Focal Adhesion Kinase Plays a Role in Osteoblast Mechanotransduction In Vitro but Does Not Affect Load-Induced Bone Formation In Vivo

Abstract

A healthy skeleton relies on bone's ability to respond to external mechanical forces. The molecular mechanisms by which bone cells sense and convert mechanical stimuli into biochemical signals, a process known as mechanotransduction, are unclear. Focal adhesions play a critical role in cell survival, migration and sensing physical force. Focal adhesion kinase (FAK) is a non-receptor protein tyrosine kinase that controls focal adhesion dynamics and can mediate reparative bone formation in vivo and osteoblast mechanotransduction in vitro. Based on these data, we hypothesized that FAK plays a role in load-induced bone formation. To test this hypothesis, we performed in vitro fluid flow experiments and in vivo bone loading studies in FAK−/− clonal lines and conditional FAK knockout mice, respectively. FAK−/− osteoblasts showed an ablated prostaglandin E2 (PGE2) response to fluid flow shear. This effect was reversed with the re-expression of wild-type FAK. Re-expression of FAK containing site-specific mutations at Tyr-397 and Tyr-925 phosphorylation sites did not rescue the phenotype, suggesting that these sites are important in osteoblast mechanotransduction. Interestingly, mice in which FAK was conditionally deleted in osteoblasts and osteocytes did not exhibit altered load-induced periosteal bone formation. Together these data suggest that although FAK is important in mechanically-induced signaling in osteoblasts in vitro, it is not required for an adaptive response in vivo, possibly due to a compensatory mechanism that does not exist in the cell culture system.

On the interactions of human bone cells with Ti6Al4V thermally oxidized by means of laser shock processing

“…Cell adhesion and osteoblast differentiation can occur in the total absence of FAK activation. Thus, FAK null osteoblasts can attach to standard tissue culture plastic (TCP), exhibiting actin fiber and focal adhesion formation, although the number of focal adhesions in these cells is lower than in wild-type cells [48]. Apart from FAK, multiple signalling proteins can be recruited to focal adhesions, including serinethreonine and tyrosine kinases, protein phosphatases, Rho GTPases and adapter proteins containing proteinprotein binding domains [49].…”

Biocompatibilidade e osteointegração de células osteoblásticas em contato com nióbio

“…De todas as moléculas que interagem com as integrinas nas adesões focais, a mais estudada e considerada fundamental na conversão do fenômeno mecânico em bioquímico é a quinase de adesão focal (FAK, focal adhesion kinase) [82]. Esta proteína permite formar múltiplos complexos proteicos com outras proteínas estruturais e de ancoragem, como demonstrado na figura 5.…”

“…O gene Ptk2, responsável pela expressão da proteína citoplasmática tirosina kinase, possui funções na mecanotransdução e outras formas de transdução de sinais para o núcleo da célula [64,82]. Sua necessidade celular é importante pelo fato de ser uma proteína que converge uma gama de sinais celulares responsáveis na manutenção, migração, sobrevivência, adesão, organização, progressão e apoptose celular.…”

“…Sua necessidade celular é importante pelo fato de ser uma proteína que converge uma gama de sinais celulares responsáveis na manutenção, migração, sobrevivência, adesão, organização, progressão e apoptose celular. Sua ausência acarretaria em vários efeitos, podendo levar a morte celular [64,82].…”

“…Os aumentos no 3º e 7º dia podem estar relacionado ao aumento na expressão do colágeno tipo I e as proteínas transmembranas integrinas α2, visto que a FAK esta associada à sinalização celular de ambas indiretamente, como evidenciado por Sato et al e Castillo et al [81,82].…”