Malignant melanomas often harbor activating mutations in BRAF (V600E) or, less frequently, in NRAS (Q61R). Intriguingly, the same mutations have been detected at higher incidences in benign nevi, which are largely composed of senescent melanocytes. Overexpression of BRAFV600E or NRASQ61R in human melanocytes in vitro has been shown to induce senescence, although via different mechanisms. How oncogene-induced senescence is overcome during melanoma progression remains unclear. Here, we report that in the majority of analysed BRAFV600E- or NRASQ61R-expressing melanoma cells, C-MYC depletion induced different yet overlapping sets of senescence phenotypes that are characteristic of normal melanocytes undergoing senescence due to overexpression of BRAFV600E or NRASQ61R, respectively. These senescence phenotypes were p16INK4A- or p53-independent, however, several of them were suppressed by genetic or pharmacological inhibition of BRAFV600E or phosphoino-sitide 3-kinase pathways, including rapamycin-mediated inhibition of mTOR-raptor in NRASQ61R-expressing melanoma cells. Reciprocally, overexpression of C-MYC in normal melanocytes suppressed BRAFV600E-induced senescence more efficiently than NRASQ61R-induced senescence, which agrees with the generally higher rates of activating mutations in BRAF than NRAS gene in human cutaneous melanomas. Our data suggest that one of the major functions of C-MYC overexpression in melanoma progression is to continuous suppress BRAFV600E- or NRASQ61R-dependent senescence programs.

The proteasome controls a plethora of survival factors in all mammalian cells analyzed to date. Therefore, it is puzzling that proteasome inhibitors such as bortezomib can display a preferential toxicity toward malignant cells. In fact, proteasome inhibitors have the salient feature of promoting a dramatic induction of the proapoptotic protein NOXA in a tumor cell-restricted manner. However, the molecular determinants that control this specific regulation of NOXA are unknown. Here, we show that the induction of NOXA by bortezomib is directly dependent on the oncogene c-MYC. This requirement for c-MYC was found in a variety of tumor cell types, in marked contrast with dispensable roles of p53, HIF-1␣, and E2F-1 (classical proteasomal targets that can regulate NOXA mRNA under stress). Conserved MYC-binding sites identified at the NOXA promoter were validated by ChIP and reporter assays. Down-regulation of the endogenous levels of c-MYC abrogated the induction of NOXA in proteasomedefective tumor cells. Conversely, forced expression of c-MYC enabled normal cells to accumulate NOXA and subsequently activate cell death programs in response to proteasome blockage. c-MYC is itself a proteasomal target whose levels or function are invariably upregulated during tumor progression. Our data provide an unexpected function of c-MYC in the control of the apoptotic machinery, and reveal a long sought-after oncogenic event conferring sensitivity to proteasome inhibition. drug selectivity ͉ melanoma ͉ oncogenes ͉ apoptosis ͉ Bcl-2 family

To further explore the role of diacylglycerol metabolism in endothelial responses, we used a degenerate reverse transcription-polymerase chain reaction method to identify diacylglycerol kinase isozymes expressed by human endothelial cells. We report the isolation of a 3.5-kilobase cDNA encoding a novel diacylglycerol kinase (hDGK) with a predicted molecular mass of 103.9 kDa. Human DGK contains two zinc fingers, an ATP binding site, and four ankyrin repeats near the carboxyl terminus. A unique feature, as compared with other diacylglycerol kinases, is the presence of a sequence homologous to the MARCKS phosphorylation site domain. From Northern blot analysis of multiple tissues, we observed that hDGK mRNA is expressed at highest levels in brain. COS-7 cells transfected with the hDGK cDNA express 117-kDa and 114-kDa proteins that react specifically with an antibody to a peptide derived from a unique sequence in hDGK. The transfected cells also express increased diacylglycerol kinase activity, which is not altered in the presence of R59949, an inhibitor of human platelet DGK activity. The hDGK displays stereoselectivity for 1,2-diacylglycerol species in comparison to 1,3-diacylglycerol, but does not exhibit any specificity for molecular species of long chain diacylglycerols.

