Upon stimulation, many proteins translocate into the nucleus in order to regulate a variety of cellular processes. The mechanism underlying the translocation is not clear since many of these proteins lack a canonical nuclear localization signal (NLS). We searched for an alternative mechanism in extracellular signal-regulated kinase (ERK)-2 and identified a 3 amino acid domain (SPS) that is phosphorylated upon stimulation to induce nuclear translocation of ERK2. A 19 amino acid stretch containing this phosphorylated domain inserts nondiffusible proteins to the nucleus autonomously. The phosphorylated SPS acts by binding to importin7 and the release from nuclear pore proteins. This allows its functioning both in passive and active ERK transports. A similar domain appears in many cytonuclear shuttling proteins, and we found that phosphorylation of similar sequences in SMAD3 or MEK1 also induces their nuclear accumulation. Therefore, our findings show that this phosphorylated domain acts as a general nuclear translocation signal (NTS).

Alterations in cellular metabolism and bioenergetics are vital for cancer cell growth and motility. Here, the role of the mitochondrial protein voltage-dependent anion channel (VDAC1), a master gatekeeper regulating the flux of metabolites and ions between mitochondria and the cytoplasm, in regulating the growth of several cancer cell lines was investigated by silencing VDAC1 expression using small interfering RNA (siRNA). A single siRNA specific to the human VDAC1 sequence at nanomolar concentrations led to some 90% decrease in VDAC1 levels in the lung A549 and H358, prostate PC-3, colon HCT116, glioblastoma U87, liver HepG2, and pancreas Panc-1 cancer cell lines. VDAC1 silencing persisted 144 hours post-transfection and resulted in profound inhibition of cell growth in cancer but not in noncancerous cells, with up to 90% inhibition being observed over 5 days that was prolonged by a second transfection. Cells expressing low VDAC1 levels showed decreased mitochondrial membrane potential and adenoside triphosphate (ATP) levels, suggesting limited metabolite exchange between mitochondria and cytosol. Moreover, cells silenced for VDAC1 expression showed decreased migration, even in the presence of the wound healing accelerator basic fibroblast growth factor (bFGF). VDAC1-siRNA inhibited cancer cell growth in a Matrigel-based assay in host nude mice. Finally, in a xenograft lung cancer mouse model, chemically modified VDAC1-siRNA not only inhibited tumor growth but also resulted in tumor regression. This study thus shows that VDAC1 silencing by means of RNA interference (RNAi) dramatically inhibits cancer cell growth and tumor development by disabling the abnormal metabolic behavior of cancer cells, potentially paving the way for a more effective pipeline of anticancer drugs.

No abstract

Pigment epithelium-derived factor (PEDF) is an endogenous inhibitor of angiogenesis and a promising anticancer agent capable of suppressing solid tumor growth in animal cancer models. We have previously shown that PEDF can be phosphorylated and that distinct phosphorylation states of this factor differentially regulate its physiologic function. Here, we report that phosphomimetic mutants of PEDF, which possess significantly increased antiangiogenic activity, are much more efficient than wild-type (WT) PEDF in inhibiting growth and neovascularization in MDA-MB-231 (breast cancer), HCT116 (colon cancer), and U87-MG (glioblastoma) xenograft models. Importantly, the antitumor activity of the phosphomimetic mutants is comparable with that of the established antiangiogenic agent bevacizumab. However, unlike bevacizumab, these compounds act in a vascular endothelial growth factor (VEGF)-independent manner, as they do not affect the levels of VEGF-A mRNA and VEGF receptor 2 phosphorylation. Further immunohistochemical analysis revealed that PEDF mutants affect mainly tumor-residing endothelial cells and prevent the formation of intratumoral vascular network by facilitating endothelial cell apoptosis. It was also found that PEDF mutants reduce survival of endothelial cells in culture much better than WT-PEDF, an effect that is apparent even in the presence of VEGF or basic fibroblast growth factor, and promote much stronger endothelial cell apoptosis. On the other hand, PEDF and its mutants did not affect survival of cultured cancer cells, indicating that the antiangiogenic activity of these agents is the foremost element of the observed antitumor effect. These findings have specific implications on improving the properties of WT-PEDF, which is currently in preclinical development, and encourage the development of PEDF mutants as specific, neovascularization-targeting anticancer agents. Cancer Res; 70(15); 6247-57. ©2010 AACR.

Pigment epithelium-derived factor (PEDF) is a potent endogenous inhibitor of angiogenesis and a promising anticancer agent. We have previously shown that PEDF can be phosphorylated and that distinct phosphorylations differentially regulate its physiological functions. We also demonstrated that triple phosphomimetic mutant (EEE-PEDF), has significantly increased antiangiogenic activity and is much more efficient than WT-PEDF in inhibiting neovascularization and tumor growth. The enhanced antiangiogenic effect was associated with a direct ability to facilitate apoptosis of tumor-residing endothelial cells (ECs), and subsequently, disruption of intratumoral vascularization. In the present report, we elucidated the molecular mechanism by which EEE-PEDF exerts more profound effects at the cellular level. We found that EEE-PEDF suppresses EC proliferation due to caspase-3-dependent apoptosis and also inhibits migration of the EC much better than WT-PEDF. Although WT-PEDF and EEE-PEDF did not affect proliferation and did not induce apoptosis of cancer cells, these agents efficiently inhibited cancer cell motility, with EEE-PEDF showing a stronger effect. The stronger activity of EEE-PEDF was correlated with a better binding to laminin receptors. Furthermore, the proapoptotic and antimigratory activities of WT-PEDF and EEE-PEDF were found regulated by differential activation of two distinct MAPK pathways, namely JNK and p38, respectively. We show that JNK and p38 phosphorylation is much higher in cells treated with EEE-PEDF. JNK leads to apoptosis of ECs, whereas p38 leads to anti-migratory effect in both EC and cancer cells. These results reveal the molecular signaling mechanism by which the phosphorylated PEDF exerts its stronger antiangiogenic, antitumor activities. Pigment epithelium-derived factor (PEDF)2 is a 46-kDa secreted glycoprotein that can act either as a neurotrophic or an antiangiogenic factor (1, 2). PEDF has been originally identified in the retina; however, it has now become evident that it is expressed throughout the human body and is constantly present in the systemic circulation (3, 4). Studies of the past decade have implicated PEDF in the pathophysiology of a wide range of angiogenesis-associated disorders, including diabetic complications (5), macular degeneration (6), and the growth of many types of solid tumors (7-10). Furthermore, the natural antiangiogenic activity of PEDF, which is far greater than that of any other known endogenously produced factor (2, 11), has turned it into a highly potent therapeutic agent for the inhibition of pathological neovascularization and, therefore, a promising tumor suppressor. To date, a number of studies have shown that exogenous administration of PEDF results in tumor regression, decreased intratumoral microvessel density, and prolonged survival in various animal cancer models (12-15). The underlying molecular mechanism of anticancer activity of PEDF has been under extensive investigation recently (13,16). Yet, the signaling pathways that mediate the effect...