**3.2. Implantation**

Implantation is a series of sequential biological events triggered after fertilization in which the blastocyst migrates from the fallopian tube into the uterus. The fertilized egg is then attached to the uterine wall and subsequently implanted in the endometrium. Implantation occurs only in a very specific time period and place during the mid-secretory phase of the uterine cycle [24]. During this period, the uterus becomes more receptive by promoting a series of cellular and molecular events favoring the implantation of the embryo. In this stage, the role of several intercellular mediators has been implicated, which include specific cytokines, growth factors, adhesion molecules, lipid mediators, steroid hormones, and Hox transcription factors [25]. Like in endometrial tissue, *HOXA10* also plays a role during embryo implantation as it has been shown that despite the fact that *HOXA10*-deficient mice (*HOXA10−/−*) exhibited normal ovulation cycle, the implantation did not occur. Interestingly, implantation was restored when embryos from *HOXA10−/−* were transferred to wild-type mice; however, wild-type embryos were not implanted in *HOXA10−/−* female mice [18], suggesting that *HOXA10* is required to have an adequate implantation environment. Moreover, *HOXA10−/−* and *HOXA11−/−* mice also exhibit poor implantation due to insufficient development of stromal glandular tissue and decidua during pregnancy [26]. In humans, the expression of both *HOXA10* and *HOXA11* genes rises gradually during the proliferative phase of the menstrual cycle, showing a peak of expression in mid-cycle, when implantation typically occurs [13, 27]. Interestingly, this peak of expression was not observed in women with endometriosis or in mice with induced endometriosis [13, 27], suggesting that HoxA10 and HoxA11 peaks require a healthy endometrium to support and continue with the implantation process. Several studies have shown that Hox10 not only promotes implantation directly but also inhibits detrimental factors such as empty spiracles homeobox 2 (EMX2), P300/ CBP-associated factor (P/CAF), and gamma-aminobutyric acid (GABA). Studies by Taylor and colleagues demonstrated that HoxA10 repressed EMX2 expression, which in turn inhibited the proliferation of endometrial cells [28], suggesting that HoxA10 is a pro-proliferative and proimplantation factor in these cells. Zhu and colleagues demonstrated that HoxA10 repressed the promoter activity of P/CAF, which impairs endometrial receptivity and embryo implantation by downregulating integrin β3 [29]. Recent studies have also shown that HoxA10 decreased mRNA levels and protein translocation of GABA receptor [30], which plays a role in the generation of uterine contractions and labor [31]. Thus, the quiescent uterus is required for adequate implantation and embryo development, along with reduced expression or activity of GABA receptor.

*HOXA9* has also been associated with the regulation of myeloid cell differentiation. The activation of HoxA9 complex favors the recruitment of CREB-binding protein (CBP/p300), histone acetylation, and activation of a number of transcription factors and proto-oncogenes,

Hox Genes in Adult Tissues and Their Role in Endothelial Cell Differentiation and Angiogenesis

http://dx.doi.org/10.5772/intechopen.76745

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Another Hox gene family member linked to hematopoiesis is *HOXA10*. The expression of this gene is high in myeloid progenitor cells, and it decreases during cell maturation [38]. Bei et al. [39] studied the expression of HoxA10 in bone marrow from patients with human acute myeloid leukemia (AML), and they observed increased expression of this gene in patients with poor prognosis. Then, they developed a HoxA10-overexpresing mouse model identifying *CDX4*, a caudal gene that contain homeodomain and code for transcription factor that plays an important role in hematopoiesis, as a *HOXA10* target gene [39]. Overall, their results demonstrated that *HOXA10* was contributing to AML pathogenesis via CDX4-positive feedback. Other groups demonstrated that HoxA13 was associated with the development of monocytes and macrophages, and its expression was observed more often in monocytic leukemia cell lines in comparison with other types of leukemia [40]. Moreover, the expression of genes *HOXB3* and *HOXB4* has been found to be altered in patients with AML with poor prognosis [41].

