UVA induces Ser381 phosphorylation of p90RSK/MAPKAP-K1 via ERK and JNK pathways

Mkp1 and Mkp2, two MAPKAP-kinase homologues in Schizosaccharomyces pombe, interact with the MAP kinase Sty1

Mkp1 ( MAPKAP kinase Schizosaccharomyces pombe 1) and Mkp2 are two members from fission yeast of the sub-class of putative MAPK-activated protein kinases in yeasts, the other known members being Rck1 and Rck2 from Saccharomyces cerevisiae. The Mkp1 protein is readily co-immunoprecipitated with Sty1 from S. pombe extracts; Mkp2 shows a weaker interaction with Sty1. In mkp1 mutants, conjugation and meiosis proceed more readily and rapidly than in wild-type cells, in analogy to what was previously found for S. cerevisiae rck1 mutants. Conversely, overexpression of mkp1(+) delays meiosis. Mkp1 is phosphorylated in vivo in a sty1(+)-dependent manner; this modification is removed when cells are starved for nitrogen, a condition that is conducive to entry into stationary phase and meiosis. Overexpression of mkp1(+), like a sty1 mutation, also causes vegetative cells to elongate. The level of Mkp1 phosphorylation drops as cells enter mitosis. We have localised Mkp1 to the cytoplasm, excluded from the nucleus, in vegetative cells. The Mkp1 protein accumulates in zygotic asci and is concentrated within spores. The mkp2(+) gene has no noticeable impact on meiosis. Mkp2 is excluded from the nucleus in vegetative cells, and is concentrated at the septa of dividing cells. Mkp2 does not accumulate in meiotic cells.

Mitogen-activated protein kinase activation in UV-induced signal transduction

Experimental evidence supported by epidemiological findings suggests that solar ultraviolet (UV) irradiation is the most important environmental carcinogen leading to the development of skin cancers. Because the ozone layer blocks UVC (wavelength, 180 to 280 nm) exposure, UVA (UVA I, 340 to 400 nm; UVA II, 320 to 340 nm) and UVB (280 to 320 nm) are probably the chief carcinogenic components of sunlight with relevance for human skin cancer. Substantial contributions to the elucidation of the specific signal transduction pathways involved in UV-induced skin carcinogenesis have been made over the past few years, and most evidence suggests that the cellular signaling response is UV wavelength-dependent. The mitogen-activated protein kinase (MAPK) signaling cascades are targets for UV and are important in the regulation of the multitude of UV-induced cellular responses. Experimental studies have used a range of UVA, UVB, UVC, and various combinations in multiple doses, and the observed effects on activation and phosphorylation of MAPKs are varied. This review focuses on the mechanistic data supporting a role for MAPKs in UV-induced skin carcinogenesis. Progress in understanding the mechanisms of UV-induced signal transduction could lead to the use of these protein kinases as specific targets for the prevention and control of skin cancer.

Protein kinases and their involvement in the cellular responses to genotoxic stress

Cells are constantly subjected to genotoxic stress, and much has been learned regarding their response to this type of stress during the past year. In general, the cellular genotoxic response can be thought to occur in three stages: (1) damage sensing; (2) activation of signal transduction pathways; (3) biological consequences and attenuation of the response. The biological consequences, in particular, include cell cycle arrest and cell death. Although our understanding of the molecular mechanisms underlying cellular genotoxic stress responses remains incomplete, many cellular components have been identified over the years, including a group of protein kinases that appears to play a major role. Various DNA-damaging agents can activate these protein kinases, triggering a protein phosphorylation cascade that leads to the activation of transcription factors, and altering gene expression. In this review, the involvement of protein kinases, particularly the mitogen-activated protein kinases (MAPKs), at different stages of the genotoxic response is discussed.

