Integrin alpha V

[[File:Integrin示意图.png|thumb|right|300px|General structure of an integrin heterodimer. Integrin alpha V (ITGAV) is an alpha subunit that typically pairs with beta 3 or beta 8.]]

Integrin alpha V (ITGAV) is a protein that in humans is encoded by the ITGAV gene. It is one of the 18 known [[Integrin alpha subunit]]s and is a crucial component of [[integrin]] receptors, which are heterodimeric cell surface proteins involved in cell-extracellular matrix (ECM) adhesion and [[Cell signaling|signaling]].

Nomenclature and Aliases

Gene: ITGAV (Integrin Subunit Alpha V)
Protein: Integrin alpha V, ITGAV
Aliases: CD51, VNR (Vitronectin Receptor Alpha Chain), MSK8

Structure

Integrins are composed of two distinct transmembrane glycoprotein subunits: an [[Integrin alpha subunit|alpha subunit]] and a [[Integrin beta subunit|beta subunit]]. Integrin alpha V is an alpha subunit that non-covalently associates with a limited number of beta subunits, most notably [[Integrin beta 3|beta 3]] (forming $\alpha_V\beta_3$) and [[Integrin beta 8|beta 8]] (forming $\alpha_V\beta_8$). Less commonly, it can also pair with [[Integrin beta 1|beta 1]], [[Integrin beta 5|beta 5]], [[Integrin beta 6|beta 6]], or [[Integrin beta 7|beta 7]].

Like other integrin alpha subunits, Integrin alpha V consists of:

Large Extracellular Domain: Responsible for binding to specific ligands in the ECM. This domain contains several subdomains, including a propeller-like headpiece with cation-binding sites essential for ligand recognition.
Single Transmembrane Domain: Anchors the protein to the cell membrane.
Short Cytoplasmic Tail: Interacts with the [[cytoskeleton]] and intracellular signaling proteins, mediating [[outside-in signaling]] and [[inside-out signaling]].

Function

Integrin alpha V-containing heterodimers play a diverse and critical role in many biological processes due to their ability to bind a wide array of ECM proteins and initiate intracellular signaling cascades.

Ligand Binding

Integrin alpha V-containing integrins are well-known for their recognition of the [[Arginine-Glycine-Aspartate motif|RGD (Arginine-Glycine-Aspartate) motif]], which is present in numerous ECM proteins. Key ligands include:

[[Vitronectin]]
[[Fibronectin]]
[[Fibrinogen]]
[[Thrombospondin]]
[[Von Willebrand factor]]
[[Osteopontin]]
[[Collagens]] (less commonly, depending on the beta subunit)

Cell Adhesion and Migration

Through their interaction with the ECM, Integrin alpha V heterodimers mediate:

Cell attachment: Anchoring cells to their surrounding matrix.
Cell migration: Facilitating cell movement during processes like wound healing, immune surveillance, and development. This is particularly important for cells like [[fibroblast]]s, [[endothelial cell]]s, and [[macrophage]]s.

Cell Signaling

Integrin alpha V acts as a bidirectional signal transducer:

Outside-in signaling: Binding of ECM ligands to the extracellular domain induces conformational changes that transmit signals into the cell, affecting [[Cell proliferation|proliferation]], [[Cell differentiation|differentiation]], [[Cell survival|survival]], and [[Gene expression|gene expression]].
Inside-out signaling: Intracellular signals can modulate the affinity and avidity of the integrin for its ligands, thereby regulating cell adhesion.

Biological Processes

Specific alpha V heterodimers are implicated in:

Angiogenesis: $\alpha_V\beta_3$ and $\alpha_V\beta_5$ are critically involved in the formation of new blood vessels, regulating endothelial cell migration and survival.
Bone remodeling: $\alpha_V\beta_3$ is highly expressed on [[osteoclast]]s and is essential for their adhesion to the bone matrix and subsequent resorption.
Wound healing and tissue repair: By mediating cell migration and adhesion, $\alpha_V$ integrins contribute to the processes of tissue regeneration.
Immune responses: Involved in the recruitment and activation of immune cells.
Development: Plays a role in embryonic development and organogenesis.

Clinical Significance

Dysregulation of Integrin alpha V function is associated with a range of pathological conditions.

Cancer

Integrin alpha V, particularly in complex with beta 3 ($\alpha_V\beta_3$) or beta 5 ($\alpha_V\beta_5$), is often overexpressed in many aggressive cancers, including [[melanoma]], [[glioblastoma]], [[breast cancer]], and [[lung cancer]]. Its high expression correlates with:

Tumor growth: Promoting cancer cell proliferation and survival.
Angiogenesis: Essential for the formation of new blood vessels that supply oxygen and nutrients to growing tumors.
Invasion and metastasis: Facilitating the migration of cancer cells away from the primary tumor and their establishment in secondary sites.
Drug resistance: Implicated in conferring resistance to various anti-cancer therapies.

Fibrosis

$\alpha_V$ integrins, especially $\alpha_V\beta_1$, $\alpha_V\beta_6$, and $\alpha_V\beta_8$, are key activators of [[TGF-beta]] (Transforming Growth Factor-beta), a major pro-fibrotic cytokine. This makes them central players in the development of various fibrotic diseases, such as [[pulmonary fibrosis]], [[liver fibrosis]], and [[kidney fibrosis]].

Other Diseases

Atherosclerosis: Involved in vascular inflammation and plaque formation.
Osteoporosis: Altered $\alpha_V\beta_3$ function on osteoclasts can contribute to imbalances in bone remodeling.
Autoimmune diseases: May play a role in inflammation and immune cell trafficking.

Therapeutic Targeting

Given its prominent role in various pathologies, Integrin alpha V has emerged as a significant therapeutic target, especially in oncology and fibrosis.

Anti-angiogenic therapies: Small molecule inhibitors and monoclonal antibodies targeting $\alpha_V\beta_3$ and $\alpha_V\beta_5$ have been developed to disrupt tumor angiogenesis (e.g., cilengitide, volociximab).
Anti-fibrotic agents: Inhibitors of $\alpha_V\beta_1$, $\alpha_V\beta_6$, and $\alpha_V\beta_8$ are under investigation for their potential to block TGF-beta activation and mitigate fibrosis.
Radioimmunotherapy: Antibodies targeting $\alpha_V\beta_3$ have been conjugated with radioisotopes for targeted delivery of radiation to integrin-expressing tumors.

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