Role of Mechanical Stimulations in Directing Mesenchymal Stem Cell Adipogenesis - The Mechanobiology of Obesity and Related Diseases - page 67

during the BMP4 treatment (commitment) period. This was observed by key
adipogenic transcription factors (PPARc, CCAAT/enhancer binding protein a or
C/EBPa, aP2) on day 1 and lipid accumulation on day 8. This result may suggest a
new perspective on how to inhibit MSC adipogenesis by cell stretch, that is,
stretching of MSCs cyclically before cells are exposed to adipogenic induction
media may function as a potent suppressor of their adipogenesis. Most of the other
studies have tested the stretching effects by applying cyclic stretch while cells
were treated with adipogenic induction media. At which stage, for instance,
commitment, differentiation, or combined (as in Fig. 2 a), stretch will be more
effective in suppressing MSC adipogenesis remains to be clarified.
Since BMP4 triggers MSCs to be committed to preadipocytes, the role of
mechanical signal in controlling this cascade may be of significant interest. The
major BMP signaling pathways are Smad pathways [ 1 ]. BMP triggers type II and
type I membrane receptors, and this in turn activates the phosphorylation of
Smad1/5/8. Activated Smad1/5/8 binds with Smad4 (co-Smad), and translocates
into nucleus to trigger transcriptional activities (such as PPARc activation for
adipogenesis). Non-Smad BMP signaling pathways include p38. It was reported
that the BMP4 activation of MSC adipogenesis may be achieved through both
Smad and p38 [ 20 ]. In our study, it was observed that cyclic stretch may not
influence the BMP4 induction of Smad1/5/8 or p38 phosphorylation (see [ 24 ]; data
not shown). On the other hand, as shown in Fig. 3 , cyclic stretching triggered
significant ERK1/2 phosphorylation relative to BMP4 treatment alone case. Fur-
ther, the stretch suppression of BMP4-induced MSC adipogenesis was signifi-
cantly deteriorated in case cells were stretched with ERK being blocked by
PD98059, an established pharmacological ERK inhibitor. Combined data strongly
suggest cyclic cell stretch may suppress the BMP4 induction of MSC adipogenesis
via upregulating ERK1/2 but not through the downregulation of Smad1/5/8 or p38.
It may be noteworthy that these data on mechanical stretch are in sharp contrast to
soluble factor-driven adipogenesis inhibition. In our other study, we showed that
retinoic acid (RA) soluble factor inhibited MSC adipogenesis via downregulating
the BMP4-induced Smad/p38 phosphorylation [ 23 ]. The data on the role of cyclic
stretch-activated ERK1/2 in downregulating MSC adipogenesis may share a
similar mechanism with that proposed for 3T3-L1 preadipocytes [ 51 ] and MSCs
[ 54 ], regardless of differences in cell type and the usage of BMP4.
Another molecular mechanism responsible for cyclic stretch-induced adipo-
genesis inhibition includes b-catenin signaling. As briefly mentioned above, the
b-catenin level and activity regulated by mechanical cue may play an important role
in inhibiting adipogenesis. In the literature, the antagonistic effect of b-catenin on
adipogenesis, while providing agonistic effect on osteogenesis, has been known
even in the absence of mechanical stimulation conditions. The canonical Wnt sig-
naling triggers b-catenin, which decreases adipogenesis via inhibiting PPARc
[ 4 , 50 ] but increases osteogenesis via triggering Runx2 [ 3 , 13 ]. Dynamic mechanical
cell stretch was found to trigger b-catenin translocation into nucleus to function to
induce osteogenesis over adipogenesis [ 43 ]. In a subsequent study, they showed that
stretch-induced b-catenin signaling may be regulated by GSK3b [ 44 ]. They