The role of p38 in UVA-induced cyclooxygenase-2 expression in the human keratinocyte cell line, HaCaT

We examined the expression of cycloxygenase-2, the rate-limiting enzyme in the production of prostaglandins, in the UVA-irradiated human keratinocyte cell line, HaCaT. UVA induced a dose-dependent increase in COX-2 at the protein level at 2 and 4 h post-irradiation and at the mRNA level at 1 and 2 h post-irradiation. Experiments using semi-quantitative RT-PCR demonstrate that UVA increased the half-life of the COX-2 message by more than fourfold in the presence of Actinomycin D (with a half life between 4 and 8 h post-irradiation), suggesting that UVA induction of COX-2 is post-transcriptionally regulated. Through the use of the specific p38 inhibitor, SB202190, increases in COX-2 message and protein levels were abrogated in UVA-irradiated cells. In UVA-irradiated cells treated with SB202190, the half-life of the COX-2 message was decreased to basal levels (between 1 and 2 h post-irradiation), indicating that p38 was responsible for the stabilization of the message. Luciferase activity was increased in UVA-irradiated cells transfected with reporter constructs containing the 3' UTR of COX-2, a region containing AU-rich elements (AREs). These regulatory sequences of AUUUA have been proposed as one mechanism of post-transcriptional regulation. Increases observed in luciferase activity could be decreased using a p38 dominant-negative construct. We report for the first that UVA can induce COX-2 expression in the human keratinocyte cell line, HaCaT. Additionally, p38 appears to play a critical role in the UVA-induced expression of COX-2 in these keratinocytes and may serve as a potential drug target in the chemoprevention of skin cancer.

TNF-alpha inhibits SP-A gene expression in lung epithelial cells via p38 MAPK

Surfactant protein A (SP-A), the major lung surfactant-associated protein, mediates local defense against pathogens and modulates inflammation in the alveolus. Tumor necrosis factor (TNF)-alpha, a proinflammatory cytokine, inhibits SP-A gene expression in lung epithelial cells. Inhibitors of the phosphatidylinositol 3-kinase pathway, i.e., wortmannin, LY-294002, and rapamycin, did not block the inhibitory effects of TNF-alpha on SP-A mRNA levels. An inhibitor of the p44/42 mitogen-activated protein kinase (MAPK) pathway, PD-98059, was also ineffective. PD-169316 and SB-203580, inhibitors of p38 MAPK, blocked the TNF-alpha-mediated inhibition of SP-A mRNA levels. TNF-alpha increased the phosphorylation of p38 MAPK within 15 min. Anisomycin, an activator of p38 MAPK, increased p38 MAPK phosphorylation and decreased SP-A mRNA levels in a dose-dependent manner. Finally, TNF-alpha increased the phosphorylation of ATF-2, a transcription factor that is a p38 MAPK substrate. We conclude that TNF-alpha downregulates SP-A gene expression in lung epithelial cells via the p38 MAPK signal transduction pathway.

Attenuation of protein kinase C and cAMP-dependent protein kinase signal transduction in the neurogranin knockout mouse

Neurogranin (Ng) is a brain-specific, postsynaptically located protein kinase C (PKC) substrate, highly expressed in the cortex, hippocampus, striatum, and amygdala. This protein is a Ca(2+)-sensitive calmodulin (CaM)-binding protein whose CaM-binding affinity is modulated by phosphorylation and oxidation. To investigate the role of Ng in neural function, a strain of Ng knockout mouse (KO) was generated. Previously we reported (Pak, J. H., Huang, F. L., Li, J., Balschun, D., Reymann, K. G., Chiang, C., Westphal, H., and Huang, K.-P. (2000) Proc. Natl. Acad. Sci. U. S. A. 97, 11232-11237) that these KO mice displayed no obvious neuroanatomical abnormality, but exhibited deficits in learning and memory and activation of Ca(2+)/CaM-dependent protein kinase II. In this report, we analyzed several downstream phosphorylation targets in phorbol 12-myristate 13-acetate- and forskolin-treated hippocampal slices from wild type (WT) and KO mice. Phorbol 12-myristate 13-acetate caused phosphorylation of Ng in WT mice and promoted the translocation of PKC from the cytosolic to the particulate fractions of both the WT and KO mice, albeit to a lesser extent in the latter. Phosphorylation of downstream targets, including mitogen-activated protein kinases, 90-kDa ribosomal S6 kinase, and the cAMP response element binding protein (CREB) was significantly attenuated in KO mice. Stimulation of hippocampal slices with forskolin also caused greater stimulation of protein kinase A (PKA) in the WT as compared with those of the KO mice. Again, phosphorylation of the downstream targets of PKA was attenuated in the KO mice. These results suggest that Ng plays a pivotal role in regulating both PKC- and PKA-mediated signaling pathways, and that the deficits in learning and memory of spatial tasks detected in the KO mice may be the result of defects in the signaling pathways leading to the phosphorylation of CREB.

