Regulation of c-Myb activity by tumor suppressor p53

Mitochondrial Hep27 is a c-Myb target gene that inhibits Mdm2 and stabilizes p53

The ever-expanding knowledge of the role of p53 in cellular metabolism, apoptosis, and cell cycle control has led to increasing interest in defining the stress response pathways that regulate Mdm2. In an effort to identify novel Mdm2 binding partners, we performed a large-scale immunoprecipitation of Mdm2 in the osteosarcoma U2OS cell line. One significant binding protein identified was Hep27, a member of the short-chain alcohol dehydrogenase/reductase (SDR) family of enzymes. Here, we demonstrate that the Hep27 preprotein contains an N-terminal mitochondrial targeting signal that is cleaved following mitochondrial import, resulting in mitochondrial matrix accumulation of mature Hep27. A fraction of the mitochondrial Hep27 translocates to the nucleus, where it binds to Mdm2 in the central domain, resulting in the attenuation of Mdm2-mediated p53 degradation. In addition, Hep27 is regulated at the transcriptional level by the proto-oncogene c-Myb and is required for c-Myb-induced p53 stabilization. Breast cancer gene expression analysis correlated estrogen receptor (ER) status with Hep27 expression and p53 function, providing a potential in vivo link between estrogen receptor signaling and p53 activity. Our data demonstrate a unique c-Myb-Hep27-Mdm2-p53 mitochondria-to-nucleus signaling pathway that may have functional significance for ER-positive breast cancers.

c-myb has a character of oxidative stress resistance in aged human diploid fibroblasts: regulates SAPK/JNK and Hsp60 pathway consequently

This study examined whether c-myb acts as a survival molecule in aged cells. A previous in vitro ageing model suggested that aged cells have a higher cell capacity for survival after exposure to oxidative stress, which involves blockage of the translocation of Hsp60 from the mitochondria to the cytoplasm followed by SAPK/JNK inactivation, than young cells. In human diploid fibroblasts (HDFs), c-myb expression increased gradually with ageing, and this increase had a significant influence on the cell survival capacity after exposure to oxidative stress. To clarify the role of c-myb in oxidative stress, young cells under 21 passages, which lacked c-myb expression, were transfected with adenovirus-mediated c-myb for express c-myb. These c-myb-over-expressed young cells showed increased cell viability upon exposure to oxidative stress to a similar extent to that of the aged cells. In addition, these c-myb-over-expressed young cells did not exhibit SAPK/JNK activation, Hsp60 displacement and cytochrome C release, as was observed in aged cells. The aged cells that had c-myb suppressed using siRNA c-myb showed reduced cell viability and increased apoptosis in a manner to that observed in young cells. From this study, c-myb blocked SAPK/JNK and Hsp60 translocation upon exposure to oxidative stress. This result suggests that c-myb might act as a modulator of cell survival in the ageing process by suppressing apoptosis in aged cells.

Regulation of Sprouty2 stability by mammalian Seven-in-Absentia homolog 2

Mammalian Sprouty (Spry) gene expression is rapidly induced upon activation of the FGF receptor signaling pathway in multiple cell types including cells of mesenchymal and epithelial origin. Spry2 inhibits FGF-dependent ERK activation and thus Spry acts as a feedback inhibitor of FGF-mediated proliferation. In addition, Spry2 interacts with the ring-finger-containing E3 ubiquitin ligase, c-Cbl, in a manner that is dependent upon phosphorylation of Tyr55 of Spry2. This interaction results in the poly-ubiquitination and subsequent degradation of Spry2 by the proteasome. Here, we describe the identification of another E3 ubiquitin ligase, human Seven-in-Absentia homolog-2 (SIAH2), as a Spry2 interacting protein. We show by yeast two-hybrid analysis that the N-terminal domain of Spry2 and the ring finger domain of SIAH2 mediated this interaction. Co-expression of SIAH2 resulted in proteasomal degradation of Spry1, 2, and to a lesser extent Spry4. The related E3 ubiquitin-ligase, SIAH1, had little effect on Spry2 protein stability when co-expressed. Unlike c-Cbl-mediated degradation of Spry2, SIAH2-mediated degradation was independent of phosphorylation of Spry2 on Tyr55. Spry2 was also phosphorylated on Tyr227, and phosphorylation of this residue was also dispensable for SIAH2-mediated degradation of Spry2. Finally, co-expression of SIAH2 with Spry2 resulted in a rescue of FGF2-mediated ERK phosphorylation. These data suggest a novel mechanism whereby Spry2 stability is regulated in a manner that is independent of tyrosine phosphorylation, and provides an addition level of control of Spry2 protein levels.

Regulation of gene expression by biotin (review)

In mammals, biotin serves as coenzyme for four carboxylases, which play essential roles in the metabolism of glucose, amino acids, and fatty acids. Biotin deficiency causes decreased rates of cell proliferation, impaired immune function, and abnormal fetal development. Evidence is accumulating that biotin also plays an important role in regulating gene expression, mediating some of the effects of biotin in cell biology and fetal development. DNA microarray studies and other gene expression studies have suggested that biotin affects transcription of genes encoding cytokines and their receptors, oncogenes, genes involved in glucose metabolism, and genes that play a role in cellular biotin homeostasis. In addition, evidence has been provided that biotin affects expression of the asialoglycoprotein receptor and propionyl-CoA carboxylase at the post-transcriptional level. Various pathways have been identified by which biotin might affect gene expression: activation of soluble guanylate cyclase by biotinyl-AMP, nuclear translocation of NF-kappaB (in response to biotin deficiency), and remodeling of chromatin by biotinylation of histones. Some biotin metabolites that cannot serve as coenzymes for carboxylases can mimic biotin with regard to its effects on gene expression. This observation suggests that biotin metabolites that have been considered "metabolic waste" in previous studies might have biotin-like activities. These new insights into biotin-dependent gene expression are likely to lead to a better understanding of roles for biotin in cell biology and fetal development.

Targeting c-Myb expression in human disease

c-Myb is a transcription factor employed in the haematopoietic system and gastrointestinal tract to regulate the exquisite balance between cell division, differentiation and survival. In its absence, these tissues either fail to form, or show aberrant biology. Mice lacking a functional c-myb gene die in utero by day 15 of development. When inappropriately expressed, as is common in leukaemia and epithelial cancers of the breast, colon and gastro-oesophagus, c-Myb appears to activate gene targets of key importance to cancer progression and metastasis. These genes include cyclooxygenase-2 (COX-2), Bcl-2, BclX(L) and c-Myc, which influence diverse processes such as angiogenesis, proliferation and apoptosis. The clinical potential for blocking c-Myb expression in malignancies is based upon strong preclinical data and some trial-based evidence. The modest clinical experience to date has been with haematopoietic malignancies, but other disease classes may be amenable to similar interventions. The frontline agents to achieve this are nuclease-resistant oligodeoxynucleotides (ODNs), which are proving to be acceptable therapeutic reagents in terms of tolerable toxicities and delivery. Nevertheless, further effort must be focused on improving their efficacy, eliminating non-specific toxicity and optimising delivery. Optimisation issues aside, it would appear that anti-c-Myb therapies will be used with most success when combined with other agents, some of which will be established cytotoxic and differentiation-inducing drugs. This review will explore the future strategic use of ODNs in vivo, focusing on a wide spectrum of diseases, including several beyond the haematopoietic malignancies, in which c-Myb appears to play a role.