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Merlin/NF-2 mediates contact inhibition of growth by suppressing recruitment of Rac to the plasma membrane.

Okada T, Lopez-Lago M, Giancotti FG - J. Cell Biol. (2005)

Bottom Line: PAK's ability to release human umbilical vein endothelial cells from contact inhibition is blocked by an unphosphorylatable form of its target Merlin, suggesting that PAK promotes mitogenesis by phosphorylating, and thus inactivating, Merlin.Small interference RNA-mediated knockdown of Merlin exerts the same effects.Dominant-negative Rac blocks PAK-mediated release from contact inhibition, implying that PAK functions upstream of Rac in this signaling pathway.

View Article: PubMed Central - PubMed

Affiliation: Cell Biology Program, Memorial Sloan-Kettering Cancer Center, New York, NY 10021, USA. t-okada@ski.mskcc.org

ABSTRACT
Introduction of activated p21-activated kinase (PAK) is sufficient to release primary endothelial cells from contact inhibition of growth. Confluent cells display deficient activation of PAK and translocation of Rac to the plasma membrane at matrix adhesions. Targeting Rac to the plasma membrane rescues these cells from contact inhibition. PAK's ability to release human umbilical vein endothelial cells from contact inhibition is blocked by an unphosphorylatable form of its target Merlin, suggesting that PAK promotes mitogenesis by phosphorylating, and thus inactivating, Merlin. Merlin mutants, which are presumed to exert a dominant-negative effect, enable recruitment of Rac to matrix adhesions and promote mitogenesis in confluent cells. Small interference RNA-mediated knockdown of Merlin exerts the same effects. Dominant-negative Rac blocks PAK-mediated release from contact inhibition, implying that PAK functions upstream of Rac in this signaling pathway. These results provide a framework for understanding the tumor suppressor function of Merlin and indicate that Merlin mediates contact inhibition of growth by suppressing recruitment of Rac to matrix adhesions.

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Hypothetical model of Merlin's function. Cadherin-initiated adhesion prevents activation of Pak in contact-inhibited cells and thereby causes accumulation of dephosphorylated Merlin. This closed form of Merlin suppresses integrin-mediated recruitment of Rac, and hence mitogenic signaling. Upon release from contact inhibition, Pak phosphorylates and inactivates Merlin, allowing recruitment of Rac to the membrane and mitogenic signaling.
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fig7: Hypothetical model of Merlin's function. Cadherin-initiated adhesion prevents activation of Pak in contact-inhibited cells and thereby causes accumulation of dephosphorylated Merlin. This closed form of Merlin suppresses integrin-mediated recruitment of Rac, and hence mitogenic signaling. Upon release from contact inhibition, Pak phosphorylates and inactivates Merlin, allowing recruitment of Rac to the membrane and mitogenic signaling.

Mentions: We examined if activated PAK rescues recruitment of Rac to integrin-dependent adhesions in confluent cells. HUVEC were transfected with GFP-Rac, alone or in combination with activated PAK, plated under confluent conditions, and treated with FN-coated beads. Fig. 6 B shows that activated PAK promotes recruitment of Rac to FN-coated beads in confluent cells. Interestingly, activated PAK accumulated underneath FN-coated beads in “rescued” cells, which is in agreement with the hypothesis that it promotes cell proliferation by inducing recruitment of Rac to matrix adhesions (Fig. 7 B). These observations indicate that PAK controls recruitment of Rac to matrix adhesions by a signaling mechanism and imply that intercellular contact interferes with this process.


Merlin/NF-2 mediates contact inhibition of growth by suppressing recruitment of Rac to the plasma membrane.

Okada T, Lopez-Lago M, Giancotti FG - J. Cell Biol. (2005)

Hypothetical model of Merlin's function. Cadherin-initiated adhesion prevents activation of Pak in contact-inhibited cells and thereby causes accumulation of dephosphorylated Merlin. This closed form of Merlin suppresses integrin-mediated recruitment of Rac, and hence mitogenic signaling. Upon release from contact inhibition, Pak phosphorylates and inactivates Merlin, allowing recruitment of Rac to the membrane and mitogenic signaling.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2171182&req=5

fig7: Hypothetical model of Merlin's function. Cadherin-initiated adhesion prevents activation of Pak in contact-inhibited cells and thereby causes accumulation of dephosphorylated Merlin. This closed form of Merlin suppresses integrin-mediated recruitment of Rac, and hence mitogenic signaling. Upon release from contact inhibition, Pak phosphorylates and inactivates Merlin, allowing recruitment of Rac to the membrane and mitogenic signaling.
Mentions: We examined if activated PAK rescues recruitment of Rac to integrin-dependent adhesions in confluent cells. HUVEC were transfected with GFP-Rac, alone or in combination with activated PAK, plated under confluent conditions, and treated with FN-coated beads. Fig. 6 B shows that activated PAK promotes recruitment of Rac to FN-coated beads in confluent cells. Interestingly, activated PAK accumulated underneath FN-coated beads in “rescued” cells, which is in agreement with the hypothesis that it promotes cell proliferation by inducing recruitment of Rac to matrix adhesions (Fig. 7 B). These observations indicate that PAK controls recruitment of Rac to matrix adhesions by a signaling mechanism and imply that intercellular contact interferes with this process.

Bottom Line: PAK's ability to release human umbilical vein endothelial cells from contact inhibition is blocked by an unphosphorylatable form of its target Merlin, suggesting that PAK promotes mitogenesis by phosphorylating, and thus inactivating, Merlin.Small interference RNA-mediated knockdown of Merlin exerts the same effects.Dominant-negative Rac blocks PAK-mediated release from contact inhibition, implying that PAK functions upstream of Rac in this signaling pathway.

View Article: PubMed Central - PubMed

Affiliation: Cell Biology Program, Memorial Sloan-Kettering Cancer Center, New York, NY 10021, USA. t-okada@ski.mskcc.org

ABSTRACT
Introduction of activated p21-activated kinase (PAK) is sufficient to release primary endothelial cells from contact inhibition of growth. Confluent cells display deficient activation of PAK and translocation of Rac to the plasma membrane at matrix adhesions. Targeting Rac to the plasma membrane rescues these cells from contact inhibition. PAK's ability to release human umbilical vein endothelial cells from contact inhibition is blocked by an unphosphorylatable form of its target Merlin, suggesting that PAK promotes mitogenesis by phosphorylating, and thus inactivating, Merlin. Merlin mutants, which are presumed to exert a dominant-negative effect, enable recruitment of Rac to matrix adhesions and promote mitogenesis in confluent cells. Small interference RNA-mediated knockdown of Merlin exerts the same effects. Dominant-negative Rac blocks PAK-mediated release from contact inhibition, implying that PAK functions upstream of Rac in this signaling pathway. These results provide a framework for understanding the tumor suppressor function of Merlin and indicate that Merlin mediates contact inhibition of growth by suppressing recruitment of Rac to matrix adhesions.

Show MeSH
Related in: MedlinePlus