Arachidonoyldiacylglycerol (20:4-DAG) is a second messenger derived from phosphatidylinositol 4,5-bisphosphate and generated by stimulation of glutamate metabotropic receptors linked to G proteins and activation of phospholipase C. 20:4-DAG signaling is terminated by its phosphorylation to phosphatidic acid, catalyzed by diacylglycerol kinase (DGK). We have cloned the murine DGK gene that showed, when expressed in COS-7 cells, selectivity for 20:4-DAG. The significance of DGK in synaptic function was investigated in mice with targeted disruption of the DGK . DGK ؊/؊ mice showed a higher resistance to eletroconvulsive shock with shorter tonic seizures and faster recovery than DGK ؉/؉ mice. The phosphatidylinositol 4,5-bisphosphate-signaling pathway in cerebral cortex was greatly affected, leading to lower accumulation of 20:4-DAG and free 20:4. Also, long-term potentiation was attenuated in perforant path-dentate granular cell synapses. We propose that DGK contributes to modulate neuronal signaling pathways linked to synaptic activity, neuronal plasticity, and epileptogenesis.

Diacylglycerol (DAG) is a second messenger that activates protein kinase C and also occupies a central role in phospholipid biosynthesis. Conversion of DAG to phosphatidic acid by DAG kinase regulates the amount of DAG and the route it takes. We used degenerate primers to amplify polymerase chain reaction products from cDNA derived from human endothelial cells. A product with a novel sequence was identified and used to clone a 2.6-kilobase cDNA from an endothelial cell library. When transfected with a truncated version of this cDNA, COS-7 cells had a marked increase in DAG kinase activity, which demonstrated clear selectivity for arachidonoyl-containing species of diacylglycerol. The open reading frame of this clone has 567 residues with a predicted protein of 64 kDa. This enzyme, which we designated DGK⑀, has two distinctive zinc finger-like structures in its N-terminal region, but does not contain the E-F hand motifs found in several other mammalian DGKs. The catalytic domain of DGK⑀, which is related to other DGKs, contains two ATP-binding motifs. Northern blotting demonstrated that DGK⑀ is expressed predominantly in testis. This unique diacylglycerol kinase may terminate signals transmitted through arachidonoyl-DAG or may contribute to the synthesis of phospholipids with defined fatty acid composition.Diacylglycerol occupies a central position in the biosynthesis of phospholipids and triglycerides. It also is an important intracellular messenger because it can bind to and activate protein kinase C, which, in turn, phosphorylates target proteins (1). This pathway has been implicated in many cellular response including growth, differentiation, and other events such as secretion. The mechanisms by which the signaling pathway and the synthesis of complex lipids are differentially regulated is not clear, but the concentration of DAG 1 within the cell is almost certainly one important component. In response to a variety of signals, the DAG level rises by the activation of one or more phospholipases C and, in some cases, a phospholipase D followed by phosphatidic acid phosphohydrolase. Either pathway causes a rise in the amount of diacylglycerol by degrading phospholipids. The level of DAG also is influenced by the rate at which it is converted into other products. One pathway for decreasing DAG is its conversion to phosphatidic acid, a reaction catalyzed by DAG kinases (EC 2.7.1.107).The stimulated rise in DAG levels is an integral component of the response of cells to a variety of stimuli that lead to growth or differentiation, and the effects of phorbol esters, which are tumor promoters, are through activation of protein kinase C. Thus, the level of DAG may be an important determinant of growth. In support of this, we found that rapidly growing endothelial cells have severalfold higher levels of DAG than quiescent cells, and others observed that transformation of cells by several oncogenes results in an increased content of DAG even in the absence of an additional stimulus (2-4). The conversion of DAG to phospha...

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Actin is a key regulator of RNA polymerase (Pol) II-dependent transcription. Positive transcription elongation factor b (P-TEFb), a Cdk9/cyclin T1 heterodimer, has been reported to play a critical role in transcription elongation. However, the relationship between actin and P-TEFb is still not clear. In this study, actin was found to interact with Cdk9, a catalytic subunit of P-TEFb, in elongation complexes. Using immunofluorescence and immunoprecipitation assays, Cdk9 was found to bind to G-actin through the conserved Thr-186 in the T-loop. Overexpression and in vitro kinase assays showed that G-actin promotes P-TEFb-dependent phosphorylation of the Pol II C-terminal domain. An in vitro transcription experiment revealed that the interaction between G-actin and Cdk9 stimulated Pol II transcription elongation. ChIP and immobilized template assays indicated that actin recruited Cdk9 to a transcriptional template in vivo and in vitro. Using cytokine IL-6-inducible p21 gene expression system, we revealed that actin recruited Cdk9 to endogenous gene. Moreover, overexpression of actin and Cdk9 increased histone H3 acetylation and acetylized histone H3 binding to a transcriptional template through the interaction with histone acetyltransferase, p300. Taken together, our results suggested that actin participates in transcription elongation by recruiting Cdk9 for phosphorylation of the Pol II C-terminal domain, and the actin-Cdk9 interaction promotes chromatin remodeling.