Role of full-length osteoprotegerin in tumor cell biology

Osteoprotegerin (OPG) is a soluble tumor necrosis factor receptor family member, which potently inhibits RANKL-mediated osteoclastogenesis. Numerous constructs have been created for therapeutic purposes in which the heparin-binding and death homology domains of OPG were removed and the remaining peptide (amino acids 22–194) was fused to the Fc domain of human IgG1 (OPG-Fc). The administration of OPG-Fc efficiently counteracted bone loss in a variety of preclinical models of cancers. However, several in vitro studies have shown that native or recombinant full-length OPG not only neuralizes RANKL, but also the death-inducing ligand TRAIL, suggesting that OPG might potentially counteract the anti-tumor activity of TRAIL. Additional evidence suggests that full-length OPG possesses RANKL- and TRAIL-independent biological properties, mainly related to the promotion of endothelial cell survival and angiogenesis. Finally, breast tumor cells overexpressing OPG have shown increased bone metastatic potential in vivo. The relevance of these apparently conflicting findings in tumor cell biology is highlighted. Received 2 September 2008; received after revision 29 September 2008; accepted 13 October 2008     

Adiponectin as a tissue regenerating hormone: more than a metabolic function

The great interest that scientists have for adiponectin is primarily due to its central metabolic role. Indeed, the major function of this adipokine is the control of glucose homeostasis that it exerts regulating liver and muscle metabolism. Adiponectin has insulin-sensitizing action and leads to down-regulation of hepatic gluconeogenesis and an increase of fatty acid oxidation. In addition, adiponectin is reported to play an important role in the inhibition of inflammation. The hormone is secreted in full-length form, which can either assemble into complexes or be converted into globular form by proteolytic cleavage. Over the past few years, emerging publications reveal a more varied and pleiotropic action of this hormone. Many studies emphasize a key role of adiponectin during tissue regeneration and show that adiponectin deficiency greatly inhibits the mechanisms underlying tissue renewal. This review deals with the role of adiponectin in tissue regeneration, mainly referring to skeletal muscle regeneration, a process in which adiponectin is deeply involved. In this tissue, globular adiponectin increases proliferation, migration and myogenic properties of both resident stem cells (namely satellite cells) and non-resident muscle precursors (namely mesoangioblasts). Furthermore, skeletal muscle could be a site for the local production of the globular form that occurs in an inflamed environment. Overall, these recent findings contribute to highlight an intriguing function of adiponectin in addition to its well-recognized metabolic action.     

mTOR signaling in neural stem cells: from basic biology to disease

The mammalian target of rapamycin (mTOR) pathway is a central controller of growth and homeostasis, and, as such, is implicated in disease states where growth is deregulated, namely cancer, metabolic diseases, and hamartoma syndromes like tuberous sclerosis complex (TSC). Accordingly, mTOR is also a pivotal regulator of the homeostasis of several distinct stem cell pools in which it finely tunes the balance between stem cell self-renewal and differentiation. mTOR hyperactivation in neural stem cells (NSCs) has been etiologically linked to the development of TSC-associated neurological lesions, such as brain hamartomas and benign tumors. Animal models generated by deletion of mTOR upstream regulators in different types of NSCs reproduce faithfully some of the TSC neurological alterations. Thus, mTOR dysregulation in NSCs seems to be responsible for the derangement of their homeostasis, thus leading to TSC development. Here we review recent advances in the molecular dissection of the mTOR cascade, its involvement in the maintenance of stem cell compartments, and in particular the implications of mTOR hyperactivation in NSCs in vivo and in vitro.     

Mesodermal fate decisions of a stem cell: the Wnt switch

Stem cells are a powerful resource for cell-based transplantation therapies in osteodegenerative disorders, but before some kinds of stem cells can be applied clinically, several aspects of their expansion and differentiation need to be better controlled. Wnt molecules and members of the Wnt signaling cascade have been ascribed a role in both these processes in vitro as well as normal development in vivo. However some results are controversial. In this review we will present the hypothesis that both canonical and non-canonical signaling are involved in mesenchymal cell fate regulation, such as adipogenesis, chondrogenesis and osteogenesis, and that in vitro it is a timely switch between the two that specifies the identity of the differentiating cell. We will specifically focus on the in vitro differentiation of adipocytes, chondrocytes and osteoblasts contrasting embryonic and mesenchymal stem cells as well as the role of Wnts in mesenchymal fate specification during embryogenesis.     

