Protein synthesis inhibitors reveal differential regulation of mitogen-activated protein kinase and stress-activated protein kinase pathways that converge on Elk-1.

Inhibitors of protein synthesis, such as anisomycin and cycloheximide, lead to superinduction of immediate-early genes. We demonstrate that these two drugs activate intracellular signaling pathways involving both the mitogen-activated protein kinase (MAPK) and stress-activated protein kinase (SAPK) cascades. The activation of either pathway correlates with phosphorylation of the c-fos regulatory transcription factor Elk-1. In HeLa cells, anisomycin stabilizes c-fos mRNA when protein synthesis is inhibited to only 50%. Under these conditions, anisomycin, in contrast to cycloheximide, rapidly induces kinase activation and efficient Elk-1 phosphorylation. However, full inhibition of translation by either drug leads to prolonged activation of SAPK activity, while MAPK induction is transient. This correlates with prolonged Elk-1 phosphorylation and c-fos transcription. Elk-1 induction and c-fos activation are also observed in KB cells, in which anisomycin strongly induces SAPKs but not MAPKs. Purified p54 SAPK alpha efficiently phosphorylates the Elk-1 C-terminal domain in vitro and comigrates with anisomycin-activated kinases in in-gel kinase assays. Thus, Elk-1 provides a potential convergence point for the MAPK and SAPK signaling pathways. The activation of signal cascades and control of transcription factor function therefore represent prominent processes in immediate-early gene superinduction.

STAT activation by the PDGF receptor requires juxtamembrane phosphorylation sites but not Src tyrosine kinase activation.

Activation of the platelet-derived growth factor (PDGF) receptor tyrosine kinase induces tyrosine phosphorylation of Signal Transducer and Activator of Transcription (STAT) proteins. Since the PDGF receptor also activates the Src tyrosine kinase, it is possible that Src mediates tyrosine phosphorylation of STATs in PDGF-treated cells. Consistent with a role for Src in STAT activation, we found that a PDGF receptor juxtamembrane tyrosine residue required for Src activation is necessary and sufficient for activation of STATs 1 and 3. To test the Src requirement further, we made other mutations in the PDGF receptor juxtamembrane region that increased or decreased Src binding. In epithelial and fibroblast cells, PDGF activated STAT1, 3 and 6 in the absence of detectable binding and activation of Src. In addition, PDGF induced c-myc RNA expression and DNA synthesis even though Src was not detectably activated. The activation of MAP kinase and the induction of c-fos gene expression both correlated with STAT but not Src activation by the receptor. We conclude that juxtamembrane tyrosine phosphorylation is necessary for both Src tyrosine kinase and STAT activation by the betaPDGF receptor, but that both processes are regulated independently by this region.

Damage to DNA by UV light and activation of transcription factors.

Bacteria react to irradiation with short wave length UV (UVC) by mounting a rescuing response which involves the synthesis of proteins engaged in DNA repair, replication and mutagenesis. We analyse here an analogous response shown by mammalian cells in culture and present experimental evidence for the chain of events induced by UV irradiation that leads to enhanced gene expression. Available results suggest that the UV induced signal cascade depends on damage to DNA and also involves components located at the plasma membrane, such as src, ras and raf. These components, upon activation by UV, signal into the cell's nucleus, thereby activating transcription factors which control the activity of UV responsive genes.

The mammalian UV response: mechanism of DNA damage induced gene expression.

DNA damage inducing treatment of cultured mammalian cells triggers the activation of transcription factors and the prolongation of the half life of p53. As the earliest event detectable in the nucleus (5 min), AP-1 (c-Jun/c-Fos) is post-translationally modified. Triggering this early event and triggering subsequent transcription factor dependent processes requires extra-nuclear components of signal transduction such as Src, Ras, Raf-1 and MAP-2 kinase. Recent efforts have concentrated on examining whether DNA damage or other secondary effects of the damaging agent generate the signal then passed on to transcription factors. Further, it has been studied whether a pathway of reverse signalling exists that originates in the nucleus and reaches the cell surface. At the cell surface the UV induced signalling chain can be interrupted experimentally. Beyond this step DNA damage and signal transduction induced by phorbol esters and growth factors merge and reach the nuclear proteins through common components.

