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It is well-established that G protein-coupled receptors (GPCRs) transduce signals into cells using G proteins as intermediary molecules. β-Arrestins are molecules involved in regulating GPCRs; however, it has recently been reported that β-arrestins can also mediate signaling through GPCRs. Signaling through G proteins or β-arrestins can be activated selectively using specific agonists; of the latter, those that can selectively activate either G proteins or β-arrestins are called biased agonists. The clinical use of biased agonists could potentially induce fewer side effects. However, partial agonists can also explain the mechanism of G protein-biased agonists; thus, appropriate assay systems must be considered. Endogenous agonists are known to bind to orthosteric and allosteric sites in the agonist binding site, and the allosteric site is associated with the activity of biased agonists. This current review presents a detailed discussion of biased agonists.

Calcium ions (Ca²(+)) play an essential role in homeostasis and the activity of cardiovascular cells. Ca²(+) influx across the plasma membrane induced by neurohumoral factors or mechanical stress elicits physiologically relevant timing and spatial patterns of Ca²(+) signaling, which leads to the activation of various cardiovascular functions, such as muscle contraction, gene expression, and hypertrophic growth of myocytes. A canonical transient receptor potential protein subfamily member, TRPC6, which is activated by diacylglycerol and mechanical stretch, works as an upstream regulator of the Ca²(+) signaling pathway required for pathological hypertrophy. We have recently found that the inhibition of cGMP-selective phosphodiesterase 5 (PDE5) suppresses agonist- and mechanical stretch-induced hypertrophy through inhibition of Ca²(+) influx in rat cardiomyocytes. The inhibition of PDE5 suppressed the increase in frequency of Ca²(+) spikes induced by receptor stimulation or mechanical stretch. Activation of protein kinase G by PDE5 inhibition phosphorylated TRPC6 proteins at Thr⁶⁹ and prevented TRPC6-mediated Ca²(+) influx. Substitution of Ala for Thr⁶⁹ in TRPC6 abolished the antihypertrophic effects of PDE5 inhibition. These results suggest that phosphorylation and functional suppression of TRPC6 underlies the prevention of cardiac hypertrophy by PDE5 inhibition. As TRPC6 proteins are also expressed in vascular smooth muscle cells and reportedly participate in vascular remodeling, TRPC6 blockade may be an effective therapeutic strategy for preventing pathologic cardiovascular remodeling.

Activation of Ca(2+) signaling in cardiomyocytes induced by receptor stimulation or mechanical stress has been implicated in the development of cardiac hypertrophy. However, it is still unclear how intracellular Ca(2+) targets specifically decode the alteration of intracellular Ca(2+) concentration ([Ca(2+)](i)) on the background of the rhythmic Ca(2+) increases required for muscle contraction. In excitable cardiomyocytes, changes in the frequency or amplitude of Ca(2+) transients evoked by Ca(2+) influx-induced Ca(2+) release have been suggested to encode signals for induction of hypertrophy, and a partial depolarization of plasma membrane by receptor stimulation will increase the frequency of Ca(2+) oscillations. We found that activation of diacylglycerol (DAG)-responsive canonical transient receptor potential (TRPC) subfamily channels (TRPC3 and TRPC6) mediate membrane depolarization induced by G(q) protein-coupled receptor stimulation. DAG-mediated membrane depolarization through activation of TRPC3/TRPC6 channels increases the frequency of Ca(2+) spikes, leading to activation of calcineurin-dependent signaling pathways. Inhibition of either TRPC3 or TRPC6 completely suppressed agonist-induced hypertrophic responses, suggesting that TRPC3 and TRPC6 form heterotetramer channels. Furthermore, we found that hypertrophic agonists increase the expression of TRPC6 proteins through activation of G(12) family proteins, leading to amplification of DAG-mediated hypertrophic signaling in cardiomyocytes. As heart failure proceeds through cardiac hypertrophy, TRPC3/TRPC6 channels may be a new therapeutic target for heart failure.

G protein-coupled receptors (GPCRs) are the most famous target proteins for medicinal drugs. So far, heterogeneity of GPCRs is mainly focused on genetic variation. However, it has been reported that the structure and function of GPCRs are modified by several mechanisms after translation. RNA editing introduces the amino acid different from that encoded in genome by changing the nucleotide. Dimer formation is another example of how heterogeneity is produced. Many receptors form homo- or hetero-dimers, and obtain different function from original receptors. Receptors are regulated by several means to modulate stimulation strength. Receptor subtype is often differentially regulated by receptor kinases and/or second messenger-regulated kinases. There is a new type of receptor that shows a novel structural feature, a long amino terminal region belonging to class B seven transmembrane receptors. The physiological function of this class of receptor is assumed to play a role in cell-cell communication. This novel structural feature may directly link GPCR to the cytoskeleton. These mechanisms to produce functional and structural heterogeneity may explain how cells evoke different responses in different tissues or cells upon the same stimulation. Thus, the post-translational mechanism to produce heterogeneity provides additional flexibility when cells respond to one extracellular stimulus.

At the beginning, beta-arrestin is believed to regulate desensitization of G protein-coupled receptors. However, it becomes apparent that beta-arrestin is involved in not only regulation but also signal transduction by identification of many proteins interacting with beta-arrestin. So far, the model of regulation and signaling of G protein-coupled receptor is built on beta 2-adrenergic receptor. However, this model does not apply to all G protein-coupled receptors. The differential regulation and signaling between beta 1- and beta 2-adrenergic receptors is presented.

Extracellular information is translated into the cellular responses by various signal transduction systems such as G protein-coupled receptor-linked and protein tyrosine kinase receptor-mediated pathways. The changes or modifications of theses signaling proteins cause uncontrolled transformation or cell death. One of the features of the signaling molecules is that these proteins contain the domains or motifs for protein-protein or protein-lipid interactions governed by several amino acids. Therefore, these amino acids may become promising targets for the drug design. In contrast with intracellular molecules, the receptors located on cellular surface are still the best candidate for the drugs. The future analysis of cellular signal transduction system allows us to develop new drugs that cannot cause serious side effects in future.