The Ras/Raf-1/MEK1/ERK Signaling Pathway Coupled to Integrin Expression Mediates Cholinergic Regulation of Keratinocyte Directional Migration

Abstract

The physiologic mechanisms that determine directionality of lateral migration are a subject of intense research. Galvanotropism in a direct current (DC) electric field represents a natural model of cell re-orientation toward the direction of future migration. Keratinocyte migration is regulated through both the nicotinic and muscarinic classes of acetylcholine (ACh) receptors. We sought to identify the signaling pathway mediating the cholinergic regulation of chemotaxis and galvanotropism. The pharmacologic and molecular modifiers of the Ras/Raf-1/MEK1/ERK signaling pathway altered both chemotaxis toward choline and galvanotropism toward the cathode in a similar way, indicating that the same signaling steps were involved. The galvanotropism was abrogated due to inhibition of ACh production by hemicholinium-3 and restored by exogenously added carbachol. The concentration gradients of ACh and choline toward the cathode in a DC field were established by highperformance liquid chromatographic measurements. This suggested that keratinocyte galvanotaxis is, in effect, chemotaxis toward the concentration gradient of ACh, which it creates in a DC field due to its highly positive charge. A time-course immunofluorescence study of the membrane redistribution of ACh receptors in keratinocytes exposed to a DC field revealed rapid relocation to and clustering at the leading edge of ␣7 nicotinic and M 1 muscarinic receptors. Their inactivation with selective antagonists or small interfering RNAs inhibited galvanotropism, which could be prevented by transfecting the cells with constitutively active MEK1. The end-point effect of the cooperative signaling downstream from ␣7 and M 1 through the MEK1/ERK was an up-regulated expression of ␣ 2 and ␣ 3 integrins, as judged from the results of real-time PCR and quantitative immunoblotting. Thus, ␣7 works together with M 1 to orient a keratinocyte toward direction of its future migration. Both ␣7 and M 1 apparently engage the Ras/Raf/MEK/ERK pathway to up-regulate expression of the "sedentary" integrins required for stabilization of the lamellipodium at the keratinocyte leading edge. Human epidermal keratinocytes (KCs)2 can synthesize and secrete acetylcholine (ACh) and use it as a local hormone for an autocrine and paracrine control of their vital functions, including motility (1). ACh and its congeners are chemotactic for KCs, neurons, and other cell types (2-8). To characterize the physiologic control of keratinocyte migration, we developed an in vitro model of skin epithelialization, termed the agarose gel keratinocyte outgrowth system (AGKOS) (9). Using AGKOS, we have demonstrated that activation of the keratinocyte nicotinic ACh receptors (nAChRs) comprised by ␣7 subunits exhibit reciprocal effects on cell motility by stimulating directional (chemotaxis) and inhibiting random (chemokinesis) migration and that ACh-gated ion channels containing ␣3 subunit stimulate chemokinesis of KCs (8).More recently, ␣7 nAChR has been shown to mediate chemotaxis of vascular smooth muscle cells to...

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