**7.2 Intracellular mechanisms pathway balancing between TGF-β and GDF11 in skin aging**

Aging of ADSCs and DF was associated to upregulation of apoptotic genes and, consequently, the number of senescent cells increased [123]. However, recent studies have demonstrated that this senescence can be induced by TGF-β/SMAD as a normal developmental process [124]. Also, in aged skin, accumulation of senescent cells and ROS likely impaired TGF-B pathway at least in DF. The mechanisms' pathways SMAD and Sirtuins are the mostly reported pathways whereby TGF-β and GDF11 acted.

The sirtuins are a family engaged in metabolism regulation and in aging-associated diseases, in addition to the mechanistic target of rapamycin (mTOR) signaling [16, 62, 125]. SIRT1 upregulation was reported to delay fibroblast's senescence and its expression is significantly reduced in aged skin [126]. Additionally, SIRT1, SIRT3, SIRT5, SIRT2, SIRT6, and SIRT7 are involved in the regulation of oxidative stress and ROS production [127, 128]. Moreover, SIRT6 is implicated in the relation of DNA repair proteins to chromatin [129].

In wound healing, phosphorylation of TGF-β/Smad2/3 intracellular pathways was increased and resulted in protein transfer to skin cell [16, 130, 131], in DF proliferation and ADSCs differentiation into fibroblasts [70], and in improvement of angiogenesis [24]. Interestingly, aging and cell differentiation have been reversed by inhibiting this pathway [132]. Indeed, activation of SMAD2/3-dependent-TGF-β signaling inhibits adipogenic and osteogenic MSCs' differentiation *in vitro* and *in vivo* [133, 134].

**53**

*Adipose-Derived Stem Cells (ADSCs) and Growth Differentiation Factor 11 (GDF11)…*

ADSCs self-renew is regulated by AKT signaling through targeting PDGFA and activating the PI3K/AKT2 axis is required for ADSCs proliferation and maintenance in the dermis [135]. Hypoxic conditions might play a pivotal role in wound repair, and the same pathway has been found leading to the activation of PI3K/AKt signaling [130]. In response to wound, ERK, AKt, and STAT-3 mechanisms pathways were activated and associated to stem cell proliferation and keratinocyte migration [80]. Interestingly, wound healing was specifically associated to microRNA and protein transfer to skin cells through the TGF-β/SMAD2 pathway, TGF-β being identified as

TGF-β signaling pathway was recently reported as the main regulator of pluripotency [137]. Although epigenetically lacking myostatin is a highly homologous factor of GDF11, muscle multipotent stem cells can be reprogrammed to became pluripotent cells as embryonic cell-like. MicroRNA participates in this regulation probably by inhibiting TGF-β/Smad2 [132]. Epithelial differentiation was achieved by ADSCs through secreting Wnt10b and Wnt3a, a modulator of the Wnt/β-catenin implicated in replicative senescence regulation, suggesting that the regenerating and rejuvenating effect of GDF11 might also act via Wnt/β-catenin activation [138]. However, further investigations should be conducted to increase the knowledge on the intracellular mechanism used by the Wnt/β-catenin to delay the senescence of

GDF11 is likely expected to act on other epidermal cells including DF, keratinocytes, and melanocytes by regulating the genetic expressions of the different proteins involved in the antiaging process. Recombinant GDF11 was likely reported to reinforce human skin by highly increasing ECM genes expression related to ECM production such as COL1A1, COL6A6, CL14A1, ELN, TGFBR3, and HAS1. Skin barrier function was likely improved by enhancing expression of ALOX12, ALOX12B, ALOXE3, DSG1, and DSP genes. Genes related to epidermal cell proliferation and differentiation were also upregulated in human skin through the

Recently, fibroblast growth factors (FGFs) have been proposed as a therapeutic option to avoid skin aging aspects and to counter the cellular responses related to aging [139, 140]. Binding to tyrosine kinase receptors, they activate the autophosphorylation of Raf-1 belonging to the family of mitogen-activated protein kinase, kinase, kinase (MAPKKK), followed by that of MAPKK, which phosphorylates ERK (belonging to MAPK). Phosphorylation of these cyclin-dependent kinases in cell

