Metabolic stress results in p53 activation, which can trigger cell-cycle arrest, ROS clearance, or apoptosis. However, what determines the p53-mediated cell fate decision upon metabolic stress is not very well understood. We show here that PGC-1α binds to p53 and modulates its transactivation function, resulting in preferential transactivation of proarrest and metabolic target genes. Thus glucose starvation results in p53-dependent cell-cycle arrest and ROS clearance, but abrogation of PGC-1α expression results in extensive apoptosis. Additionally, prolonged starvation results in PGC-1α degradation concomitant with induction of apoptosis. We have also identified RNF2, a Polycomb group (PcG) protein, as the cognate E3 ubiquitin ligase. Starvation of mice where PGC-1α expression is abrogated results in loss of p53-mediated ROS clearance, enhanced p53-dependent apoptosis, and consequent severe liver atrophy. These findings provide key insights into the role of PGC-1α in regulating p53-mediated cell fate decisions in response to metabolic stress.

A critical unresolved issue about the genotoxic stress response is how the resulting activation of the p53 tumor suppressor can lead either to cell-cycle arrest and DNA repair or to apoptosis. We show here that hematopoietic zinc finger (Hzf), a zinc-finger-containing p53 target gene, modulates p53 transactivation functions in an autoregulatory feedback loop. Hzf is induced by p53 and binds to its DNA-binding domain, resulting in preferential transactivation of proarrest p53 target genes over its proapoptotic target genes. Thus, p53 activation results in cell-cycle arrest in Hzf wild-type MEFs, while in Hzf(-/-) MEFs, apoptosis is induced. Exposure of Hzf null mice to ionizing radiation resulted in enhanced apoptosis in several organs, as compared to in wild-type mice. These findings provide novel insights into the regulation of p53 transactivation function and suggest that Hzf functions as a key player in regulating cell fate decisions in response to genotoxic stress.

SIRT6 (sirtuin 6) is a member of sirtuin family of deacetylases involved in diverse processes including genome stability, metabolic homeostasis, and tumorigenesis. However, the role of SIRT6 deacetylase activity in its tumor-suppressor functions is not well understood. Here we report that SIRT6 binds to and deacetylates nuclear PKM2 (pyruvate kinase M2) at the lysine 433 residue. PKM2 is a glycolytic enzyme with nonmetabolic nuclear oncogenic functions. SIRT6-mediated deacetylation results in PKM2 nuclear export. We further have identified exportin 4 as the specific transporter mediating PKM2 nuclear export. As a result of SIRT6-mediated deacetylation, PKM2 nuclear protein kinase and transcriptional coactivator functions are abolished. Thus, SIRT6 suppresses PKM2 oncogenic functions, resulting in reduced cell proliferation, migration potential, and invasiveness. Furthermore, studies in mouse tumor models demonstrate that PKM2 deacetylation is integral to SIRT6-mediated tumor suppression and inhibition of metastasis. Additionally, reduced SIRT6 levels correlate with elevated nuclear acetylated PKM2 levels in increasing grades of hepatocellular carcinoma. These findings provide key insights into the pivotal role of deacetylase activity in SIRT6 tumor-suppressor functions.) is a member of the highly conserved sirtuin family of NAD + -dependent enzymes and plays a key role in DNA repair, telomere maintenance, and cellular metabolic processes. It exhibits diverse enzymatic activities including NAD + -dependent deacetylation and mono-ADP ribosylation. SIRT6 deacetylates telomeric histone H3 at lysine 9 (H3K9) and lysine 56 residues (H3K56) (1, 2). SIRT6-mediated deacetylation of telomeric H3K9 is required for the stable association of the Werner syndrome protein with telomeric chromatin for proper telomere function. SIRT6 also interacts with the NF-κB RELA subunit, deacetylates H3K9 at NF-κB target gene promoters, and attenuates NF-κB-mediated apoptosis and senescence (3). Likewise, SIRT6 also binds to hypoxia-inducible factor 1-alpha (HIF1α), deacetylates H3K9 at HIF1α target gene promoters, and regulates glucose homeostasis (4). SIRT6 also interacts with and deacetylates CtIP [C-terminal binding protein (CtBP) interacting protein] to promote the repair of DNA double-strand breaks by homologous recombination (5). SIRT6 mono-ADP ribosylates PARP1 to stimulate the repair of DNA double-strand breaks in response to oxidative stress (6).Several recent studies report that SIRT6 functions as a tumor suppressor. It was observed that loss of SIRT6 promotes tumor formation even without the activation of known oncogenes (7). Furthermore, SIRT6 was found to be down-regulated in pancreatic and colorectal cancers. Down-regulation of SIRT6 also has been reported in hepatocellular carcinoma (8, 9). Elevated c-JUN levels in hepatocellular carcinoma down-regulate SIRT6 expression in a c-FOS-dependent manner (8). Recently, naturally occurring cancer-associated point mutations were identified in SIRT6 that result in its loss of tumor-su...

Discoidin domain receptor 1 (DDR1) is a receptor tyrosine kinase activated by various types of collagens and is known to play a role in cell attachment, migration, survival, and proliferation. However, little is known about the molecular mechanism(s) underlying the role of DDR1 in cancer. We report here that DDR1 induces cyclooxygenase-2 (Cox-2) expression resulting in enhanced chemoresistance. Depletion of DDR1-mediated Cox-2 induction using short hairpin RNA (shRNA) results in increased chemosensitivity. We also show that DDR1 activates the nuclear factor-KB (NF-KB) pathway and blocking this activation by an IKB superrepressor mutant results in the ablation of DDR1-induced Cox-2, leading to enhanced chemosensitivity, indicating that DDR1-mediated Cox-2 induction is NF-KB dependent. We identify the upstream activating kinases of the NF-KB pathway, IKKB and IKK;, as essential for DDR1-mediated NF-KB activation, whereas IKKA seems to be dispensable. Finally, shRNAmediated inhibition of DDR1 expression significantly enhanced chemosensitivity to genotoxic drugs in breast cancer cells. Thus, DDR1 signaling provides a novel target for therapeutic intervention with the prosurvival/antiapoptotic machinery of tumor cells. (Cancer Res 2006; 66(16): 8123-30)