Integrins regulate cellular behaviors through signaling pathways, including Rho GTPases and kinases. CD98 heterodimers, comprised of a heavy chain (CD98hc, SLC3A2) and one of several light chains, interact with integrins through CD98hc. CD98hc overexpression leads to anchorage-independent cell growth and tumorigenesis in 3T3 fibroblasts and activates certain integrin-regulated signaling pathways. To establish the biological function of CD98hc, we disrupted the gene and analyzed CD98hc-null cells. Here we report that CD98hc contributes to integrin-dependent cell spreading, cell migration, and protection from apoptosis. Furthermore, CD98hc is required for efficient adhesion-induced activation of Akt and Rac GTPase, major contributors to the integrin-dependent signals involved in cell survival and cell migration. CD98 promotes amino acid transport through its light chains; however, a CD98hc mutant that interacts with ␤1 integrins, but not CD98 light chains, restored integrin-dependent signaling and protection from apoptosis. ␤1 integrins are involved in the pathogenesis of certain cancers. CD98hc deletion markedly impaired the ability of embryonic stem cells to form teratocarcinomas in mice; teratocarcinoma formation was reconstituted by reexpression of CD98hc or of the mutant that interacts exclusively with integrins. Thus, CD98hc is an integrinassociated protein that mediates integrin-dependent signals, which promote tumorigenesis.amino acid transport ͉ signal transduction ͉ cell adhesion ͉ apoptosis ͉ cancer

The integrin family of adhesion receptors are involved in cell growth, migration and tumour metastasis. Integrins are heterodimeric proteins composed of an alpha and a beta subunit, each with a large extracellular, a single transmembrane, and a short cytoplasmic domain. The dynamic regulation of integrin affinity for ligands in response to cellular signals is central to integrin function. This process is energy dependent and is mediated through integrin cytoplasmic domains. However, the cellular machinery regulating integrin affinity remains poorly understood. Here we describe a genetic strategy to disentangle integrin signalling pathways. Dominant suppression occurs when overexpression of isolated integrin beta1 cytoplasmic domains blocks integrin activation. Proteins involved in integrin signalling were identified by their capacity to complement dominant suppression in an expression cloning scheme. CD98, an early T-cell activation antigen that associates with functional integrins, was found to regulate integrin activation. Furthermore, antibody-mediated crosslinking of CD98 stimulated beta1 integrin-dependent cell adhesion. These data indicate that CD98 is involved in regulating integrin affinity, and validate an unbiased genetic approach to analysing integrin signalling pathways.

CD98 is a cell surface heterodimer formed by the covalent linkage of CD98 heavy chain (CD98hc) with several different light chains to form amino acid transporters. CD98hc also binds specifically to the integrin ␤ 1A cytoplasmic domain and regulates integrin function. In this study, we examined the relationship between the ability of CD98hc to stimulate amino acid transport and to affect integrin function. By constructing chimeras with CD98hc and a type II transmembrane protein (CD69), we found that the cytoplasmic and transmembrane domains of CD98hc are required for its effects on integrin function, while the extracellular domain is required for stimulation of isoleucine transport. Consequently, the capacity to promote amino acid transport is not required for CD98hc's effect on integrin function. Furthermore, a mutant of CD98hc that lacks its integrin binding site can still promote increased isoleucine transport. Thus, these two functions of CD98hc are separable and require distinct domains of the protein.

Abstract-Multiple signaling pathways have been implicated in the hypertrophic response of ventricular myocytes, yet the importance of cell-matrix interactions has not been extensively examined. Integrins are cell-surface molecules that link the extracellular matrix to the cellular cytoskeleton. They can function as cell signaling molecules and transducers of mechanical information in noncardiac cells. Given these properties and their abundance in cardiac cells, we evaluated the hypothesis that ␤ 1 integrin function is involved in the ␣ 1 -adrenergic mediated hypertrophic response of neonatal rat ventricular myocytes. The hypertrophic response of this model required interaction with extracellular matrix proteins. Specificity of these results was confirmed by demonstrating that ventricular myocytes plated onto an anti-␤ 1 integrin antibody supported the hypertrophic gene response. Adenovirus-mediated overexpression of ␤ 1 integrin augmented the myocyte hypertrophic response when assessed by protein synthesis and atrial natriuretic factor production, a marker gene of hypertrophic induction. DNA synthesis was not altered by integrin overexpression. Transfection of cultured cardiac myocytes with either the ubiquitously expressed ␤ 1A integrin or the cardiac/skeletal muscle-specific ␤ 1 isoform (␤ 1D ) activated reporter expression from both the atrial natriuretic factor and myosin light chain-2 ventricular promoters, genetic markers of ventricular cell hypertrophy. Finally, suppression of integrin signaling by overexpression of free ␤ 1 integrin cytoplasmic domains inhibited the adrenergically mediated atrial natriuretic factor response. These findings show that integrin ligation and signaling are involved in the cardiac hypertrophic response pathway. (Circ Res. 1998;82:1160-1172.)Key Words: hypertrophy Ⅲ myocardium Ⅲ extracellular matrix Ⅲ integrin Ⅲ myocyte M echanical stress on the heart from either pressure or volume loading can cause hypertrophy of the myocardium to optimize cardiac performance. Results from in vivo studies and cell culture models have indicated that morphological hypertrophy coincides with a cascade of intracellular biochemical signals.