These FGFs are derived from fibroblasts and are decreased during their proliferative and metabolic activities, thus reducing ECM proteins' production. The decrease in FGF production also converge on the reduced amounts of collagen, decrease of dermal and epidermal integrity and elasticity and strength as signs of aging [141]. In addition to FGF secretion, fibroblasts are also targeted by TFG-β and GDF11 to activate the SMAD pathway leading them to have a pivotal role in skin regeneration. This suggests that FGF was expected to act independently but simultaneously to TGF-β and GDF11 to improve skin structure and aging aspects. Having a high mitogenic activity and high stability, rFGF-1 has been reported to strongly activate fibroblasts and keratinocytes proliferation with a potential use in angiogenesis, wound healing, and skin antiaging [142, 143]. In the same way, FGF-2 or b-FGF and keratinocyte growth factor (KGF) reduced wrinkles and increased proliferation of fibroblasts and keratinocytes modulating normal process of angiogenesis, tissue repair, and wound healing as well as significantly

*DOI: http://dx.doi.org/10.5772/intechopen.91233*

a "mediator" [16, 132, 136].

ADSCs and other targeted cells.

Smad2/3mechanisms pathways [24].

nucleus is involved in controlling cell division.

improving the antiaging process [27, 144, 145].

#### *Adipose-Derived Stem Cells (ADSCs) and Growth Differentiation Factor 11 (GDF11)… DOI: http://dx.doi.org/10.5772/intechopen.91233*

ADSCs self-renew is regulated by AKT signaling through targeting PDGFA and activating the PI3K/AKT2 axis is required for ADSCs proliferation and maintenance in the dermis [135]. Hypoxic conditions might play a pivotal role in wound repair, and the same pathway has been found leading to the activation of PI3K/AKt signaling [130]. In response to wound, ERK, AKt, and STAT-3 mechanisms pathways were activated and associated to stem cell proliferation and keratinocyte migration [80]. Interestingly, wound healing was specifically associated to microRNA and protein transfer to skin cells through the TGF-β/SMAD2 pathway, TGF-β being identified as a "mediator" [16, 132, 136].

TGF-β signaling pathway was recently reported as the main regulator of pluripotency [137]. Although epigenetically lacking myostatin is a highly homologous factor of GDF11, muscle multipotent stem cells can be reprogrammed to became pluripotent cells as embryonic cell-like. MicroRNA participates in this regulation probably by inhibiting TGF-β/Smad2 [132]. Epithelial differentiation was achieved by ADSCs through secreting Wnt10b and Wnt3a, a modulator of the Wnt/β-catenin implicated in replicative senescence regulation, suggesting that the regenerating and rejuvenating effect of GDF11 might also act via Wnt/β-catenin activation [138]. However, further investigations should be conducted to increase the knowledge on the intracellular mechanism used by the Wnt/β-catenin to delay the senescence of ADSCs and other targeted cells.

GDF11 is likely expected to act on other epidermal cells including DF, keratinocytes, and melanocytes by regulating the genetic expressions of the different proteins involved in the antiaging process. Recombinant GDF11 was likely reported to reinforce human skin by highly increasing ECM genes expression related to ECM production such as COL1A1, COL6A6, CL14A1, ELN, TGFBR3, and HAS1. Skin barrier function was likely improved by enhancing expression of ALOX12, ALOX12B, ALOXE3, DSG1, and DSP genes. Genes related to epidermal cell proliferation and differentiation were also upregulated in human skin through the Smad2/3mechanisms pathways [24].

Recently, fibroblast growth factors (FGFs) have been proposed as a therapeutic option to avoid skin aging aspects and to counter the cellular responses related to aging [139, 140]. Binding to tyrosine kinase receptors, they activate the autophosphorylation of Raf-1 belonging to the family of mitogen-activated protein kinase, kinase, kinase (MAPKKK), followed by that of MAPKK, which phosphorylates ERK (belonging to MAPK). Phosphorylation of these cyclin-dependent kinases in cell nucleus is involved in controlling cell division.

