Lifespan extension by peroxidase and dual oxidase-mediated ROS signaling through pyrroloquinoline quinone in C. elegans.

Reactive oxygen species (ROS), originally characterized based on their harmful effects on cells or organisms, are now recognized as important signal molecules regulating various biological processes. In particular, low levels of ROS released from mitochondria extend lifespan. Here, we identified a novel mechanism of generating appropriate levels of ROS at the plasma membrane through a peroxidase and dual oxidase (DUOX) system, which could extend lifespan in Caenorhabditis elegans A redox co-factor, pyrroloquinoline quinone (PQQ), activates the C. elegans DUOX protein BLI-3 to produce the ROS H2O2 at the plasma membrane, which is subsequently degraded by peroxidase (MLT-7), eventually ensuring adequate levels of ROS. These ROS signals are transduced mainly by the oxidative stress transcriptional factors SKN-1 (Nrf2 or NFE2L2 in mammals) and JUN-1, and partially by DAF-16 (a FOXO protein homolog). Cell biology experiments demonstrated a similarity between the mechanisms of PQQ-induced activation of human DUOX1 and DUOX2 and that of C. elegans BLI-3, suggesting that DUOXs are potential targets of intervention for lifespan extension. We propose that low levels of ROS, fine-tuned by the peroxidase and dual oxidase system at the plasma membrane, act as second messengers to extend lifespan by the effect of hormesis.

Co-occurrence of tetraspanin and ROS generators: Conservation in protein cross-linking and other developmental processes.

The nematode exoskeleton, commonly called the cuticle, is a highly structured extracellular matrix mainly composed of collagen. Secreted collagen molecules from the underlying epidermal cells are cross-linked via their tyrosyl residues. Reactive oxygen species (ROS) are required for the cross-linking reaction to produce tyrosyl radicals. The conserved ROS generator enzyme in C. elegans, BLI-3/CeDUOX1, a homolog of dual oxidases (DUOXs), is responsible for production of hydrogen peroxide. The ROS generation system must be properly controlled since ROS are highly reactive molecules that irreversibly inhibit the functions of cellular components such as nucleic acids and proteins. We recently reported that the ROS generation system directed by BLI-3 requires the tetraspanin protein, TSP-15. Herein we outline the process of cuticle development with a focus on the molecular roles of TSP-15 in the BLI-3 system. We also propose the co-occurrence of tetraspanin and ROS generators by convergent evolution.

Tetraspanin is required for generation of reactive oxygen species by the dual oxidase system in Caenorhabditis elegans.

Reactive oxygen species (ROS) are toxic but essential molecules responsible for host defense and cellular signaling. Conserved NADPH oxidase (NOX) family enzymes direct the regulated production of ROS. Hydrogen peroxide (H(2)O(2)) generated by dual oxidases (DUOXs), a member of the NOX family, is crucial for innate mucosal immunity. In addition, H(2)O(2) is required for cellular signaling mediated by protein modifications, such as the thyroid hormone biosynthetic pathway in mammals. In contrast to other NOX isozymes, the regulatory mechanisms of DUOX activity are less understood. Using Caenorhabditis elegans as a model, we demonstrate that the tetraspanin protein is required for induction of the DUOX signaling pathway in conjunction with the dual oxidase maturation factor (DUOXA). In the current study, we show that genetic mutation of DUOX (bli-3), DUOXA (doxa-1), and peroxidase (mlt-7) in C. elegans causes the same defects as a tetraspanin tsp-15 mutant, represented by exoskeletal deficiencies due to the failure of tyrosine cross-linking of collagen. The deficiency in the tsp-15 mutant was restored by co-expression of bli-3 and doxa-1, indicating the involvement of tsp-15 in the generation of ROS. H(2)O(2) generation by BLI-3 was completely dependent on TSP-15 when reconstituted in mammalian cells. We also demonstrated that TSP-15, BLI-3, and DOXA-1 form complexes in vitro and in vivo. Cell-fusion-based analysis suggested that association with TSP-15 at the cell surface is crucial for BLI-3 activation to release H(2)O(2). This study provides the first evidence for an essential role of tetraspanin in ROS generation.

