The calcium/calmodulin-dependent phosphatase calcineurin plays a central role in the control of cardiomyocyte hypertrophy in response to pathological stimuli. Although calcineurin is present at high levels in normal heart, its activity appears to be unaffected by calcium during the course of a cardiac cycle. The mechanism(s) whereby calcineurin is selectively activated by calcium under pathological conditions has remained unclear. Here, we demonstrate that diverse signals for cardiac hypertrophy stimulate expression of canonical transient receptor potential (TRPC) channels. TRPC consists of a family of seven membrane-spanning nonselective cation channels that have been implicated in the nonvoltage-gated influx of calcium in response to G protein-coupled receptor signaling, receptor tyrosine kinase signaling, and depletion of internal calcium stores. TRPC3 expression is up-regulated in multiple rodent models of pathological cardiac hypertrophy, whereas TRPC5 expression is induced in failing human heart. We demonstrate that TRPC promotes cardiomyocyte hypertrophy through activation of calcineurin and its downstream effector, the nuclear factor of activated T cells transcription factor. These results define a novel role for TRPC channels in the control of cardiac growth, and suggest that a TRPC-derived pool of calcium contributes to selective activation of calcineurin in diseased heart. Cardiac hypertrophy is an adaptive response of the heart to many forms of cardiac disease, including hypertension, mechanical load abnormalities, myocardial infarction, valvular dysfunction, cardiac arrhythmias, endocrine disorders, and genetic mutations in cardiac contractile protein genes. Whereas the hypertrophic response is thought to be an initially compensatory mechanism that augments cardiac performance, sustained hypertrophy is maladaptive and frequently leads to ventricular dilation and the clinical syndrome of heart failure. Accordingly, cardiac hypertrophy has been established as an independent risk factor for cardiac morbidity and mortality (1).Abnormal calcium handling, characterized by elevated intracellular diastolic calcium levels, is a hallmark of cardiac hypertrophy and heart failure. Elevated intracellular calcium not only impairs the contractile performance of the heart, but also activates calcium-dependent signaling pathways that mediate maladaptive cardiac remodeling (2). One such pathway is regulated by the calcium-calmodulin-dependent phosphatase calcineurin, which has been shown to be sufficient, and in some cases, necessary for pathological hypertrophy (3-5). Activated calcineurin dephosphorylates the transcription factor nuclear factor of activated T-cells (NFAT), 4 facilitating translocation of NFAT to the nucleus where it acts in concert with other proteins to mediate expression of prohypertrophic genes. The activity of calcineurin is tightly regulated in vivo by a negative feedback mechanism; one of the most highly sensitive NFAT target genes encodes a potent calcineurin inhibitor, modulatory calcineuri...

The final step in Hedgehog (Hh) signal transduction is post-translational regulation of the transcription factor, Cubitus interruptus (Ci). Ci resides in the cytoplasm in a latent form, where Hh regulates its processing into a transcriptional repressor or its nuclear access as a transcriptional activator. Levels of latent Ci are controlled by degradation, with different pathways activated in response to different levels of Hh. Here, we describe the roadkill (rdx) gene, which is expressed in response to Hh. The Rdx protein belongs to a conserved family of proteins that serve as substrate adaptors for Cullin3-mediated ubiquitylation. Overexpression of rdx reduced Ci levels and decreased both transcriptional activation and repression mediated by Ci. Loss of rdx allowed excessive accumulation of Ci. rdx manipulation in the eye revealed a novel role for Hh in the organization and survival of pigment and cone cells. These studies identify rdx as a limiting factor in a feedback loop that attenuates Hh responses through reducing levels of Ci. The existence of human orthologs for Rdx raises the possibility that this novel feedback loop also modulates Hh responses in humans.

No abstract

AT1 receptors are selectively downregulated in failing human ventricles, similar to the selective downregulation of beta 1 receptors. The relative lack of AT1 downregulation in ISC hearts may be related to differences in the degree of ventricular dysfunction.

Diverse pathological insults trigger a cardiac remodeling process during which myocytes undergo hypertrophy, with consequent decline in cardiac function and eventual heart failure. Multiple transcriptional regulators of pathological cardiac hypertrophy are controlled at the level of subcellular distribution. For example, prohypertrophic transcription factors belonging to the nuclear factor of activated T cells (NFAT) and GATA families are subject to CRM1-dependent nuclear export but are rapidly relocalized to the nucleus in response to cues for hypertrophic growth. Here, we demonstrate that the antihypertrophic chromatin-modifying enzyme histone deacetylase 5 (HDAC5) is shuttled out of the cardiomyocyte nucleus via a CRM1-mediated pathway in response to diverse signals for hypertrophy. CRM1 antagonists block the agonist-mediated nuclear export of HDAC 5 and repress pathological gene expression and associated hypertrophy of cultured cardiomyocytes. Conversely, CRM1 activity is dispensable for nonpathological cardiac gene activation mediated by thyroid hormone and insulin-like growth factor 1, agonists that fail to trigger the nuclear export of HDAC5. These results suggest a selective role for CRM1 in derepression of pathological cardiac genes via its neutralizing effects on antihypertrophic factors such as HDAC5. Pharmacological approaches targeting CRM1-dependent nuclear export in heart muscle may have salutary effects on cardiac function by suppressing maladaptive changes in gene expression evoked by stress signals.A common mechanism controlling gene expression involves altering the subcellular distribution of transcriptional regulators. A multitude of transcription factors and cofactors possess nuclear localization sequences (NLSs) and nuclear export signals (NESs) that mediate entry into and exit from the nucleus, respectively. Frequently, signal transduction pathways that impinge on transcriptional regulators function by positively or negatively affecting the activities of these intrinsic targeting domains.For proteins over ϳ40 kDa, passage into and out of the nucleus is governed by the nuclear pore complex (NPC), a multisubunit structure embedded in the nuclear envelope (27). Positively charged NLSs are bound by importins ␣ and ␤, which tether cargo to the cytosolic face of the NPC and facilitate translocation of proteins into the nucleus. The CRM1 protein, also referred to as exportin, mediates the transit of proteins out of the nucleus (16), although CRM1-independent mechanisms for nuclear export exist (25, 33). CRM1 binds hydrophobic NESs together with the small GTP binding protein Ran, and these ternary complexes are shuttled out of the nucleus through a series of interactions with the NPC.