More than just scanning: the importance of cap-independent mRNA translation initiation for cellular stress response and cancer

The scanning model for eukaryotic mRNA translation initiation states that the small ribosomal subunit, along with initiation factors, binds at the cap structure at the 5′ end of the mRNA and scans the 5′ untranslated region (5′UTR) until an initiation codon is found. However, under conditions that impair canonical cap-dependent translation, the synthesis of some proteins is kept by alternative mechanisms that are required for cell survival and stress recovery. Alternative modes of translation initiation include cap- and/or scanning-independent mechanisms of ribosomal recruitment. In most cap-independent translation initiation events there is a direct recruitment of the 40S ribosome into a position upstream, or directly at, the initiation codon via a specific internal ribosome entry site (IRES) element in the 5′UTR. Yet, in some cellular mRNAs, a different translation initiation mechanism that is neither cap- nor IRES-dependent seems to occur through a special RNA structure called cap-independent translational enhancer (CITE). Recent evidence uncovered a distinct mechanism through which mRNAs containing N ⁶-methyladenosine (m⁶A) residues in their 5′UTR directly bind eukaryotic initiation factor 3 (eIF3) and the 40S ribosomal subunit in order to initiate translation in the absence of the cap-binding proteins. This review focuses on the important role of cap-independent translation mechanisms in human cells and how these alternative mechanisms can either act individually or cooperate with other cis-acting RNA regulons to orchestrate specific translational responses triggered upon several cellular stress states, and diseases such as cancer. Elucidation of these non-canonical mechanisms reveals the complexity of translational control and points out their potential as prospective novel therapeutic targets.     

Interaction between PGE2 and EGF receptor through MAPKs in mouse embryonic stem cell proliferation

Identifying the small molecules that permit precise regulation of embryonic stem (ES) cell proliferation should further support our understanding of the underlying molecular mechanisms of self renewal. In the present study, we showed that PGE₂ increased [³H]-thymidine incorporation in a time and dose dependent manner. In addition, PGE₂ increased the expression of cell cycle regulatory proteins, the percentage of cells in S phase and the total number of cells. PGE₂ obviously increased E-type prostaglandin (EP) receptor 1 mRNA expression level compare to 2, 3, 4 subtypes. EP1 antagonist also blocked PGE₂-induced cell cycle regulatory protein expression and thymidine incorporation. PGE₂ caused phosphorylation of protine kinase C, Src, epidermal growth factor (EGF) receptor, phosphatidylinositol 3-kinase (PI3K)/Akt phosphorylation, and p44/42 mitogen-activated protein kinase (MAPK), which were blocked by each inhibitors. In conclusion, PGE₂-stimulated proliferation is mediated by MAPK via EP1 receptor-dependent PKC and EGF receptor-dependent PI3K/Akt signaling pathways in mouse ES cells. Received 30 January 2009; received after revision 03 March 2009; accepted 10 March 2009     

New imaging probes to track cell fate: reporter genes in stem cell research

Cell fate is a concept used to describe the differentiation and development of a cell in its organismal context over time. It is important in the field of regenerative medicine, where stem cell therapy holds much promise but is limited by our ability to assess its efficacy, which is mainly due to the inability to monitor what happens to the cells upon engraftment to the damaged tissue. Currently, several imaging modalities can be used to track cells in the clinical setting; however, they do not satisfy many of the criteria necessary to accurately assess several aspects of cell fate. In recent years, reporter genes have become a popular option for tracking transplanted cells, via various imaging modalities in small mammalian animal models. This review article examines the reporter gene strategies used in imaging modalities such as MRI, SPECT/PET, Optoacoustic and Bioluminescence Imaging. Strengths and limitations of the use of reporter genes in each modality are discussed.     

p27 small interfering RNA induces cell death through elevating cell cycle activity in cultured cortical neurons: a proof-of-concept study

Recent research has demonstrated that cell cycle-associated molecules are activated in multiple forms of cell death in mature neurons, and raised a hypothesis that unscheduled cell cycle activity leads to neuronal cell death. But there is little evidence that changes in endogenous level of these molecules are causally associated with neuronal cell death. Here we transfected small interfering RNA (siRNA) targeting cyclin-dependent kinase (CDK) inhibitor p27, which plays an important role in cell cycle arrest at G1-S phase, into cultured cortical neurons. Transfection of p27 siRNA reduced neuronal viability in a time-dependent manner. p27 siRNA induced phosphorylation of retinoblastoma protein (Rb), a marker of cell cycle progression at late G1 phase. Moreover, phosphorylation of Rb and neuronal cell death provoked by p27 siRNA were abrogated by pharmacological CDK inhibitors, olomoucine and purvalanol A. Our data demonstrate that a decrease in endogenous p27 induces neuronal cell death through elevating cell cycle activity.     

Pyruvate kinase inhibited by L-cysteine as a marker of tumorigenic human urothelial cell lines

Summary It was found that a decrease in electrophoretic mobility of pyruvate kinase (PK) isoenzyme, and an increase of the sensitivity of this enzyme to L-cysteine, were markers of immortalization and tumorigenic properties, respectively, in human urothelial cell lines characterized by different grades of transformation (TGr) in vitro.     

Pyruvate kinase inhibited by L-cysteine as a marker of tumorigenic human urothelial cell lines

It was found that a decrease in electrophoretic mobility of pyruvate kinase (PK) isoenzyme, and an increase of the sensitivity of this enzyme to L-cysteine, were markers of immortalization and tumorigenic properties, respectively, in human urothelial cell lines characterized by different grades of transformation (TGr) in vitro.