1. ¹Department of Periodontics and Oral Medicine, School of Dentistry, University of Michigan, Ann Arbor, MI, USA
2. ²Norlux Neuro-Oncology, Department of Biomedicine, University of Bergen, Bergen, Norway
3. ³Cancer Research Center, The Burnham Institute for Medical Research, La Jolla, CA, USA

Correspondence: YL Kapila, Department of Periodontics and Oral Medicine, School of Dentistry, University of Michigan, 1101 N University Avenue, Ann Arbor, MI 48109-1078, USA. Tel: +734 615 2295; Fax: +734 763 5503; E-mail: ykapila@umich.edu

Received 13 September 2007; Revised 16 January 2008; Accepted 21 January 2008; Published online 22 February 2008.

Abstract

Disruption of cell–matrix interactions can lead to anoikis – apoptosis due to loss of matrix contacts. Altered fibronectin (FN) induces anoikis of primary human fibroblasts by a novel signaling pathway characterized by reduced phosphorylation of focal adhesion kinase (FAK). However, the receptors involved are unknown. FAK phosphorylation is regulated by nerve/glial antigen 2 (NG2) receptor signaling through PKC a point at which signals from integrins and proteoglycans may converge. We found that an altered FN matrix induced anoikis in fibroblasts by upregulating NG2 and downregulating integrin 4. Suppressing NG2 expression or overexpressing 4 rescued cells from anoikis. NG2 overexpression alone induced apoptosis and, by reducing FAK phosphorylation, increased anoikis induced by an altered matrix. NG2 overexpression or an altered matrix also suppressed PKC expression, but overexpressing integrin 4 enhanced FAK phosphorylation independently of PKC. Cotransfection with NG2 cDNA and integrin 4 siRNA did not lower PKC and pFAK levels more than transfection with either alone. PKC was upstream of FAK phosphorylation, as silencing PKC decreased FAK phosphorylation. PKC overexpression reversed this behavior and rescued cells from anoikis. Thus, NG2 is a novel proapoptotic receptor, and NG2 and integrin 4 oppositely regulate anoikis in fibroblasts. NG2 and integrin 4 regulate FAK phosphorylation by PKC-dependent and -independent pathways, respectively.