Targeting protein kinases for tumor therapy.

Phosphorylation is the most common biochemical modification of cellular molecules, regulating fundamental cellular processes like growth, differentiation, proliferation, movement, and death. It is now clear that protein and lipid kinases as well as phosphatases are causally involved in human disease, especially in cancer. The first part of this review tries to compile our current knowledge about the involvement of protein kinases in human cancer. Phosphatases as well as phospholipid kinases will be omitted from this article for the purpose of simplification. In the second part an updated list of ongoing clinical trials involving protein kinases as targets for tumor therapy will be given, together with a brief summary of technical approaches in targeting protein kinases for therapy.

UV irradiation-induced interleukin-1 and basic fibroblast growth factor synthesis and release mediate part of the UV response.

UV irradiated cells release into the culture medium factors that induce, when given to nonirradiated cells, the transcription of several UV-inducible genes (collagenase I, human immunodeficiency virus type 1, metallothionein IIA). We identify here the active factors released from UV-treated HeLa cells, as interleukin 1 alpha and basic fibroblast growth factor. UV irradiation leads to increased mRNA levels for both factors and to their enhanced synthesis. Experiments with the drug suramin, which inhibits growth factor-growth factor receptor interactions and with antibodies directed against interleukin 1 alpha and basic fibroblast growth factor, suggest that growth factors do not only transduce the UV-induced signal to nonirradiated cells but act on the producer cell thus establishing an obligatory growth factor loop for at least part of the UV response.

Involvement of growth factor receptors in the mammalian UVC response.

Irradiation of HeLa cells with short-wavelength ultraviolet light (UVC) induces the modification and activation of the preexisting transcription factors c-Fos-c-Jun (AP-1) and TCF/Elk-1, as well as the protein synthesis independent transcriptional activation of the c-fos and c-jun genes. This response to UVC is mediated via obligatory cytoplasmic signal transduction, involving Ras and Raf, Src, and MAP kinases. The UVC response is inhibited by prior down-modulation of growth factor receptor signaling upon growth factor prestimulation, by suramin (an inhibitor of receptor activation) or by expression of a dominant negative epidermal growth factor (EGF) receptor mutant. These data suggest the involvement of several growth factor receptors in the UVC response. Indeed, UVC induces the suramin-inhibitable immediate tyrosine phosphorylation of the EGF receptor.

Src family kinases are required for integrin but not PDGFR signal transduction.

Src family kinases (SFKs) have been implicated as important regulators of ligand-induced cellular responses including proliferation, survival, adhesion and migration. Analysis of SFK function has been impeded by extensive redundancy between family members. We have generated mouse embryos harboring functional null mutations of the ubiquitously expressed SFKs Src, Yes and Fyn. This triple mutation leads to severe developmental defects and lethality by E9.5. To elucidate the molecular mechanisms underlying this phenotype, SYF cells (deficient for Src, Yes and Fyn) were derived and tested for their ability to respond to growth factors or plating on extracellular matrix. Our studies reveal that while Src, Yes and Fyn are largely dispensable for platelet-derived growth factor (PDGF)-induced signaling, they are absolutely required to mediate specific functions regulated by extracellular matrix proteins. Fibronectin-induced tyrosine phosphorylation of focal adhesion proteins, including the focal adhesion kinase FAK, was nearly eliminated in the absence of Src, Yes and Fyn. Furthermore, consistent with previous reports demonstrating the importance of FAK for cell migration, SYF cells displayed reduced motility in vitro. These results demonstrate that SFK activity is essential during embryogenesis and suggest that defects observed in SYF triple mutant embryos may be linked to deficiencies in signaling by extracellular matrix-coupled receptors.

A human ubiquitin-conjugating enzyme homologous to yeast UBC8.

