Purpose: Contact inhibition is an important mechanism for maintaining corneal
endothelium in a non-replicative state. Protein tyrosine phosphatases (PTPs) play a
role in regulating the integrity of cell-cell contacts, differentiation, and growth. In this
study, we aimed to evaluate whether phosphatases are involved in the maintenance of
contact-dependent inhibition of proliferation in corneal endothelial cells and to
identify candidate PTPs that are expressed in these cells and might be involved in
regulation of contact inhibition.
Methods: Confluent cultures of rat corneal endothelial cells or endothelium in ex
vivo corneas were treated with the general phosphatase inhibitor, sodium
orthovanadate (SOV). Immunocytochemistry (ICC) evaluated the effect of SOV on
cell-cell contacts by staining for ZO-1, and on cell cycle progression by staining for
Ki67. Transverse sections of rat cornea and cultured rat corneal endothelial cells were
used to test for expression of the candidate PTPs: PTP-mu, PTP-LAR, PTP1B, SHP-1,
SHP-2, and PTEN using ICC and either Western blots or RT-PCR.
Results: ZO-1 staining demonstrated that SOV induced a time-dependent release of
cell-cell contacts in confluent cultures of corneal endothelial cells and in the
endothelium of ex vivo corneas. Staining for Ki67 indicated that SOV promoted
limited cell cycle progression in the absence of serum. PTP-mu, PTP1B, SHP-1,
SHP-2, and PTEN, but not PTP-LAR, were expressed in rat corneal endothelial cells
in situ and in culture. The subcellular location of PTP-mu and PTP1B differed in
subconfluent and confluent cells, while that of SHP-1, SHP-2, and PTEN was similar,
regardless of confluent status. Western blots confirmed the expression of PTP1B,
SHP-1, SHP-2, and PTEN. RT-PCR confirmed expression of PTP-mu mRNA.
Conclusion: Phosphatases are involved in regulation of junctional integrity and of
cell proliferation in corneal endothelial cells. PTP-mu, PTP1B, SHP-1, SHP-2, and
PTEN are expressed in rat corneal endothelium and may be involved in regulation of
contact inhibition in these normally non-proliferating cells.
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