The Membrane-associated Form of the DNA Repair Protein Ku is Involved in Cell Adhesion to Fibronectin

Abstract

The Ku heterodimer (Ku70/Ku80) plays a central role in DNA double-strand breaks recognition and repair. However, Ku is expressed also on the surface of different types of cells along with its intracellular pool within the nucleus and the cytoplasm. Participation of membrane-associated Ku in cell-cell interaction has been reported recently. Here, we describe a novel function of cell-surface Ku as an adhesion receptor for fibronectin (Fn). The role of Ku in cell adhesion was investigated by comparing the Ku80 deficient Chinese hamster ovary (CHO) cell line, xrs-6, with clones transfected stably with either the hamster or human Ku80 cDNA. Ku expression in transfectant cells resulted in a significant increased adhesion on Fn and type IV collagen as compared to control cells. The observed increase in cell adhesion relied on Ku cell-surface expression, since antibodies directed against Ku70 or Ku80 subunit inhibited adhesion on Fn of Ku80, but not control vector, transfected xrs-6 cells. In addition, both Ku70 and Ku80 present a structural relationship with integrin I (or A) domains and the A1 and A3 domains of von Willebrand factor, domains known to be involved in Fn binding. Both Ku70 and Ku80 exhibit a complete set of residues compatible in their position and chemical nature with the formation of a metal ion-dependent adhesion (MIDAS) site implicated in ligand binding and integrin activation. Taken together, these functional and structural approaches support a new role for Ku as an adhesion receptor for Fn.

Cell polarity protein Par3 complexes with DNA-PK via Ku70 and regulates DNA double-strand break repair

“…Whether Par3 can direct its own nuclear entry or uses an associated partner remains to be determined. Although Ku proteins are predominantly nuclear proteins, they are also reported to localize on the membrane and in the cytosol in different cell lines [40][41][42][43]. Although we were able to detect both Ku and DNA-PKcs in the membrane and cytosol fractions using sub-fractionation and western blot assays, Par3 was associated with Ku70/Ku80, but not with DNA-PKcs in these fractions.…”

The Double Life of the Ku Protein: Facing the DNA Breaks and the Extracellular Environment

“…As if the large number of cell surface proteins devoted to cell-cell or cell/ECM interaction were not quite sufficient, recent reports demonstrate that some mammalian cells are using the DNA repair protein Ku in a membrane-associated form for this purpose. [1][2][3] Furthermore, our recent data suggest that Ku might also have function in proteolytic processes and highlight the unexpected function of this DNA repair protein in the interaction between cells and the microenvironment.…”

“…Indeed, antibodies directed against Ku70 or Ku80 specifically inhibit adhesion of CD-40L-treated, but not untreated, myeloma cells to fibronectin (Fn) and we have recently shown by using Ku deficient and proficient cell lines, that Ku present at the cell surface supports the adhesion of fibroblasts on fibronectin and collagen IV. 2 Additional results, although mainly descriptive, raised the idea that Ku might also have a role in cell migration and invasion. In fact, hypoxic exposure of Kelly neuroblastoma and MCF7 breast carcinoma resulted in a upregulation of Ku cell surface expression coincident with an increased ability of these tumours cells to invade endothelial monolayers and this enhanced invasion could be partially inhibited by an antibody directed against Ku 80.…”

Análise da expressão de proteínas envolvidas no controle do ciclo celular, apoptose, angiogênese, invasão e migração de células C6 in vitro e in vivo, após o tratamento com o ácido g-linolênico (GLA) e com um novo complexo dirutênico contendo Ibuprofeno (Ru-Ibp).

“…Sabe-se que o complexo heterodimérico Ku70/80 pode regular a adesão célula-célula e célula-matriz extracelular, participando de processos de adesão, migração e invasão celular funcionando como receptor para a fibronectina (Monferran et al, 2004a), além de possuir uma função proteolítica através da interação com a MMP-9 (Monferran et al, 2004b). Dessa forma, com a diminuição dos níveis de Ku80 causada pelo GLA e a conseqüente diminuição da formação do heterodímero Ku70/80, é possível especular uma resposta pela qual o GLA exerça a sua ação antimigratória demonstrada em estudos anteriores in vitro .…”

“…Recentemente verificou-se que usando um antisense para Ku70 mediado por um adenovírus, foi possível tornar células de glioma radiosensíveis (Li et al, 2003). Um estudo interessante revelou ainda que o complexo heterodimérico Ku70/80 pode regular a adesão célula-célula e célula-matriz extracelular participando de processos de adesão, migração e invasão celular funcionando como receptor para a fibronectina (Monferran et al, 2004a), além de possuir uma função proteolítica através da interação com a metaloprotease-9 (MMP-9) (Monferran et al, 2004b) sendo assim um importante alvo para terapias antimigratórias e antiinvasivas.INTRODUÇÃO 31 Esquema 4. Ku como uma oncoproteína ou uma proteína supressora de tumor.…”