Involvement of c-jun NH(2)-terminal kinases in resveratrol-induced activation of p53 and apoptosis

Resveratrol, a constituent of grapes and other foods, is one of the most promising agents for cancer prevention. In a previous study, we showed that the antitumor activity of resveratrol occurs through extracellular signal-regulated protein kinases (ERKs) and p38 kinase-mediated p53 activation. In this study, we also determined that c-jun NH(2)-terminal kinases (JNKs) are involved in resveratrol-induced p53 activation and induction of apoptosis. In the JB6 mouse epidermal cell line, resveratrol activated JNKs dose-dependently within a dose range of 10-40 microM, the same dosage responsible for the inhibition of tumor promoter-induced cell transformation. Stable expression of a dominant negative mutant of JNK1 or disruption of the Jnk1 or Jnk2 gene markedly inhibited resveratrol-induced p53-dependent transcription activity and induction of apoptosis. Furthermore, resveratrol-activated JNKs were shown to phosphorylate p53 in vitro, but this activity was repressed in the cells expressing a dominant negative mutant of JNK1 or in Jnk1 or Jnk2 knockout (Jnk1(-/-) or Jnk2(-/-)) cells. These data suggested that JNKs act as mediators of resveratrol-induced activation of p53 and apoptosis, which may occur partially through p53 phosphorylation.

UVA irradiation-induced activation of activator protein-1 is correlated with induced expression of AP-1 family members in the human keratinocyte cell line HaCaT

To determine whether the transcription factor activator protein-1 (AP-1) could be modulated by ultraviolet A (UVA) exposure, we examined AP-1 DNA-binding activity and transactivation after exposure to UVA in the human immortalized keratinocyte cell line HaCaT. Maximal AP-1 transactivation was observed with 250 kJ/m2 UVA between 3 and 4 h after irradiation. DNA binding of AP-1 to the target 12-O-tetradecanoylphorbol-13-acetate response element sequence was maximally induced 1-3 h after irradiation. Both de novo transcription and translation contributed to the UVA-induced AP-1 DNA binding. c-Fos was implicated as a primary component of the AP-1 DNA-binding complex. Other components of the complex included Fra-2, c-Jun, JunB and JunD. UVA irradiation induced protein expression of c-Fos, c-Jun, Fra-1 and Fra-2. Phosphorylated forms of these induced proteins were determined at specific time points. A strong correlation existed between UVA-induced AP-1 activity and accumulation of c-Fos, c-Jun and Fra-1 proteins. UVA irradiation also induced c-fos and c-jun mRNA expression and transcriptional activation of the c-fos gene promoter. These results demonstrate that UVA irradiation activates AP-1 and that c-fos induction may play a critical role in the response of these human keratinocytes to UVA irradiation.

Role of MAP kinases in UVB-induced phosphorylation of p53 at serine 20

Phosphorylation of the p53 tumor suppressor protein is one of the key regulatory steps in its activation process. Serine 20 phosphorylation of p53 has been shown to be required for the activation of p53 following UV radiation, but the signaling pathway mediating UV-induced phosphorylation is unknown. Here, we determined the role of MAP kinases in UVB-induced phosphorylation and found that JNKs are directly involved in the phosphorylation of p53 at serine 20. In a mouse JB6 epidermal cell line, dominant negative JNK1 abrogated UVB-induced phosphorylation of p53 at serine 20, whereas dominant negative p38 kinase or its inhibitor, SB202190, partially attenuated the phosphorylation. In contrast, dominant negative ERK2 or the MEK1 inhibitor, PD98059, had no effect on p53 phosphorylation at serine 20. Importantly, UVB-activated or active recombinant JNK1/2, or the p38 kinase downstream target, MAPKAPK-2, but not ERKs or p38 kinase, phosphorylated p53 at serine 20 in vitro. Furthermore, phosphorylation of p53 at serine 20 by UVB-activated JNKs and UVB-induced p53-dependent transcriptional activity were suppressed in Jnk1 or Jnk2 knockout (Jnk1(-/-) or Jnk2(-/-)) cells. Additionally, Jnk1(-/-), Jnk2(-/-), and p53-deficient (p53(-/-)) cells, as well as re-introduction of a p53 mutant with substitution of serine 20 to alanine into p53(-/-) cells, were defective for UVB-induced apoptosis. These findings strongly suggest that JNKs are the major direct signaling mediators of UVB-induced p53 phosphorylation at serine 20.

