**2.5. HIV-1 structural proteins**

The HIV-1 gp120 is the key protein for viral entry by binding to the CD4 receptor and to the co-receptor CCR5 or CXCR4. The HIV-1 matrix protein p17 (p17) is a myristoylated protein that exerts many important and crucial functions during the virus cell cycle. It contributes to nuclear localization of the pre-integration complex after HIV-1 entry and promotes virus maturation and assembly [29]. In addition to its key role in the virus life cycle, p17 exerts a chemokine-like activity by binding to the chemokine receptor CXCR1 and CXCR2 and mimics some of the biological activities of IL-8, the CXCR1 and CXCR2 natural ligand.

Binding of gp120 and p17 to their receptors and/or co-receptors alters the biological activity of different cells. Extracellularly, p17 alters immune responses by activating different immune cells such as CD4<sup>+</sup> T cells, CD8<sup>+</sup> T cells, NK cells, plasmacytoid dendritic cells, monocytes, and B cells and contributing to the production and release of pro-inflammatory molecules and to the development of an inflammatory microenvironment [30–32]. Furthermore, p17 stimulates the rapid adhesion and chemotaxis of monocytes and B cells through activation of the Rho/

**Figure 2.** Role of HIV-1 structural and regulatory proteins in the pathogenesis of inflammation and endothelial cell (EC) dysfunction: endothelin-1 (ET-1), interleukin-6 (IL-6), monocyte chemoattractant protein-1 (MCP-1), HIV-1 glycoprotein-120 (gp120), HIV-1 matrix protein (p17), HIV-1 transactivator of transcription (Tat), and HIV-1 negative regulatory factor (Nef).

ROCK signaling pathway [33], suggesting that p17 may recruit activated monocytes and B cells in different tissues and organs to participate and/or sustain inflammatory processes.

**2.4. Role of HIV-1 proteins in the pathogenesis of endothelial dysfunction**

highlighting the need to target them for therapeutic benefit.

T cells, CD8<sup>+</sup>

**2.5. HIV-1 structural proteins**

352 Endothelial Dysfunction - Old Concepts and New Challenges

cells such as CD4<sup>+</sup>

regulatory factor (Nef).

The HIV-1 genome encodes a total of three structural proteins, two envelope proteins, three enzymes, and six accessory proteins. HIV-1 has designed its structural and regulatory/accessory proteins to better adapt to the human host and to promote virus replication and transmission. Among the many functions in the virus life cycle, a major role played by different HIV-1 proteins in directly driving inflammation and EC dysregulation is strengthening (**Figure 2**), thus

The HIV-1 gp120 is the key protein for viral entry by binding to the CD4 receptor and to the co-receptor CCR5 or CXCR4. The HIV-1 matrix protein p17 (p17) is a myristoylated protein that exerts many important and crucial functions during the virus cell cycle. It contributes to nuclear localization of the pre-integration complex after HIV-1 entry and promotes virus maturation and assembly [29]. In addition to its key role in the virus life cycle, p17 exerts a chemokine-like activity by binding to the chemokine receptor CXCR1 and CXCR2 and mimics

Binding of gp120 and p17 to their receptors and/or co-receptors alters the biological activity of different cells. Extracellularly, p17 alters immune responses by activating different immune

B cells and contributing to the production and release of pro-inflammatory molecules and to the development of an inflammatory microenvironment [30–32]. Furthermore, p17 stimulates the rapid adhesion and chemotaxis of monocytes and B cells through activation of the Rho/

**Figure 2.** Role of HIV-1 structural and regulatory proteins in the pathogenesis of inflammation and endothelial cell (EC) dysfunction: endothelin-1 (ET-1), interleukin-6 (IL-6), monocyte chemoattractant protein-1 (MCP-1), HIV-1 glycoprotein-120 (gp120), HIV-1 matrix protein (p17), HIV-1 transactivator of transcription (Tat), and HIV-1 negative

T cells, NK cells, plasmacytoid dendritic cells, monocytes, and

some of the biological activities of IL-8, the CXCR1 and CXCR2 natural ligand.

On the other hand, gp120 is known to induce dysfunction of T cells, macrophages, cardiomyocytes, ECs, and central nervous system cells, when expressed on the viral particle, on the surface of infected cells, or as a viral-free soluble protein [34].

Endothelial dysfunction mediated by these two HIV-1 structural proteins results to occur through different mechanisms: gp120 is considered a direct and indirect proapoptotic factor favoring EC death, whereas p17 is a potent angiogenic and lymphangiogenic factor.

EC death by gp120 is mediated by its interaction with CXCR4 expressed on the endothelial cell surface that triggers different downstream effects, as activation of the CXCR4-dependent caspase and the mitogen-activated protein kinase (MAPK), or through protein kinase C (PKC) activation [35]. The indirect mechanism of gp120 apoptosis is based on the increased secretion of ET-1 [36, 37], inhibition of NO synthase [38], and a higher surface expression of endothelial monocyte-activating polypeptide II (EMAPII) [39]. In particular, EMAPII acts as proapoptotic factor following different types of stress including hypoxia and mechanical stress. It is worth noting that after its interaction with CXCR4, gp120 promotes p38 MAPK signaling pathway activation and a rapid surface expression and release of EMAPII, thus favoring apoptosis through a paracrine mechanism. In the context of an inflammatory microenvironment, gp120 may also contribute to reduce the EC-derived NO synthesized by the NO synthase that is a major mediator of endothelium-dependent vasorelaxation and endothelial dysfunction.

P17 is a potent angiogenic and lymphangiogenic molecule both in vitro and in vivo. Activity of p17 is dependent on its interaction with the chemokine receptors CXCR1 and CXCR2, expressed on ECs [40–42]. Angiogenesis and lymphangiogenesis promoted by p17 after its interaction with CXCR1 and/or CXCR2 involve activation of both MAPK/ERK and PI3K/ Akt signaling pathways [40–42]. Lymphangiogenesis induced by p17 was found to be partly mediated by the selective release of the pro-angiogenic/lymphangiogenic factor ET-1 [42], which binds to its B receptor (ETBR) expressed on lymph node-derived ECs (LECs) and activates the downstream PI3K/Akt and MAPK/ERK signaling pathways.

Interestingly, many studies demonstrated a long-term persistence of these two structural HIV-1 proteins in lymph node germinal centers and lymphoid tissue of HIV<sup>+</sup> patients, even during successful cART and in the absence of any detectable viral replication [43, 44].

Interestingly, p17 is continuously released in the extracellular space even in the absence of viral replication and viral protease activity [45] and is detected at nanomolar concentrations in the blood of HIV<sup>+</sup> patients even in the presence of anti-p17 antibodies [46].

Altogether, these findings suggest that gp120 and p17 are released by infected cells even during cART, bind to ECs, and drive cell activation, angiogenesis, and/or apoptosis, leading to vascular disease. In addition, the capability of p17 to stimulate the immune system and promote a pro-inflammatory status highlights the key role played by this protein in driving endothelial dysfunction.
