhibited induction of c-Jun by physiological stimuli and repressed c-jun target genes (cyclin A, beta-PAK, and stathmin). DACH1 bound c-Jun and inhibited AP-1 transcriptional activity. c-jun and c-fos were transcriptionally repressed by DACH1, requiring the conserved N-terminal (dac and ski/sno [DS]) domain. c-fos transcriptional repression by DACH1 requires the SRF site of the c-fos promoter. DACH1 inhibited c-Jun transactivation through the delta domain of c-Jun. DACH1 coprecipitated the histone deacetylase proteins (HDAC1, HDAC2, and NCoR), providing a mechanism by which DACH1 represses c-Jun activity through the conserved delta domain. An oncogenic v-Jun deleted of the delta domain was resistant to DACH1 repression. Collectively, these studies demonstrate a novel mechanism by which DACH1 blocks c-Jun-mediated contact-independent growth through repressing the c-Jun delta domain.

Ecdysone-inducible expression of oncogenic Ha-Ras in NIH 3T3 cells leads to transient nuclear localization of activated extracellular signal-regulated kinase regulated by mitogen-activated protein kinase phosphatase-1

The Ras family of GTP-binding proteins are key transducers of extracellular signals, particularly through the mitogen-activated protein kinase (MAPK) pathway. Constitutively active forms of Ras are found in a variety of tumours, suggesting an important role for this pathway in cancer. Here we report that initial cellular exposure to oncogenic Ras chronically activated the MAPK pathway in the cytoplasm, but transiently activated the same pathway in the nucleus. Nuclear-activated extracellular signal-regulated kinase (ERK) was rapidly dephosphorylated, with consequent short-term activation of the Elk-1 transcription factor and expression of the c-fos gene. Additional experiments suggested that the regulatory mechanism involved requires the calcium-dependent protein phosphotyrosine phosphatase MAPK phosphatase-1 (MKP-1). This is the first report on the ability of Ras, in the absence of growth factors, to transiently activate the MAPK pathway in the nucleus and show an involvement of MKP-1 in nuclear ERK2 regulation. In addition we show that transient activation of the MAPK pathway is sufficient to drive chronic cell-cycle progression. We conclude that, whereas the MAPK pathway is necessary to initiate cellular proliferation and transformation, the transient nature of the MAPK pathway activation suggests the involvement of additional signalling pathway(s) regulated by Ras.

Jun, the oncoprotein

Cellular Jun (c-Jun) and viral Jun (v-Jun) can induce oncogenic transformation. For this activity, c-Jun requires an upstream signal, delivered by the Jun N-terminal kinase (JNK). v-Jun does not interact with JNK; it is autonomous and constitutively active. v-Jun and c-Jun address overlapping but not identical sets of genes. Whether all genes essential for transformation reside within the overlap of the v-Jun and c-Jun target spectra remains to be determined. The search for transformation-relevant targets of Jun is moving into a new stage with the application of DNA microarrays technology. Genetic screens and functional tests remain a necessity for the identification of genes that control the oncogenic phenotype.

ERK signalling in metastatic human MDA-MB-231 breast carcinoma cells is adapted to obtain high urokinase expression and rapid cell proliferation

Increased urokinase plasminogen activator (u-PA) production is associated with tumor invasion and metastasis in several malignancies, including breast cancer. The mechanisms underlying constitutive u-PA expression are not well understood. We examined the relationship between the signal strength of the ERK pathway and the level of u-PA expression in the metastatic human breast cancer cell line MDA-MB-231. Treatment with the MEK1 inhibitor PD98059 resulted in decreased ERK1/2 phosphorylation and decreased u-PA mRNA and protein expression. Inhibition of ERK1/2 activity also led to decreased cell proliferation and to decreased cyclin D1 expression. Less than 5% of total ERK1/2 was phosphorylated in exponentially growing MDA-MB-231 cells, and ERK1/2 activity could be stimulated by okadaic acid. Okadaic acid did not stimulate u-PA expression, but induced strong expression of the cdk-inhibitor p21Cip1. These findings suggest that ERK1/2 signaling is tuned to a level which results in high u-PA expression and rapid cell proliferation.