The development of the vascular system involves two processes called vasculogenesis and angiogenesis [42]. During vasculogenesis, angioblasts derived from different sources, including mesodermal embryonic layer or bone marrow, differentiate into endothelial cells and subsequently form a primitive network of tubular structures called blood vessels [43]. Vasculogenesis occurs largely during embryonic development; however, the presence of a population of circulating endothelial progenitor cells (EPCs) derived from the bone marrow in adults strongly suggests that this process may occur in the postnatal period [44]. In contrast, angiogenesis refers to the formation of new blood vessels from preexisting vessels by cell migration and remodeling of the primitive vascular network [45]. Vasculogenesis and angiogenesis are involved in the development of the functional vascular system in the embryo and the formation of blood vessels in the postnatal period. Both vasculogenesis and angiogenesis are under the regulation of several growth factors, which include vascular endothelial growth factor (VEGF), fibroblast growth factor 2 (FGF2), platelet-derived growth factor (PDGF), and transforming growth factor β1 (TGF-β1), among others [45]. Interestingly, different research groups have found that Hox genes regulate the expression of these growth factors and, in turn, endothelial cell differentiation. In the next section, we will describe supporting evidence about the role of Hox genes in

The *HOXA3* gene is required for modeling the anterior body plan during embryogenesis, but they can also play a role in promoting angiogenesis [46, 47]. It has been shown that activation of *HOXA3* favors the migration of endothelial cells and keratinocytes, associated with increased expression of urokinase-type plasminogen activator receptor (uPAR) in

including Erg, FLT3, and SOX4 Myb, which regulate hematopoiesis [37].

**4. Hox genes in vascularity and angiogenesis**

endothelial differentiation, vasculogenesis, and angiogenesis (**Figure 2**).

**4.1. HOXA3**

#### **3.3. Hematopoiesis**

Hox genes are highly expressed in hematopoietic stem cells (HSC) and immature progenitor cells [32]; however, this expression is gradually decreased upon cell differentiation. Moreover, overexpression of genes from the *HOXA* cluster impairs B and T lymphocyte differentiation, affects erythropoiesis, and reduces stem cell bone marrow homing, favoring the induction of myeloproliferative disorders and leukemias [33]. In fact, overexpression of *HOXA1*, *HOXA4*, and *HOXA6* genes has been shown to favor the generation of permanent cell lines [34]. Studies by Wang et al. showed increased proliferation and higher self-growth and self-renewal of hematopoietic stem progenitor cells (HSC) (Line 9 and Line H1) when HoxA6 was overexpressed compared to normal conditions [34]. The authors observed that overexpression of this gene sustained HSC self-renewal and multipotency by promoting mature erythroid lineage cells and partial apoptosis of erythroid progenitors.

Another gene involved in this process is *HOXA5*. Overexpression of HoxA5 in HSC isolated from umbilical cord blood, bone marrow [35], or mice [36] promotes a significant shift toward myeloid differentiation in relation to erythroid differentiation when compared to respective control cells [35, 36]. Then, the authors evaluated genes affected by HoxA5, and they observed downregulation of several genes involved in cell proliferation, differentiation, and metabolism [35, 36].

*HOXA9* has also been associated with the regulation of myeloid cell differentiation. The activation of HoxA9 complex favors the recruitment of CREB-binding protein (CBP/p300), histone acetylation, and activation of a number of transcription factors and proto-oncogenes, including Erg, FLT3, and SOX4 Myb, which regulate hematopoiesis [37].

Another Hox gene family member linked to hematopoiesis is *HOXA10*. The expression of this gene is high in myeloid progenitor cells, and it decreases during cell maturation [38]. Bei et al. [39] studied the expression of HoxA10 in bone marrow from patients with human acute myeloid leukemia (AML), and they observed increased expression of this gene in patients with poor prognosis. Then, they developed a HoxA10-overexpresing mouse model identifying *CDX4*, a caudal gene that contain homeodomain and code for transcription factor that plays an important role in hematopoiesis, as a *HOXA10* target gene [39]. Overall, their results demonstrated that *HOXA10* was contributing to AML pathogenesis via CDX4-positive feedback. Other groups demonstrated that HoxA13 was associated with the development of monocytes and macrophages, and its expression was observed more often in monocytic leukemia cell lines in comparison with other types of leukemia [40]. Moreover, the expression of genes *HOXB3* and *HOXB4* has been found to be altered in patients with AML with poor prognosis [41].
