The importance of endoglin in fibrosis has been widely accredited 
. Fibrosis is the end result of a succession of events that occur after mechanical damage to the epithelium and/or endothelium 
. Fibrotic diseases may be attributable to a variety of causes, but it is generally thought that an initiating injury event activates repair processes that aim to restore the original tissue architecture, and a failure to finely tune the repair process leads to persistent fibroblast activation and tissue destruction 
. During physiological dermal wound healing, the activity of activated fibroblasts is terminated when the tissue is repaired. In pathological wound healing, however, activated fibroblasts persist and lead to tissue deformation and fibrosis 
. The potential role of endoglin in fibroplasia during wound healing has been proposed 
since a significant elevation in fibroblast-associated endoglin levels was observed between days 4 and 10 of wound healing 
. To investigate the functional role of endoglin in wound healing induced fibrosis we have used mice heterozygous for endoglin (Eng+/−
) and their wild-type littermates (Eng+/+
. Endoglin knockout mice die at midgestation because of defective angiogenesis 
mice have allowed to analyze the involvement of endoglin in different processes such as angiogenesis 
, cardiovascular function 
and tumor development 
. In this work, we show that Eng+/−
mice exhibit persistence of activated fibroblasts in the wounds, as detected by α-SMA staining. In addition, primary cultured fibroblasts from these mice display higher expression of ECM-related molecules, increased proliferation and migration rates and diminished responses to apoptotic signals, whereas endoglin over-expression results in an inhibition of these processes. According to these results endoglin deficiency would lead to an increased fibrosis post-wounding. Although some studies suggest a profibrotic role for endoglin 
, our results are consistent with many previous works that demonstrate that endoglin acts as an antifibrotic molecule 
This antifibrotic effect of endoglin seems to be mediated by PI3K/Akt signaling pathway. Thus, under basal conditions Akt phosphorylation was increased in Eng+/−
dermal fibroblasts and this effect was inhibited after endoglin overexpression. Akt/PKB is an intermediate signaling component of the PI3K pathway that is activated by phosphorylation in Thr and Ser residues and that is involved in several cellular processes, including growth, metabolism, reproduction, and life span 
. In particular, Akt has been described as a fibroblast proliferation promoter 
. Here, we show that PI3K pathway inhibition reduced the faster proliferation of endoglin-haploinsufficient dermal fibroblasts to levels similar to those of control cells. Moreover, the differences in cell proliferation due to endoglin expression in NIH3T3 fibroblasts were abolished after PI3K inhibition. These results suggest that the difference in proliferation found in endoglin-haploinsufficient cells is a direct consequence of different Akt activation. Recently, the role of GSK-3β in wound healing has been analyzed 
; GSK-3β is a downstream member of the PI3K pathway that is degraded after Akt-mediated phosphorylation. Interestingly, GSK-3β knock-down results in increased in vivo
activated fibroblasts accumulation in the wound area and enhanced proliferation and migration in vitro
. Our work, together with that of the above authors, highlights the relevance of PI3K/Akt pathway as an important mediator during wound healing and its importance in the regulation of activated fibroblasts accumulation after repair.
Whatever the mechanism of endoglin regulating Akt activation this effect seems to be independent of TGFβ1 signaling. It is widely accepted that endoglin has an important role regulating TGFβ signaling. Endoglin expression promotes TGFβ-mediated ALK1-Smad1/5 activation, in contrast with the classical ALK5-Smad2/3 activation. This model has mainly been described in endothelial cells 
, and myoblasts 
. However, our study supports a previously proposed idea that endoglin has cellular effects independent of TGFβ 
. There are only a few works in the literature that relates endoglin and PI3K-Akt pathway and almost all of them refer a regulation of endoglin expression by PI3K-Akt pathway 
. According with our results, very recently, Lee et al.
shown the PI3K/Akt pathway as an endoglin target in regulating the stabilization of capillary sprouts and endothelial cell survival. They proposed a model in which GIPC could be the mediator coupling endoglin to the PI3K subunits and Akt at the plasma membrane and could be implicated in a differential response to TGFβ1 and BMP-9. In the presence of endoglin, TGFβ1 inhibits Akt phosphorylation with a modest enhancement by BMP-9 treatment 
. Our data support these results as we also find a regulation of Akt phosphorylation depending on endoglin expression. Moreover, endoglin regulated PI3K/Akt signaling pathway could be a broad mechanism involved in the regulation of further physiologic processes and in different cell types.
In summary, our work analyzing the involvement of endoglin in activated fibroblasts accumulation may contribute to a better understanding of the pathophysiological processes of wound healing that give rise to fibrogenesis. Moreover, we proposed the PI3K/Akt pathway as the mechanism of action of endoglin on the regulation of post-wound healing fibrosis. Therefore, according to our results, endoglin and the PI3K/Akt pathway may be therapeutic targets for the treatment of fibrotic wounds.