In mammalian cells, the MYC oncoprotein binds to thousands of promoters. During mitogenic stimulation of primary lymphocytes, MYC promotes an increase in the expression of virtually all genes. In contrast, MYC-driven tumour cells differ from normal cells in the expression of specific sets of up- and downregulated genes that have considerable prognostic value. To understand this discrepancy, we studied the consequences of inducible expression and depletion of MYC in human cells and murine tumour models. Changes in MYC levels activate and repress specific sets of direct target genes that are characteristic of MYC-transformed tumour cells. Three factors account for this specificity. First, the magnitude of response parallels the change in occupancy by MYC at each promoter. Functionally distinct classes of target genes differ in the E-box sequence bound by MYC, suggesting that different cellular responses to physiological and oncogenic MYC levels are controlled by promoter affinity. Second, MYC both positively and negatively affects transcription initiation independent of its effect on transcriptional elongation. Third, complex formation with MIZ1 (also known as ZBTB17) mediates repression of multiple target genes by MYC and the ratio of MYC and MIZ1 bound to each promoter correlates with the direction of response.

Genetic analysis in mice has demonstrated a crucial role of the Met tyrosine kinase receptor and its ligand, hepatocyte growth factor͞ scatter factor (HGF͞SF), in development of the liver, muscle, and placenta. Here, we use conditional mutagenesis in mice to analyze the function of Met during liver regeneration, using the Mx-cre transgene to introduce the mutation in the adult. After partial hepatectomy in mice carrying the Mx-cre-induced Met mutation, regeneration of the liver is impaired. Comparison of signal transduction pathways in control and mutant livers indicates that Met and other signaling receptors cooperate to fully activate particular signaling molecules, for instance, the protein kinase Akt. However, activation of the Erk1͞2 kinase during liver regeneration depends exclusively on Met. Signaling crosstalk is thus an important aspect of the regulation of liver regeneration. Analysis of cell cycle progression of hepatocytes in conditional Met mutant mice indicates a defective exit from quiescence and diminished entry into S phase. Impaired liver regeneration is accompanied by compensatory physiological responses that include prolonged up-regulation of HGF͞SF and IL-6 in peripheral blood. Our data demonstrate that the HGF͞SF͞Met signaling system is essential not only during liver development but also for the regeneration of the organ in the adult.

Deregulated expression of the MYC oncoprotein contributes to the genesis of many human tumours, yet strategies to exploit this for a rational tumour therapy are scarce. MYC promotes cell growth and proliferation, and alters cellular metabolism to enhance the provision of precursors for phospholipids and cellular macromolecules 1,2 . Here we show in human and murine cell lines that oncogenic levels of MYC establish a dependence on AMPK-related kinase 5 (ARK5; also known as NUAK1) for maintaining metabolic homeostasis and for cell survival. ARK5 is an upstream regulator of AMPK and limits protein synthesis via inhibition of the mammalian target of rapamycin 1 (mTORC1) signalling pathway. ARK5 also maintains expression of mitochondrial respiratory chain complexes and respiratory capacity, which is required for efficient glutamine metabolism. Inhibition of ARK5 leads to a collapse of cellular ATP levels in cells expressing deregulated MYC, inducing multiple pro-apoptotic responses as a secondary consequence. Depletion of ARK5 prolongs survival in MYC-driven mouse models of hepatocellular carcinoma, demonstrating that targeting cellular energy homeostasis is a valid therapeutic strategy to eliminate tumour cells that express deregulated MYC.To identify kinases that are specifically required for the viability of cells expressing deregulated MYC, we used U2OS cells expressing c-MYC fused to the oestrogen receptor ligand binding domain (MYC-ER) (Fig. 1a). Activation of MYC-ER by 4-hydroxytamoxifen (OHT) had little effect on apoptosis when cells were grown at low density in the presence of growth factors. Under these conditions, we performed a short interfering (si)RNA screen of the human kinome, using automated microscopy to identify siRNAs that induced poly-ADP-ribose-polymerase cleavage specifically in the presence of OHT. This screen yielded two hits, ARK5 and AMPK (Supplementary Table 1).Depletion of ARK5 induced the accumulation of MYC-expressing cells that stained positive for annexin V and propidium iodide (Fig. 1a and Supplementary Fig. 1a). Similarly, expressing different short hairpin (sh)RNAs targeting ARK5 induced levels of MYC-dependent death that correlated with the degree of knockdown (Fig. 1b). Titration of OHT revealed that levels of MYC that cause a dependence on ARK5 are higher than those required to promote proliferation ( Supplementary Fig. 1b). Depletion of ARK5 induced death in U2OS cells constitutively expressing MYC and suppressed propagation of MRC5 fibroblasts in a MYCdependent manner (Fig. 1c and Supplementary Fig. 1c). Expression of murine ARK5, which is not targeted by the shRNAs used, prevented death upon depletion of human ARK5 (Fig. 1d). This rescue required LKB1-dependent phosphorylation of T212, but not AKT-dependent phosphorylation of S601 (refs 3, 4). Mutation of K85 within the ATPbinding domain blocked the ability of murine ARK5 to prevent death, demonstrating that rescue requires ARK5 catalytic activity. Accordingly, a small-molecule inhibitor of ARK5, BX795, mimicked the effects o...

Checkpoints that limit stem cell self-renewal in response to DNA damage can contribute to cancer protection but may also promote tissue aging. Molecular components that control stem cell responses to DNA damage remain to be delineated. Using in vivo RNAi screens, we identified basic leucine zipper transcription factor, ATF-like (BATF) as a major component limiting self-renewal of hematopoietic stem cells (HSCs) in response to telomere dysfunction and γ-irradiation. DNA damage induces BATF in a G-CSF/STAT3-dependent manner resulting in lymphoid differentiation of HSCs. BATF deletion improves HSC self-renewal and function in response to γ-irradiation or telomere shortening but results in accumulation of DNA damage in HSCs. Analysis of bone marrow from patients with myelodysplastic syndrome supports the conclusion that DNA damage-dependent induction of BATF is conserved in human HSCs. Together, these results provide experimental evidence that a BATF-dependent differentiation checkpoint limits self-renewal of HSCs in response to DNA damage.