Regulation of the p53 cofactor JMY

The anti-prohferative function of the p53 tumour suppressor depends primarily on its ability to act as a transcription factor and to coordinate the cellular response to various stress signals. p53 can induce several cellular responses, including cell-cycle arrest, DNA repair and apoptosis. The choice of the p53 response is thought to depend on many factors, including the action of p53-cooperating molecules. The cofactor JMY has been demonstrated to cooperate with the p300 coactivator to augment p53 activity. The aim of this study was to investigate the mechanisms regulating JMY function to further understand its role in the stress response. Results presented here indicate that the subcellular localisation of JMY is regulated by the cellular environment. Whilst DNA damage stimulated the translocation of JMY from the cytoplasm to the nucleus, removal of proliferative and survival signals by serum deprivation induced rapid nuclear export of JMY and compromised its ability to affect cell cycle progression and stimulate apoptosis. The Mdm2 oncoprotein negatively regulates p53 activity at multiple levels including regulation of the intracellular location of p53 and inhibition of its interaction with the transcriptional coactivator CBP. Mdm2 interacts with JMY in cells, and enhances its cytoplasmic localisation without promoting its degradation. Leptomycin B blocked Mdm2-dependent redistribution of JMY, implying the involvement of a nuclear export mechanism. DNA damage was found to abrogate the effect of Mdm2 on the subcellular localisation of JMY, and to reduce the interaction between the two proteins. Mutations in the ubiquitin ligase domain of Mdm2, as well as removal of the proline-rich region in JMY abolished the Mdm2- mediated cytoplasmic translocation of JMY. These data suggest the existence of a functional relationship between the p53 cofactor JMY, and its major inhibitor Mdm2, which is likely to impact on p53 activity. Importantly, regulation of the intracellular localisation of JMY might provide an additional mechanism of modulating the p53 response

Mdm2 targets the p53 transcription cofactor JMY for degradation

We define here a new mechanism through which Mdm2 (mouse double minute 2) regulates p53 activity, by targeting the p53 transcription cofactor JMY. DNA damage causes an increase in JMY protein, and, in a similar manner, small molecule inhibitors of Mdm2 activity induce JMY in unperturbed cells. At a mechanistic level, Mdm2 regulation of JMY requires the Mdm2 RING (really interesting new gene) finger, which promotes the ubiquitin-dependent degradation of JMY. However, regulation of JMY occurs independently of the p53-binding domain in Mdm2 and p53 activity. These results define a new functional relationship between the p53 cofactor JMY and Mdm2, and indicate that transcription cofactors that facilitate p53 activity are important targets for Mdm2 in suppressing the p53 response

Prolyl hydroxylases as regulators of cell metabolism

Cellular response to oxygen depletion is mediated by HIF (hypoxia-inducible factor). HIF is a heterodimer consisting of a constitutively expressed subunit (HIF beta) and an oxygen-regulated subunit (HIF alpha). HIF alpha stability is regulated by prolyl hydroxylation by PHD (prolyl hydroxylase domain-containing protein) family members. PHD activity depends on the availability of molecular oxygen, making PHDs the oxygen-sensing system in animal cells. However, PHDs have recently been shown to respond to stimuli other than oxygen, such as 2-oxoglutarate (alpha-ketoglutarate), succinate or fumarate, as illustrated by the pseudo-hypoxic response in succinate dehydrogenase- or fumarate dehydrogenase-deficient tumours. Moreover, HIF alpha is not the sole PHD effector, suggesting that PHDs have functions that extend beyond oxygen sensing. Currently, we are investigating the role of PHDs in the cellular response to amino acid deprivation, a process regulated by mTOR (mammalian target of rapamycin). The precise mechanism whereby amino acids are signalling to mTOR is not fully understood. Given that 2-oxoglutarate is a limiting co-substrate for PHD activity during normoxia and that 2-oxoglutarate levels depend on amino acid availability, it is possible that PHD activity depends not only on oxygen, but also on amino acid availability, suggesting a global metabolic sensor function for PHDs which could be signalling not only to HIF, but also to mTOR

Succinate links TCA cycle dysfunction to oncogenesis by inhibiting HIF- prolylhydroxylase

Several mitochondrial proteins are tumor suppressors. These include succinate dehydrogenase (SDH) and fumarate hydratase, both enzymes of the tricarboxylic acid (TCA) cycle. However, to date, the mechanisms by which defects in the TCA cycle contribute to tumor formation have not been elucidated. Here we describe a mitochondrion-to-cytosol signaling pathway that links mitochondrial dysfunction to oncogenic events: succinate, which accumulates as a result of SDH inhibition, inhibits HIF-α prolyl hydroxylases in the cytosol, leading to stabilization and activation of HIF-1α. These results suggest a mechanistic link between SDH mutations and HIF-1α induction, providing an explanation for the highly vascular tumors that develop in the absence of VHL mutations