Histologic sections were also stained with wheat germ agglutininCTRITC to determine cross-sectional areas of cardiac myocytes (Figure ?(Figure9B,9B, bottom). a major contributor to diastolic dysfunction of the heart and symptomatic congestive heart failure in Silvestrol humans and Silvestrol is a potent prognostic indicator of all-cause and cardiac mortality independent of symptomatology (2). Sustained cardiac hypertrophy develops in response to a variety of pathologic stimuli, including pressure overload, hypertension, and contractile protein abnormalities. In order for any form of hypertrophic remodeling to occur, stress stimuli must activate specific signaling pathways that lead to an increase in size of cardiac myocytes, protein synthesis, sarcomeric assembly and organization, and activation of CACN2 a fetal cardiac gene expression program (3). In recent years, substantial progress has been made in defining intracellular signaling pathways that control transduction of hypertrophic stimuli. Among these pathways, a role for the calcineurin/nuclear factor of activated T cellsCdependent (calcineurin/NFATCdependent) signaling pathway Silvestrol as a regulator of cardiac hypertrophy has been clearly established (4). Calcineurin A is a calcium-activated serine/threonine phosphatase that dephosphorylates and activates NFAT family members. First described as a regulator of immune function in T cells, calcineurin A lies downstream of G proteinCcoupled receptor activation in a signaling cascade that leads to cardiac hypertrophy. Calcineurin A is tethered to -actinin-2 at the Z-disc in myocytes, which may allow coupling of contractile status and calcineurin activation (5). In cultured cardiomyocytes, the calcineurin inhibitors cyclosporin A and FK506 prevent hypertrophy of myocytes in the presence of hypertrophic agonists (1). In transgenic mice, cardiac-specific overexpression of activated calcineurin A promotes a hypertrophic response that progresses to heart failure and death (4). Conversely, calcineurin ACdeficient mice show a dramatic reduction in calcineurin activity and hypertrophic responses, induced by pressure overload or agonist infusion (6). Moreover, proteins that inhibit calcineurin activation Cabin/Cain, A kinaseCanchoring protein 79 (AKAP79), calcineurin B homology protein, and modulatory calcineurinCinteracting protein 1 (MCIP1) can abrogate cardiac hypertrophy in response to hypertrophic stimuli both in vitro and in vivo Silvestrol (7C10). However, the mechanisms through which calcineurin activity is regulated in cardiomyocytes remain Silvestrol unclear at the present time. Atrogin-1, also known as muscle atrophy F-box, is a skeletal muscleC and cardiac muscleCspecific F-box protein that binds to Skp1, Cul1, and Roc1, components of the SCF family of ubiquitin ligases (11, 12). The F-box proteins function as adaptors that bind specific substrates through protein-protein interaction domains to target them for ubiquitin-dependent degradation (13, 14). Atrogin-1 lacks common domains found in other F-box proteins that are known to interact with substrates (e.g., WD-40 repeats, leucine-rich repeats), so that identification of putative atrogin-1 targets has proven elusive. However, atrogin-1 contains conserved domains such as a PDZ-binding motif and a cytochrome family heme-binding site signature at its carboxyl terminus that may be involved in protein-protein interactions (12). Overexpression of atrogin-1 in skeletal myotubes leads to atrophy, and mice deficient in atrogin-1 are resistant to denervation atrophy of skeletal muscle (11, 12). These observations suggest that atrogin-1 may regulate muscle cell size via its participation in a muscle-specific ubiquitin ligase complex, although such an activity is yet to be proven and signaling pathways regulated by atrogin-1 have not been determined. In addition, neither the function of atrogin-1 nor its downstream targets in the heart are known. It is logical to speculate that atrogin-1 should regulate pathways that are shared by skeletal and cardiac muscle. In order to resolve these issues, we have searched for potential partner proteins of atrogin-1 that might.
