Alternative strategies are urgently required to fight obesity and
associated metabolic disorders including diabetes and cardiovascular diseases.
Brown and brown-like beige adipocytes (BAs) store fat, but in contrast to white
adipocytes, they are equipped to dissipate energy stored. Therefore, BAs
represent promising cell targets to counteract obesity. However, the scarcity
of BAs in adults is a major limitation for a BA-based therapy of obesity, and
the notion to increase the BA mass by transplanting BA progenitors (BAPs) in
obese patients recently emerged. The capacity of human induced pluripotent stem
cells (hiPSCs) to generate BAPs at a high efficiency offers the opportunity to
produce an unlimited number of patient-matched BAs. However, hiPSC-BAPs display
a low adipogenic capacity that hampered their use both in cell-based therapy
and basic research. Recently we, and others, have identified the critical role
of TGFβ pathway in switching off differentiation of hiPSC-BAPs in classical 2D
culture and also in a 3D beige adiposphere model better mimicking adipocytes in vivo. Inhibition of TGFβ pathway
unlocks differentiation of hiPSC-BAPs making this cell model a suitable tool
for therapeutic transplantation. In contrast to BAPs derived from human iPSCs,
inhibition of TGF β pathway is not a requisite for differentiation of
preadipocytes derived from adult adipose tissues. This observation suggests
that hiPSC-BAPs and adult adipose tissue-preadipocytes are at different stages
of the adipose progenitor hierarchy.
Keywords: TGFβ pathway,
Human induced pluripotent stem cells, Beige adipocytes, Adipocyte progenitors,
Stem cell-based therapy, Obesity
Abbreviations: TGF: Transforming Growth Factor; BA: Brown
and Beige Adipocyte; BAP: Brown and Beige Adipocyte Progenitor; hiPSC: Human
Induced Pluripotent Stem Cell
THE
TGF β PATHWAY GOVERNS THE DIFFERENTIATION OF hiPSCs INTO BAs
Induced pluripotent stem cells (iPSCs) represent
an abundant source of multiple cell types of therapeutic interest for drug
screening as well as for transplantation [8,9]. Nakao’s group was the first to
demonstrate the capacity of hiPSCs to generate white adipocytes [10]. Total
differentiated hiPSC populations, but not purified adipose progenitors, were
transplanted into mice. Indeed, differentiated hiPSC cultures can be enriched
with adipocytes, but still contain other cell types that are unsuitable for
transplantation, including undifferentiated hiPSCs that can form teratomas. An
alternative to eliminate hiPSC capacity to form teratomas consists in purifying
progenitors of interest during hiPSC differentiation. Ahfeldt et al. [11] and
Mohsen-Kanson et al. [12] were able to generate pure BAs from hiPSCs that
displayed a high adipogenic capacity but only following transduction with
adipogenesis master genes. The need to genetically modify hiPSCs-derived
progenitors to generate adipocytes clearly illustrates the low adipogenic potential
of hiPSCs. This feature represented a bottleneck hampering their clinical use
[13].
Several factors, such as ascorbic acid, EGF and
hydrocortisone have been shown to regulate hiPSC-BAPs differentiation
[12,14,15]. However, TGFβ signaling holds a pivotal role. Members of the TGFβ
family are expressed in various tissues where they have been shown to regulate
various biological processes including regulation of apoptosis, proliferation
and differentiation of different cell types [16]. The TGFβ pathway emerged as a
critical anti-adipogenic player through the activation of Smad 2/3 [17-19].
Deletion of TGFβ receptor 1 in mice has been shown to promote beige
adipogenesis within white adipose tissue, supporting a model where TGFβ
receptor signalling play a role in regulating the pool of beige adipose
progenitors [20]. It has been shown that Smad2/3 pathway was active during
hiPSC-BAP differentiation suggesting that bioactive TGFβ family members were
secreted that might lock differentiation [14]. In agreement with this
hypothesis, Su et al. [21] showed more recently that expression of TGFβ-ligands
and receptors increased from the differentiation of FOXF1 mesoderm progenitors
towards adipocytes during in vitro
development of hiPSCs [21]. Then, the anti adipogenic role of the TGFβ pathway
has been functionally demonstrated thanks to the use of the TGFβ inhibitor
SB431542 [22]. Inhibition of active Smad 2/3 pathway upon SB431542 addition
during hiPSC-BAP differentiation induced a dramatically increased of UCP1 expression
and of the number of mature beige adipocytes [14,15,21,23]. In addition,
inhibition of TGFβ signaling in hiPSC-mesenchymal stem cells, i.e., before
induction of adipogenic differentiation, promoted the generation of adipocytes
[21]. Altogether, these data underline the critical role of TGFβ pathway in the
commitment of hiPSC into the adipogenic lineage. They indicate that TGFβ
signalling inhibition enhances the conversion of mesenchymal stem cells into
adipogenic progenitors and switches on the differentiation of progenitors into
mature beige adipocytes.
INHIBITION
OF TGFβ PATHWAY IS REQUIRED ONLY DURING THE FIRST DAYS OF DIFFERENTIATION OF
hiPSC-3D BEIGE ADIPOSPHERES
Inhibition of TGFβ pathway is required to induce
differentiation of hiPSC-derived adipose progenitor cells into adipocytes,
whereas is not for the differentiation of progenitors derived from human adult
adipose tissues. The low hiPSC-BAP adipogenic capacity compared to adult-BAPs
is reminiscent of an observation reported by Han et al. [24]. These authors
observed that epididymal adipose tissue, which undergoes early development in
mouse, is composed of progenitor cells that lack their adipogenic capacity once
isolated from the tissue. In contrast to cells derived from other fat pads that
developed later, epididymal fat cells required a 3D structure and a different
micro-environment to undergo differentiation. Therefore, among the reasons to
explain the weak efficacy of hiPSC-BAP differentiation, one can mention the
culture conditions that do not mimic the phenotype of the cells and their
physiological microenvironment within the adipose tissue. Cells are classically
grown as monolayer, which poorly reflects the in vivo situation [25]. In contrast, the cell-cell and
cell-extracellular matrix interactions are promoted in 3D configurations.
Therefore, 3D cultures represent a bridge between traditional cell culture and
live tissue. HiPSC-BAPs can form 3D spheroids able to differentiate into beige
adipospheres expressing UCP-1 (Figure 1B,
Yao X and Dani C, unpublished data). In fact, beige adipospheres revealed a
TGFβ pathway depend phase only during the first days of spheroid
differentiation.
CONCLUSION
Interestingly, Seale group has recently proposed a
mesenchymal progenitor cell hierarchy in adipose tissue where the multipotent
progenitor cell required inhibition of TGFβ pathway for its differentiation
into preadipocytes. Then, preadipocytes are refractory to the anti adipogenic
action of TGFβ to differentiate into adipocytes (Figure 1A, [26]). As discussed above, differentiation of hiPSC-BAP
spheroids display also a sensitive and a refractory phase to anti adipgenic
effect of TGFβ. It is tempting to speculate that BAPs derived from hiPSCs
resemble the multipotent progenitor subpopulation in adult adipose tissue at
the origin of preadipocytes. Further analyses are required to test this
hypothesis. Numerous other issues have also to be solved before a therapeutic
use of iPSCs in the obesity field, but identification of pathways governing the
differentiation of BAPs at a high level as well as their capacity to form 3D
adipospheres open the opportunity of using hiPSCs advantages for anti-obesity
therapy.
ACKNOWLEDGEMENT
The work has been supported by the
ANR-18-CE18-0006-01.
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