**7. Current research on the RAS in pancreatic stem cells**

**5. The RAS and endothelial progenitor cells**

162 Renin-Angiotensin System - Past, Present and Future

**6. The vascular RAS and erythropoiesis**

The identification of circulating endothelial progenitor cells (EPCs) has introduced the concept of postnatal vasculogenesis. EPCs could originate from haematopoietic stem cells (HSCs) or MSCs [22, 23]. Also, the EPCs existing in the adventitial layer of vessels have the ability to differentiate into adult endothelial cells [24]. Different factors such as ischaemia, vascular damage and even physical exercise result in the recruitment of circulating EPCs and thus neovascularization and restoration of endothelial functionality [25, 26]. In this context, the improvement of myocardial perfusion after EPC transplantation has been observed in clinical trials [27]. Several mechanisms have been suggested regarding EPCs mobilization. For instance, it was observed that ischaemic lesions release angiogenic factors like VEGF and

activate MAPK or the RAS-signalling pathways [28], which increase EPCs migration.

Despite the important role of vascular endothelium in cardiovascular disease (CVD), their limited regeneration capacity remains a vital problem. EPCs improve angiogenesis and participate in endothelium recovery subsequent to vascular injuries [29]. Cardiovascular diseases (CVDs) are directly related to both the decline of EPC mobilization and the number of EPCs present in the damaged site. In this context, Ang II stimulates EPCs migration to ischaemic regions and commences vascularization through VEGF-associated endothelial nitric oxide synthase [30]. The activation of NAPH and subsequent ROS (reactive oxygen species) generation constitutes the stimulatory impact of Ang II on EPCs that is required for normal EPC function. However, the long-term activation of NADPH and oxidative stress is concomitant with cell senescence [31]. Moreover, acute high-dose exposure to Ang II has been shown to negatively modulate EPC function in the hind limb ischaemic rat model [32].

RAS has been shown to result in progenitor cell senescence and suppression of differentiation and adherence in bone marrow-derived EPCs in Ang II infusion models. This inhibitory impact could be attenuated by the administration of AT1 receptor antagonists [31]. Previous reports have proved the crucial role of Ang II during erythropoiesis [33]. In studies using transgenic mice expressing human renin and angiotensinogen, a drastic rise in levels of erythropoietin was observed, which is a glycoprotein hormone that controls erythropoiesis. Genetic ablation of AT1 receptor from these mice reduced erythropoietin levels and restored haematocrit levels [34]. Also, ACE blockade has been concomitant with haematocrit decrease in vivo [35]. The idea of ACE and/or Ang II being contributed to erythropoiesis was further confirmed by a recent research in which ACE marked haematopoietic stem cells from human embryonic, fetal and adult haematopoietic tissues [36]. However, the mechanism of Ang II-associated regulation of erythropoiesis is mainly unclear. Most of these effects are observed during early phases of erythropoiesis [37]. As mentioned above, some researchers imply that Ang II acts indirectly via its effect on erythropoietin levels [38], whereas others do not agree with this link [39]. The other possible mechanism is proposed to be the involvement of JAK (Janus kinase)/STAT (signal transducer and activator of

transcription) pathway. JAK/STAT pathway is known to be activated by Ang II [40].

The local RAS is not only involved in the physiology of pancreas, but it also influences the pancreatic stem cell (PSC) functionality. RAS has been shown to be associated with pancreatic islet cell function and proliferation and differentiation of PSCs/progenitor cells during development [41]. Different stem/progenitor cells have been reported to be differentiated into insulin-expressing cells, which make them appropriate candidates for islet cell transplantation. Regarding the potential role of RAS in stem cell differentiation, it is possible that RAS-modulated stem cell could be a new source of pancreatic β-cells. Both exocrine and endocrine pancreas are known to have local RAS components [42]. In exocrine part, AT1 receptor activation turns on signalling pathways such as ROS generation and activation of pro-inflammatory, vasoactive and growth factor receptors [43, 44]. Therefore, Ang II might result in fibrosis and inflammation of exocrine pancreas through the AT1 receptor. Hence, blockade of RAS has been considered a potential therapeutic opportunity for some pancreas disorders.

In the endocrine portion of pancreas, RAS has been shown to be a key regulator of insulin and islet physiology [43]. AT1 receptor stimulation leads to β-cells, decreased islet blood flow and insulin secretion, while AT2 receptor activation results in β-cell proliferation and islet blood flow and insulin secretion enhancement [19]. Moreover, the ACE2/Ang-(1–7)/Mas axis, which has been attracting more research attention recently, is present in several local tissues and mainly acts as a negative modulator of ACE/Ang II/AT1R signalling. Similar to AT2 receptor activation, ACE2 overexpression in the pancreas of type 2 diabetic animals restored glucose homeostasis, as evidenced by diminished blood glucose levels, elevated insulin secretion and β-cell proliferation [45].

PSCs exist in both developing and adult pancreas in three major pancreas sections, that is, ductal endothelium, islet and acinar tissues [46]. Embryo, foetus and adult pancreas as well as bone marrow-derived MSCs are probable sources for PSCs. Transplantation of mouse or human PSCs into diabetic mice has been revealed to reduce their diabetes [46].

A novel well-defined area of research is the developmental control of RAS on cell proliferation in tumours and in tissue regeneration. Both the ACE/AngII/AT1R signalling and the alternative RAS arm (ACE2/Ang-(1–7)/Mas) interact with different growth factors; hence, they might contribute to cell proliferation and angiogenesis in neoplasms, including pancreatic cancers [47–49]. It has been demonstrated that RAS inhibition seems to be a promising therapeutic approach for the mitigation of pathophysiological circumstances of the pancreas including diabetes [50], pancreatitis [43] and pancreatic cancer [51]. Transplantation of human fetal pancreatic progenitor cell has been shown to reverse hyperglycaemia and glucose intolerance in diabetic mice [52]. ROS production has a close relation with RAS activation, and ROS-signalling pathway is associated with stem/progenitor cell proliferation, differentiation and function [53]. So, it is an interesting probability that the elevation of RAS-induced differentiation of pancreatic progenitor cells towards an endocrine lineage might offer a basis for therapy in terms of islet replacement treatments for diabetes.

MSCs have been suggested as an appropriate substitute to islet transplantation for promoting regeneration of endogenous pancreatic progenitor cells to achieve permanent normal blood glucose level in patients with type 1 diabetes [54]. Local RAS in pancreatic islet could regulate PSC differentiation and thus lead to the beneficial outcomes following MSC transplantation. In a study, these kinds of pancreatic progenitor cells have shown to differentiate into insulinsecreting cells.

RAS components like angiotensinogen and renin are expressed after the beginning of pancreatic progenitor cells differentiation, but they are not present in undifferentiated cells. These results indicate that a functional RAS exists in pancreatic progenitor cells and in mature islets that could be modulating cellular differentiation. The mitogenic behaviour related to the Ang II bindings of AT1 receptors has been proposed to regulate reprogramming of pancreatic cells and the differentiation plasticity [55]. However, it is unclear whether AT2 receptor activation reveals counter-regulatory role in this context. Furthermore, it is hypothesized that the ACE2/Ang-(1–7)/Mas axis plays an essential role in pancreatic stem cell differentiation as previous studies have shown the involvement of ACE2 arm in the proliferation and differentiation of other stem cells [56].