These FGFs are derived from fibroblasts and are decreased during their proliferative and metabolic activities, thus reducing ECM proteins' production. The decrease in FGF production also converge on the reduced amounts of collagen, decrease of dermal and epidermal integrity and elasticity and strength as signs of aging [141]. In addition to FGF secretion, fibroblasts are also targeted by TFG-β and GDF11 to activate the SMAD pathway leading them to have a pivotal role in skin regeneration. This suggests that FGF was expected to act independently but simultaneously to TGF-β and GDF11 to improve skin structure and aging aspects. Having a high mitogenic activity and high stability, rFGF-1 has been reported to strongly activate fibroblasts and keratinocytes proliferation with a potential use in angiogenesis, wound healing, and skin antiaging [142, 143]. In the same way, FGF-2 or b-FGF and keratinocyte growth factor (KGF) reduced wrinkles and increased proliferation of fibroblasts and keratinocytes modulating normal process of angiogenesis, tissue repair, and wound healing as well as significantly improving the antiaging process [27, 144, 145].

*Regenerative Medicine*

than the elderly [18].

cell-free therapy [109, 121].

**and GDF11 in skin aging**

GDF11 acted.

interfere with the aging process [24, 95, 101].

of DNA repair proteins to chromatin [129].

DF compared to the neonatal ones [95].

donors [115]. In the same manner, using platelets-rich plasma (PRP) for anti-wrinkle and anti-aging skin aspects appeared legitime related to its higher quantities of GDF11 [116]. Interestingly, GDF11 expression and activity were reduced in adult

Fibroblasts were also recognized to play a crucial role in skin regeneration through GDF11 secretion in both neonatal and adult cells [95]. MSCs derived from placenta and umbilical cord blood promote fibroblasts plasticity [117] probably through GDF11 release, thus stimulating the rejuvenation of human skin [118]. These authors have effectively demonstrated that GDF11 activated fibroblasts to increase ECM proteins' production and especially collagen I and III and fibronectin [23]. Also, MSCs have proven their proliferative superiority in young donors rather

In an animal model, transplanted autologous ADSCs improved skin-graft survival through secreting factors presenting anti-apoptotic activity [119]. In addition to ADSC, DF also appeared attractive in terms of protein secretion [109]. ADSC-conditioned media were anti-apoptotic and ensured skin tissue regeneration [119]; their protective and antiaging properties have been demonstrated on DF by preventing their oxidative stress and increasing their superoxide dismutase and glutathione peroxidase activities [120]. These cells act through their different and directed secretome to improve and induce tissue repair, consolidating their place as better candidate for regenerative medicine and opening recently the way for a new

Moreover, by increasing collagenase matrix metalloproteinase-9 (MMP-9) secretion, rGDF11 participated in matrix remodeling maybe through interaction of MMP-9 with TGF-β1 to facilitate skin wound closure [97, 122]. These cell interactions reveal the role of the TFG-β and GDF11 mechanisms used by ADSCs to

Aging of ADSCs and DF was associated to upregulation of apoptotic genes and, consequently, the number of senescent cells increased [123]. However, recent studies have demonstrated that this senescence can be induced by TGF-β/SMAD as a normal developmental process [124]. Also, in aged skin, accumulation of senescent cells and ROS likely impaired TGF-B pathway at least in DF. The mechanisms' pathways SMAD and Sirtuins are the mostly reported pathways whereby TGF-β and

The sirtuins are a family engaged in metabolism regulation and in aging-associated diseases, in addition to the mechanistic target of rapamycin (mTOR) signaling [16, 62, 125]. SIRT1 upregulation was reported to delay fibroblast's senescence and its expression is significantly reduced in aged skin [126]. Additionally, SIRT1, SIRT3, SIRT5, SIRT2, SIRT6, and SIRT7 are involved in the regulation of oxidative stress and ROS production [127, 128]. Moreover, SIRT6 is implicated in the relation

In wound healing, phosphorylation of TGF-β/Smad2/3 intracellular pathways was increased and resulted in protein transfer to skin cell [16, 130, 131], in DF proliferation and ADSCs differentiation into fibroblasts [70], and in improvement of angiogenesis [24]. Interestingly, aging and cell differentiation have been reversed by inhibiting this pathway [132]. Indeed, activation of SMAD2/3-dependent-TGF-β signaling inhibits adipogenic and osteogenic MSCs' differentiation *in vitro* and *in vivo* [133, 134].

**7.2 Intracellular mechanisms pathway balancing between TGF-β**

**52**
