Dexamethasone-associated cross-linked actin network formation in human trabecular meshwork cells involves ß3 integrin signaling.

ABSTRACT

PURPOSE: To determine whether cross-linked actin networks (CLANs) formed in dexamethasone (DEX)-treated human trabecular meshwork (HTM) cells are structurally similar to those formed after ß3 integrin activation and involve αvß3 integrin signaling. METHODS: Two HTM cell strains and an αvß3 integrin-overexpressing immortalized TM cell line were used. DEX- or ethanol-pretreated HTM cells were plated on fibronectin with or without ß3 integrin-activating mAb AP-5. Immunofluorescence microscopy was used to identify phalloidin-labeled CLANs and to ascertain the presence of α-actinin, PIP(2), and syndecan-4 within them. ß3 Integrin signaling involvement was determined using a PI3-kinase (LY294002) or Rac1 (NSC23766) inhibitor. αvß3 Integrin expression levels and the ß3 integrin activation state were determined by fluorescence-activated cell sorter analysis and immunofluorescence microscopy. RESULTS: CLANs associated with either DEX treatment or ß3 integrin activation contained syndecan-4, PIP(2), and α-actinin. In the absence of mAb AP-5, LY294002 did not affect DEX-associated CLAN formation, whereas NSC23766 decreased the percentage of CLAN-positive cells by 80%. In the presence of mAb AP-5, both inhibitors decreased DEX-associated CLAN formation. DEX pretreatment increased ß3 integrin-induced CLAN formation nearly sixfold and the level of αvß3 integrin expression and activation threefold compared with control cells. Activated ß3 integrin-positive adhesions increased nearly fivefold in DEX-treated cells. αvß3 Integrin overexpression in TM-1 cells increased CLAN formation twofold. CONCLUSIONS: DEX-associated CLANs were structurally similar to those induced by mAb AP-5 and involved both increased expression and activation of αvß3 integrins. Thus, glucocorticoid-induced CLAN formation may involve enhanced ß3 integrin signaling in HTM cells, possibly by an inside-out signaling mechanism.