Integrin affinity modulation and survival signalling.
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Date
2008Author
Elliott, Paul Anthony
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Abstract
Integrins are heterodimeric transmembrane proteins that provide a bi-directional link
between the cell’s internal biological mechanisms and the extracellular environment.
During inside-out signalling, intracellular messages converge on the integrin
cytoplasmic domain to induce a conformational change. This is transmitted to the
extracellular domain where it results in an alteration in affinity for integrin ligands
such as fibronectin and laminin. In this way the cell has developed the ability to
modulate the critical functions of adhesion and cell movement. In outside-in
signalling, the integrin performs a more complex function than simple adhesion;
upon binding to ligand, the integrin extracellular domain undergoes a conformational
change which is transmitted to the cytoplasmic domain. This alters the integrin’s
cytoplasmic domain affinity for intracellular signalling proteins and results in the
activation of intracellular second messenger pathways. In this way, the extracellular
milieu is able to influence intracellular signalling including those involved in
apoptosis.
This thesis demonstrates data which provide original insights into bi-directional
integrin signalling:
Inside-out signalling: Constitutively active Notch1 increases β3-integrin affinity and
abrogates Hras-mediated integrin suppression without increasing expression of β3-
integrin. Dominant-Negative Rras blocks Notch-mediated integrin activation and
Notch1-mediated reversal of Hras and Raf-mediated integrin suppression and this is
independent of erk phosphorylation. Notch1 induces Rras activation. Functional
adhesion assays confirm that Notch1IC increases K562 adhesion in a β1-integrin
dependent manner and this is abrogated by Dominant-Negative Rras. This data
supports a mechanism in which Notch1 increases integrin affinity via activation of
Rras.
Outside-in signalling: Evidence is presented demonstrating that extracellular matrix
proteins, laminin and fibronectin, activate β1-integrins to protect SCLC cells against
the apoptotic effects of etoposide and ionizing radiation via PI3Kinase activation.
This occurs in two ways: 1) PI3Kinase-dependent β1-integrin signalling resulting in
phosphorylation of Bad and reduced caspase-9 cleavage and 2) a β1-integrinmediated
over-riding of etoposide and radiotherapy-induced cell cycle S phase delay
and G2/M arrest. β1-integrin-mediated outside-in survival signalling was
investigated further in the in vivo setting; MatrigelTM, a basement membrane product
rich in extracellular matrix proteins, promoted SCLC xenograft survival and growth
in a β1-integrin and tyrosine kinase-dependent manner.
This data provides novel insights into the critical functions that integrins play in
adhesion and survival signalling.