Requirement of ATM in UVA-induced signaling and apoptosis

Solar UVA, but not UVC, reaches the earth's surface and therefore is an important etiological factor for the induction of human skin cancer. ATM kinase is an important regulator of cell survival and cell cycle checkpoints. Here, we observe that UVA, unlike UVC, triggers ATM kinase activity, and the activation may occur through reactive oxygen species produced after irradiation of cells with UVA. We also show that ATM activation is involved in the apoptotic response to UVA but not UVC. Furthermore, we provide evidence that ATM-dependent p53 and c-Jun N-terminal kinase (JNK) pathways are linked to UVA-induced apoptosis. On the other hand, UVC-induced apoptosis occurs through ATR-dependent p53 phosphorylation as well as the JNK pathway. Therefore, these results suggest that ATM, like p53, is involved in the UVA-induced apoptosis to suppress carcinogenesis.

Induction of EGFR-dependent and EGFR-independent signaling pathways by ultraviolet A irradiation

Most of the signal pathways involved in ultraviolet (UV)-induced skin carcinogenesis are thought to originate at plasma membrane receptors. However, UVA-induced signal transduction to downstream ribosomal protein S6 kinases, p70(S6K) and p90(RSK), is not well understood. In this report, we show that UVA stimulation of the epidermal growth factor receptor (EGFR) may lead to activation of p70(S6K)/p90(RSK) through phosphatidyl isositol (PI)-3 kinase and extracellular receptor-activated kinases (ERKs). Evidence is provided that phosphorylation and activation of p70(S6K)/p90(RSK) induced by UVA were prevented in Egfr(-/-) cells and were also markedly inhibited by the EGFR-specific tyrosine kinase inhibitors AG1478 and PD153035. Furthermore, EGFR tyrosine kinase inhibitors and EGFR deficiency significantly suppressed activation of PI-3 kinase and ERKs in regulating activation of p90(RSK)/p70(S6K) but had no effect on activation of c-Jun NH(2)-terminal kinases (JNKs) and p38 kinase in response to UVA. Thus, our results suggest that UVA-induced EGFR signaling may be required for activation of p90(RSK)/p70(S6K), PI-3 kinase, and ERKs but not JNKs or p38 kinase.

Angiotensin II promotes the phosphorylation of cyclic AMP-responsive element binding protein (CREB) at Ser133 through an ERK1/2-dependent mechanism

In cells from the adrenal medulla, angiotensin II (AII) regulates both the activity and mRNA levels of catecholamine biosynthetic enzymes whose expression is thought to be under the control of cAMP-responsive element (CRE) binding protein (CREB). In this study, we evaluated the effect of AII stimulation on CREB phosphorylation at Ser133 (pCREB) in bovine adrenal chromaffin cells (BACC). We found that AII produces a rapid and AII type-1 receptor (AT1)-dependent increase in pCREB levels, which is blocked by the MEK1/2 inhibitor U0126 but not by H-89, SB203580 or KN-93, suggesting that it is mediated by the extracellular-regulated protein kinases 1 and 2 (ERK1/2) and not by cAMP-dependent protein kinase (PKA), p38 mitogen-activated protein kinase (p38MAPK) or Ca(2+)/calmodulin-dependent protein kinases (CaMKs) dependent pathways. Gel-shift experiments showed that the increase in pCREB levels is accompanied by an ERK1/2-dependent upregulation of CRE-binding activity. We also found that AII promotes a rapid and reversible increase in the activity of the non-receptor tyrosine kinase Src and that the inhibition of this enzyme completely blocks the AII-induced phosphorylation of ERK1/2, the CREB kinase (p90)RSK and CREB. Our data support the hypothesis that in BACC, AII upregulates CREB functionality through a mechanism that requires Src-mediated activation of ERK 1/2 and (p90)RSK.