Ubiquitin-conjugating enzymes catalyze the covalent attachment of ubiquitin to cellular substrates. Here we describe the isolation of a novel ubiquitin-conjugating enzyme from human placenta and the cloning of the corresponding cDNA. DNA sequencing revealed that this gene, UbcH2, encodes a protein with significant sequence similarity to yeast UBC8. In contrast to a previous report (Qin, S., Nakajima, B., Nomura, M., and Arfin, S. M. (1991) J. Biol. Chem. 266, 15549-15554), we discovered that UBC8 is interrupted by a single intron bearing an unusual branch point sequence. The revised amino acid sequence of yeast UBC8 exhibits 54% amino acid sequence identity to human UbcH2. Moreover, full-length UbcH2 and UBC8 enzymes expressed from their cDNAs show similar enzymatic activities in vitro by catalyzing the ubiquitination of histones, suggesting that the two enzymes may fulfill similar functions in vivo. Interestingly, comparison of the enzymatic activities of a truncated UBC8 (Qin, S., Nakajima, B., Nomura, M. and Arfin, S. M. (1991) J. Biol. Chem. 266, 15549-15554) and of the full-length enzyme (this report) suggests, that the first 12 amino-terminal residues of UBC8 are required for ubiquitination of histones in vitro but not for thiolester formation with ubiquitin. This suggests that the NH2 terminus of UBC8 may be necessary either for substrate recognition or for the transfer of ubiquitin onto substrates. The UbcH2 gene is located on chromosome 7 and shows a complex expression pattern with at least five different mRNAs.

CREB is activated by UVC through a p38/HOG-1-dependent protein kinase.

Changes in environmental conditions such as the addition of growth factors or irradiation of cells in culture first affect immediate response genes. We have shown previously that short wavelength UV irradiation (UVC) elicits massive activation of several growth factor receptor-dependent pathways. At the level of the immediate response gene c-fos, these pathways activate the transcription factor complex serum response factor (SRF)-p62TCF which mediates part of the UV-induced transcriptional response. These studies have, however, suggested that more that one pathway is required for full UV responsiveness of c-fos. Using appropriate promoter mutations and dominant-negative cAMP response element (CRE)-binding protein (CREB), we now find that UVC-induced transcriptional activation depends also on the CRE at position -60 of the c-fos promoter and on the functionality of a CREB. Upon UV irradiation, CREB and ATF-1 are phosphorylated at serines 133 and 63, respectively, preceded by and dependent on activation of p38/RK/HOG-1 and of a p38/RK/HOG-1-dependent p108 CREB kinase. Although p90RSK1 and MAPKAP kinase 2 are also activated by UV, p90RSK1 does not, at least not decisively, participate in this signalling pathway to CREB and ATF-1 as it is not p38/RK/HOG-1 dependent, and CREB is a poor substrate for MAPKAP kinase 2 in vitro. On the basis of resistance to the growth factor receptor inhibitor suramin and of several types of cross-refractoriness experiments, the UVC-induced CREB/ATF-1 phosphorylation represents an as yet unrecognized route of UVC-induced signal transduction, independent of suramin-inhibitable growth factor receptors and different from the Erk 1,2-p62TCF pathway.

UV-induced activation of AP-1 involves obligatory extranuclear steps including Raf-1 kinase.

Irradiation of cells with ultraviolet light (UV) leads to modifications of c-Jun resembling those elicited by phorbol esters or oncogenes, and to enhanced transcription of AP-1-dependent genes. The UV-induced signal also triggers activation of Raf-1 and MAP-2 kinases. A dominant-negative Raf-1 kinase mutant strongly interferes with both phorbol ester and UV-induced AP-1 activation, indicating obligatory involvement of identical components in cytoplasmic signal transduction. Thus, from a presumably nuclear site of energy absorption, a signal needs to be transmitted to the cytoplasm in order to achieve activation of a nuclear transcription factor. Further, signals elicited from different primary sites merge prior to or at the level of activation of Raf-1 kinase.