The Cell–Surface Interaction - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 21

ECM-integrin binding [ 97 ] and mechanical loading [ 98 ]. Additionally, integrin-
mediated activation of the ERK/MAPK pathway results in phosphorylation and
stimulation of the osteoblast differentiation master control gene RUNX2 [ 99 ].
Focal adhesion kinase (FAK) is a non-receptor kinase that is linked to the b
integrin subunit and is principally involved in integrin-dependent signalling. FAK
is recruited to the focal adhesion site through integrin clustering. FAK influences
transcriptional events through adhesion-dependent phosphorylation of downstream
signalling molecules which results in the binding and activation of tyrosine kinase,
which subsequently activates MAPK signalling pathways. MAPK pathways are
essentially a chain of proteins involved in signal propagation from focal adhesion
sites to the nucleus. Binding a 5 b 1 to fibronectin activates ERK1/2, a subclass of
MAPK, which functions as a mediator of cellular differentiation [ 100 ]. It is likely,
however, that other MAPKs may also be involved. For example, FAK is required
for signalling through Jun NH 2 -terminal kinase for cell cycle regulation. Conse-
quently, integrins can differentially modulate cell proliferation and phenotypic
expression through distinct pathways.
Surface topography has been identified as an influential factor in ERK/MAPK
signalling changes [ 101 ], essentially through modulation of integrin clustering and
adhesion formation. Hamamura et al. [ 102 ] recently showed that geometrical
alterations in ECM environments can alter the phosphorylation pattern of p130Cas,
FAK, ERK1/2 and p38 MAPK for osteoblast cells cultured on 3D collagen
matrices. Specifically, they showed this using a whole-genome array that revealed
that cells grown in the 3D collagen matrix partly suppress genes associated with
cell adhesion and cell cycling. Furthermore, Western blot analysis revealed that
the expression of phosphorylated p130Cas, FAK and ERK1/2 was decreased in
cells grown in a 3D collagen matrix. Conversely, the phosphorylation of p38
MAPK was at an elevated level in the 3D matrix and its upregulation was linked to
an increase in mRNA levels of dentin matrix protein 1 and bone sialoprotein.
Prior to this, Kokobu et al. [ 103 ] demonstrated that for human gingival fibro-
blasts ERK 1/2 is translocated to the nucleus in cells in manner dependent on
surface topography and culture time. It appears that surface topography also dif-
ferentially influences Src involvement in the ERK pathway [ 104 ]. Schwartz et al.
[ 105 ] showed that ERK/MAPK activation is required for maintenance of control
levels of alkaline phosphatase; however, they did not find this outcome to be
reliant on surface microroughness.
Other studies indicate that FAK and ERK phosphorylation can also be affected
by nanotopography. Salaszynk et al. [ 106 ] suggested that FAK activity is neces-
sary for osteoblast differentiation of MSCs. In support of this, Biggs et al. [ 107 ]
showed that osteoblast differentiation and function is correlated to focal adhesion
growth and FAK-mediated activation of the ERK/MAPK pathway in MSCs.
Interestingly, it also appears that the ERK/MAPK pathway is involved in the
molecular response to aseptic loosening of implants. One study suggests that the
MAPK signalling pathway controls NF-jB-mediated transcriptional activation in
response to wear debris particles [ 108 ]. Beidelschies et al. [ 109 ] revealed that
activation of ERK1/2/Egr-1 and NF-jB pathways is responsible for the ability of