No abstract

The initiation of apoptosis often transpires in the presence of agents that regulate cell survival. This study evaluated the effects of stress-induced ceramide on the anti-apoptotic activity of the phosphoinositide-3 kinase [PI(3)K] pathway. PI(3)K activity is directly down-regulated by stress-induced ceramide in a dose-dependent manner with rapid kinetics and high specificity. Ceramide inhibition of PI(3)K is dependent on acid-sphingomyelinase. Down-regulation of PI(3)K by ceramide results in inhibition of the kinase Akt and decreased phosphorylation of the death effector Bad. Thus, ceramide levels could act as a general apoptotic rheostat controlling cell survival by regulating PI(3)K anti-apoptotic effector mechanisms.

Low oxygen tension-mediated transcription by hypoxiainducible factors (HIF) has been reported to facilitate tumor progression, therapeutic resistance, and metastatic adaptation. One previously described target of hypoxia-mediated transcription is the cytokine/growth factor macrophage migration inhibitory factor (MIF). In studies designed to better understand hypoxia-stimulated MIF function, we have discovered that not only is MIF induced by hypoxia in pancreatic adenocarcinoma but MIF is also necessary for maximal hypoxia-induced HIF-1A expression. Cells lacking MIF are defective in hypoxia-and prolyl hydroxylase inhibitor-induced HIF-1A stabilization and subsequent transcription of glycolytic and angiogenic gene products. Moreover, COP9 signalosome subunit 5 (CSN5), a component of the COP9 signalosome previously reported to functionally interact with MIF, has recently been shown to interact with and stabilize HIF-1A. Our results indicate that MIF interacts with CSN5 in pancreatic cancer cells and that MIF-depleted cells display marked defects in hypoxia-induced CSN5/HIF-1A interactions. This functional interdependence between HIF-1A and MIF may represent an important and previously unrecognized protumorigenic axis. [Cancer Res 2007;67(1):186-93]

In the last several years, multiple lines of evidence have suggested that the COP9 signalosome (CSN) plays a significant role in the regulation of multiple cancers and could be an attractive target for therapeutic intervention. First, the CSN plays a key role in the regulation of Cullin-containing ubiquitin E3 ligases that are central mediators of a variety of cellular functions essential during cancer progression. Second, several studies suggest that the individual subunits of the CSN, particularly CSN5, might regulate oncogenic and tumor suppressive functions independently of, or coordinately with, the CSN holocomplex. Thus, deregulation of CSN subunit function can have a dramatic effect on diverse cellular functions, including the maintenance of DNA fidelity, cell cycle control, DNA repair, angiogenesis, and microenvironmental homeostasis that are critical for tumor development. Additionally, clinical studies have suggested that the expression or localization of some CSN subunits correlate to disease progression or clinical outcome in a variety of tumor types. Although the study of CSN function in relation to tumor progression is in its infancy, this review will address current studies in relation to cancer initiation, progression, and potential for therapeutic intervention. (Mol Cancer Res 2005;3(12):645 -53)

Mammalian oxygen homeostasis is dependent on the HIF family of transcription factors. The CSN subunit, CSN5, binds both the CODD of HIF-1␣ and the pVHL tumor suppressor. High CSN5 expression generates a pVHL-independent form of CSN5 that stabilizes HIF-1␣ aerobically by inhibiting HIF-1␣ prolyl-564 hydroxylation. Aerobic CSN5 association with HIF-1␣ occurs independently of the CSN holocomplex, leading to HIF-1␣ stabilization independent of Cullin 2 deneddylation. CSN5 weakly associates with HIF-1␣ under hypoxia, but is required for optimal hypoxia-mediated HIF-1␣ stabilization. These results indicate that CSN5 regulates aerobic as well as hypoxic HIF-1␣ stability by different mechanisms during oncogenesis. Received December 18, 2003; revised version accepted March 1, 2004. Adaptation to hypoxia in humans requires the hypoxiainducible factor (HIF) family of transcriptional regulators (for review, see Kim and Kaelin 2003). Various isoforms of both HIF-␣ and HIF-␤ exist, with HIF-1␣ and HIF-2␣ protein stability being regulated by dioxygen levels (Kim and Kaelin 2003). HIF-1␣ is targeted for ubiquitylation and degradation under aerobic conditions by trans-4-hydroxylation of highly conserved LXXLAP motifs in oxygen-dependent degradation domains (ODDs) at P564 (CODD) and P402 (NODD; Kim and Kaelin 2003). The 4-prolyl-hydroxylation is performed by a conserved family of O 2 -, Fe(II)-, and 2-oxoglutarate-dependent prolylhydroxylase isozymes termed prolyl hydroxylase domain enzymes (PHD1-3; Kim and Kaelin 2003). Prolyl-hydroxylation allows recognition of HIF-1␣ by the von HippelLindau tumor suppressor protein (pVHL), which serves as the recognition component of an E3 ubiquitin ligase complex (pVHL/Elongin C/Elongin B [VCB], Cullin 2 [Cul2], and Rbx-1). Limiting oxygen availability or direct inhibition of the PHD enzymes by large divalent metal ions, iron chelators, certain 2-oxoglutarate analogs, or the absence of ascorbate allows HIF-␣ to escape hydroxylation and recognition by pVHL. Aerobic HIF-1␣ stabilization can also be induced by a variety of growth factors and oncogenes (Kim and Kaelin 2003).The COP9 signalosome (CSN) holocomplex is composed of eight subunits designated CSN1-CSN8 (for review, see Wolf et al. 2003). Until recently, the function of the CSN was obscure, although it appeared to be involved in regulating protein stability. Mutational analysis in Saccharomyces pombe revealed that CSN disruption resulted in the accumulation of neddylated cullins (Wolf et al. 2003). The conjugation of the ubiquitin-like protein Nedd8 to cullins is thought to be required for ubiquitin E2 recruitment. CSN5 is found both in the nucleus and cytoplasm and contains a JAB-1/MPN domain metalloenzyme motif (JAMM) that forms the catalytic region of an isopeptidase (Wolf et al. 2003). CSN5 requires assembly into the CSN holocomplex to function as a Nedd8 isopeptidase. The CSN5 JAMM domain is responsible for the cleavage of Nedd8 from cullins, resulting in an inability of cullin complexes to catalyze ubiquitin ligation. Ongoing C...