1.17.7 Integrin Definition

Integrin Definition is a description of a class of cell-matrix adhesion molecule composed of two distinct protein subunits joined together to form a single functional receptor, which spans the cell membrane and mediates physical attachment between the cell and specific components of the surrounding extracellular matrix, while also transmitting signals bidirectionally between the extracellular matrix and the cell's internal signaling machinery.

Conceptual Basis

A Two-Subunit Receptor Structure

An integrin is defined by its composition from two distinct protein subunits that associate with one another to form a single functional adhesion receptor, with the specific combination of subunits present determining the particular extracellular matrix component that the resulting integrin is capable of binding.

A Receptor Mediating Both Adhesion and Signaling

An integrin functions simultaneously as a structural adhesion molecule, providing physical attachment to the extracellular matrix, and as a signaling receptor, transmitting information regarding the state of that attachment into the cell's interior, distinguishing integrins from adhesion molecules that serve a purely structural, non-signaling role.

Structural Basis

Extracellular Matrix-Binding Region

The portion of an integrin extending outward from the cell surface is structured to specifically recognize and bind particular components of the extracellular matrix, with the precise binding specificity of a given integrin determined by the particular combination of subunits forming that integrin.

Intracellular Region Linked to the Cytoskeleton

The portion of an integrin extending into the interior of the cell is coupled, through intracellular adaptor proteins, to the internal cytoskeletal framework, mechanically linking the extracellular matrix-binding event to the cell's internal structural network.

Bidirectional Signaling

Outside-In Signaling

Engagement of an integrin with its extracellular matrix binding partner can trigger signals transmitted from the outside of the cell toward the interior, termed outside-in signaling, through which the physical state of matrix attachment influences intracellular processes including proliferation and survival.

Inside-Out Signaling

Conversely, signals originating within the cell can alter the binding affinity of an integrin for its extracellular matrix partner, termed inside-out signaling, through which the cell actively regulates the strength of its own matrix attachment in response to internal signaling conditions rather than passively responding only to external cues.

Functional Significance

Regulating the Strength and Dynamics of Matrix Attachment

Because integrin binding affinity can be actively regulated through inside-out signaling, integrins allow a cell to dynamically adjust the strength of its attachment to the surrounding matrix, transitioning between states of stable anchorage and states more conducive to movement, according to the cell's current functional requirements.

Coupling Matrix Attachment to Cellular Behavior

Through outside-in signaling, integrins provide a mechanism through which the physical characteristics of the surrounding extracellular matrix, including its composition and mechanical properties, can influence broader aspects of cellular behavior, linking the cell's adhesive environment directly to its ongoing signaling state.

Relationship to Cell-Matrix Adhesion and Cancer Cell Biology

A Principal Mediator of Cell-Matrix Adhesion

Integrins constitute a principal class of adhesion molecule mediating cell-matrix adhesion, providing the specific binding interactions through which cells attach to defined components of the surrounding extracellular matrix.

Relevance to Altered Cancer Cell Adhesion

Because integrins govern both the physical strength of matrix attachment and the intracellular signaling consequences that follow from it, altered expression, altered binding specificity, or altered regulation of integrins is closely associated with the changed patterns of matrix adhesion and matrix-dependent signaling characteristic of cancer cell adhesion.