MIG-13 controls anteroposterior cell migration by interacting with UNC-71/ADM-1 and SRC-1 in Caenorhabditis elegans.

The transmembrane protein MIG-13 is a key regulator required for anterior migration of neural cells in Caenorhabditis elegans, but the signaling mechanisms involved remain unknown. Here, we isolated a suppressor mutation in the unc-71/adm-1 gene, which rescued the AVM neuron migration defect in mig-13 mutants. Genetic analyses revealed that UNC-71 at least partly acts downstream of MIG-13 and has an inhibitory effect on the anterior cell migration. The unc-71 mutation also rescued the anterior migration defect of AVM neuron in src-1 mutants. These findings suggest that MIG-13 controls anteroposterior cell migration by interacting with UNC-71 and SRC-1 in C. elegans.

HB-EGF and PDGF mediate reciprocal interactions of carcinoma cells with cancer-associated fibroblasts to support progression of uterine cervical cancers.

Tumor stroma drives the growth and progression of cancers. A heparin-binding epidermal growth factor-like growth factor, HB-EGF, is an EGF receptor ligand that stimulates cell growth in an autocrine or paracrine fashion. While elevated expression of HB-EGF in cancer cells and its contribution to tumor progression are well documented, the effects of HB-EGF expression in the tumor stroma have not been clarified. Here, we show that HB-EGF is expressed in stromal fibroblasts where it promotes cancer cell proliferation. In uterine cervical cancers, HB-EGF was detected immunohistochemically in the stroma proximal to the cancer epithelium. Proliferation of cervical cancer cells in vitro was enhanced by coculture with fibroblasts isolated from tumor tissues of patients with cervical cancer. Inhibition of HB-EGF function or treatment with platelet-derived growth factor (PDGF) inhibitors abrogated cancer cell growth enhanced by cervical cancer-associated fibroblast (CCF) coculture. Furthermore, tumor formation in a mouse xenograft model was enhanced by cotransplantation of CCF or mouse embryonic fibroblasts, but not with embryonic fibroblasts from HB-EGF-deficient mice. Conversely, conditioned medium from cancer cells induced HB-EGF expression in CCF. Mechanistic investigations established that PDGF was the primary factor responsible. Together, our findings indicate that HB-EGF and PDGF reciprocally mediate the interaction of cancer cells with cancer-associated fibroblasts, promoting cancer cell proliferation in a paracrine manner that has implications for novel combinatorial cancer therapies.

Tetraspanin protein (TSP-15) is required for epidermal integrity in Caenorhabditis elegans.

Epidermal integrity is essential for animal development and survival. Here, we demonstrate that TSP-15, a member of the tetraspanin protein family, is required for epithelial membrane integrity in Caenorhabditis elegans. Reduction of tsp-15 function by mutation or by RNA interference elicits abnormalities of the hypodermis, including dissociation of the cuticle and degeneration of the hypodermis. Lethality during molting often results. Examination of GFP transgenic animals, genetic mosaic analysis and rescue assays revealed that TSP-15 functions in hyp7, a large syncytium that composes most of the hypodermis. Assays with a membrane-impermeable dye or leakage analysis of a hypodermal-specific marker indicate that the barrier function of the hypodermal membrane is impaired owing to the loss or reduction of TSP-15. These results indicate that TSP-15 functions in the maintenance of epithelial cell integrity.

Mice with defects in HB-EGF ectodomain shedding show severe developmental abnormalities.

Heparin-binding EGF-like growth factor (HB-EGF) is first synthesized as a membrane-anchored form (proHB-EGF), and its soluble form (sHB-EGF) is released by ectodomain shedding from proHB-EGF. To examine the significance of proHB-EGF processing in vivo, we generated mutant mice by targeted gene replacement, expressing either an uncleavable form (HBuc) or a transmembrane domain-truncated form (HBdeltatm) of the molecule. HB(uc/uc) mice developed severe heart failure and enlarged heart valves, phenotypes similar to those in proHB-EGF null mice. On the other hand, mice carrying HBdeltatm exhibited severe hyperplasia in both skin and heart. These results indicate that ectodomain shedding of proHB-EGF is essential for HB-EGF function in vivo, and that this process requires strict control.