MSK1 and JNKs mediate phosphorylation of STAT3 in UVA-irradiated mouse epidermal JB6 cells

Phosphorylation of Tyr(705) and Ser(727) of signal transducer and activator of transcription 3 (STAT3) are known to be required for maximal activation by diverse stimuli. Tyr(705) phosphorylation is generally accepted to be mediated by the Janus kinase family. But the mechanism for STAT3 (Ser(727)) phosphorylation is not well understood. Here, we provide evidence that UVA-induced phosphorylation of STAT3 at Ser(727) is inhibited by pretreatment of JB6 cells with PD98059 or SB202190. Phosphorylation of STAT3 (Ser(727)) is also markedly prevented by a dominant negative mutant of ERK2, c-Jun N-terminal kinase 1 (JNK1), or p38 kinase and in knockout Jnk1(-/-) or Jnk2(-/-) cells. Furthermore, STAT3 (Ser(727)) phosphorylation is suppressed by C- or N-terminal "kinase-dead" mutants of mitogen- and stress-activated protein kinase 1 (MSK1), a downstream kinase of ERKs and p38 kinase, and H89, a potential MSK1 inhibitor. In vitro experiments showed that active MSK1 and JNKs, but not ERKs or p38 kinase, phosphorylate STAT3 (Ser(727)). Additionally, the role of MAPKs in mediating UVA-stimulated DNA binding activity of STAT3 was investigated. Overall, these results suggest that UVA-induced Ser(727) phosphorylation of STAT3 may occur through MSK1 and JNKs.

Phosphorylation of raf-1 by kinase suppressor of ras is inhibited by "MEK-specific" inhibitors PD 098059 and U0126 in differentiating HL60 cells

Determination of the involvement of MAP kinase cascades in signaling cell growth or differentiation is aided by the use of the inhibitors PD 098059 [2-(2'-amino-3'-methoxyphenyl)oxananphthalen-4-one] and U0126 [1,4-diamino-2,3-dicyano-1,4-bis(2-aminophenylthio)butadiene], believed to be MEK-specific kinase inhibitors. We report here that the activity of kinase suppressor of ras (KSR-1), a kinase upstream of raf-1, is inhibited by both these compounds at concentrations similar to those that inhibit MEK-1. Further, in HL60 cells induced to differentiate with 1,25-dihydroxyvitamin D(3) raf-1 and p90RSK, but not ERK1/2, are coregulated, and their expression as well as monocytic differentiation is inhibited in parallel by PD 098059. Thus, in this system raf-1 is phosphorylated by KSR-1, and PD 098059 as well as U0126 inhibits this phosphorylation. This suggests great caution in the interpretation of experiments that utilize these pharmacological inhibitors of kinase activity as evidence for a role for the MEK--ERK module in ras or raf-1 signaling.

Sodium butyrate induces transcription from the G alpha(i2) gene promoter through multiple Sp1 sites in the promoter and by activating the MEK-ERK signal transduction pathway

Sodium butyrate, an erythroid differentiation inducer and a histone deacetylase inhibitor, increases G alpha(i2) levels in differentiating K562 cells. Here we show that sodium butyrate induces G alpha(i2) gene transcription via sequences at -50/-36 and -92/-85 in the G alpha(i2) gene promoter. Both sequences contain core sequence motif for Sp1 binding; electrophoretic mobility shift as well as supershift assays confirmed binding to Sp1. Transcription from the G alpha(i2) gene promoter was also activated by two other histone deacetylase inhibitors, trichostatin A and Helminthsporium carbonium toxin (HC toxin), which also induce erythroblastic differentiation in K562 cells. However, hydroxyurea, a potent erythroid differentiation inducer in these cells, did not activate transcription from this gene promoter, indicating that promoter activation is inducer-specific. Mutations within the Sp1 sites at -50/-36 and -92/-85 in the G alpha(i2) gene promoter substantially decreased transcriptional activation by sodium butyrate, trichostatin A, or HC toxin. Transfection with constitutively activated ERKs indicated that this promoter can be activated through the MEK-ERK signal transduction pathway. Inhibition of the MEK-ERK pathway with U0126 or reduction in the expression of endogenous ERK with an antisense oligonucleotide to ERK significantly inhibited sodium butyrate- and HC toxin-induced transcription but had no effect on trichostatin A-induced transcription. Inhibition of the JNK and p38 MAPKs, using selective inhibitors, had no effect on sodium butyrate-induced transcription. In cells in which sodium butyrate induction of promoter activation had been inhibited by various concentrations of U0126, constitutively activated ERK2 reversed this inhibition. These results show that the MEK-ERK signal transduction pathway is important in butyrate signaling, which eventually converges in the cell nucleus.