The α9β1 integrin enhances cell migration by polyamine-mediated modulation of an inward-rectifier potassium channel

Abstract

The ␣9␤1 integrin accelerates cell migration through binding of spermidine/spermine acetyltransferase (SSAT) to the ␣9 cytoplasmic domain. We now show that SSAT enhances ␣9-mediated migration specifically through catabolism of spermidine and/or spermine. Because spermine and spermidine are effective blockers of K ؉ ion efflux through inward-rectifier K ؉ (Kir) channels, we examined the involvement of Kir channels in this pathway. The Kir channel inhibitor, barium, or knockdown of a single subunit, Kir4.2, specifically inhibited ␣9-dependent cell migration. ␣9␤1 and Kir4.2 colocalized in focal adhesions at the leading edge of migrating cells and inhibition or knockdown of Kir4.2 caused reduced persistence and an increased number of lamellipodial extensions in cells migrating on an ␣9␤1 ligand. These results identify a pathway through which the ␣9 integrin subunit stimulates cell migration by localized polyamine catabolism and modulation of Kir channel function.T he ␣9␤1 integrin is widely expressed and binds to a number of ligands, including the extracellular matrix protein tenascin-C, the vascular cell adhesion molecule, VCAM-1, and several members of the ADAM family (1-3). ␣9␤1 mediates enhanced cell migration, an effect that specifically depends on the ␣9 cytoplasmic domain (4, 5). The closely related integrin ␣4 subunit also accelerates cell migration, through reversible interaction with the scaffolding protein, paxillin (6, 7). The mechanism by which the ␣9 subunit cytoplasmic domain accelerates cell migration is completely different (4, 5), and requires binding of the enzyme, spermidine/ spermine-N 1 -acetyltransferase (SSAT) (5). The mechanisms by which SSAT binding modulates migration were until now unknown. SSAT specifically catalyzes catabolism of the higher order polyamines, spermidine and spermine, to the lower order polyamine, putrescine (8), thereby increasing intracellular levels of putrescine, and decreasing those of spermidine and spermine (9, 10).Spermine and spermidine are potent blockers of outward potassium (K ϩ ) currents from inward rectifier K ϩ (Kir) channels (11-13). These channels conduct larger inward currents at membrane voltages below the resting potential than outward currents at more positive voltages. The long, positively charged polyamines, spermine (ϩ4) or spermidine (ϩ3), mediate rectification by binding to negatively charged residues in the channel pore (12,14). The catabolized polyamine putrescine (ϩ2) is less effective at blocking outward flow of K ϩ ions (11, 12). Although Kir channels have been implicated in the regulation of many different physiological processes, including heart rate and membrane excitability (15), there are currently no reports suggesting their involvement in the regulation of cell migration. However, numerous studies have suggested that K ϩ efflux is a critical factor in modulating cell migration (16)(17)(18)(19). In this study, we show that both the catalytic activity of SSAT and the downstream catabolism of polyamines are specifically required for ␣9...

Integrin α9β1 in airway smooth muscle suppresses exaggerated airway narrowing

“…We have previously shown that integrin α 9 β 1 -mediated enhancement of cell migration is caused by altered rectification of a specific Kir channel, Kir4.2 (25), and have found that Kir4.2 is also the major Kir channel expressed in airway smooth muscle (our unpublished observations). However, we found that inhibiting Kir channels by barium chloride had no effect on force generation by tracheal rings or lung slices (our unpublished observations).…”

“…We have previously shown that another effect of integrin α 9 β 1 , accelerated cell migration, depends on the interaction between the integrin α 9 cytoplasmic domain and SSAT (24,25). SSAT is the rate-limiting step in catabolism of higher-order polyamines (48), which converts spermine to spermidine and then spermidine to putrescine.…”

“…We have previously shown that the integrin α 9 cytoplasmic domain specifically binds to the polyamine catabolizing enzyme spermidine/spermine N 1 -acetyltransferase (SSAT) and that this effect is critical for integrin α 9 β 1 -dependent cell migration (24,25). To determine whether the interaction of the integrin α 9 cytoplasmic domain with SSAT is also important for integrin α 9 β 1 -mediated modulation of airway smooth muscle contraction, we evaluated the effects of stabilizing SSAT (and thus effectively enhancing SSAT activity) on airway contraction using the polyamine analog N 1 ,N 11 -bis(ethyl)norspermine tetrahydrochloride (BE3-3-3), which prevents proteasome-mediated SSAT degradation (26).…”

Integrin α9β1 mediates enhanced cell migration through nitric oxide synthase activity regulated by Src tyrosine kinase

“…The exact region within the CDs of the integrins that is required for NO induction remains to be determined. Of note, not all signaling pathways for cell migration are shared by these two integrins because paxillin is required for migration mediated by integrin α4β1 but not by integrin α9β1 (Liu et al, 1999;Young et al, 2001;Nishiya et al, 2005), and SSAT (Chen et al, 2004;deHart et al, 2008) is exclusively utilized by integrin α9β1. In the light of such differences in signaling mechanism, the minimal effect of iNOS inhibition on integrin-α4β1-induced migration (Fig.…”

Polyamine-dependent activation of Rac1 is stimulated by focal adhesion-mediated Tiam1 activation

“…19,20 Recently, deHart et al have shown that local changes in polyamine concentrations caused by association of spermidine/spermine acetyltransferase (SSAT) with α9β1 integrin increased potassium efflux and enhanced migration. 21 Furthermore, a report by Makitie et al showed that transglutaminase-catalyzed polyamination of RhoA regulates its activity and that ODC influenced RhoA distribution in the cells. 22 These studies suggested a role for polyamines in global as well localized effects on migration.…”