5-HT1B Receptor-Mediated Activation of ERK1/2 Requires Both Gαi/o and β-Arrestin Proteins

β-Arrestin1 is involved in hepatocellular carcinoma metastasis via extracellular signal-regulated kinase-mediated epithelial-mesenchymal transition

BACKGROUND AND AIM

Hepatocellular carcinoma (HCC) is a malignant disease worldwide. It is implicated in high cancer-related mortality rates in humans. β-Arrestin1 (ARRB1) has been demonstrated to be related to the development of several cancers, while the relationship between ARRB1 and metastasis in HCC is unknown.

METHODS

A tissue microarray of 68 tissues from HCC patients with or without metastasis was collected. Wild-type and ARRB1 knockout mice were used to examine the role of ARRB1 in metastasis in vivo. The level of ARRB1 in HCC tissues, mouse liver tissues, and cell lines was determined by quantitative reverse transcription-polymerase chain reaction, Western blot, and immunohistochemistry. Migration, invasion, and motility capacities of HCC cells were determined by transwell assay and wound healing assay. Vein injection of nude mice model was used to reveal the metastatic abilities of HCC cell lines. For the mechanism study, we investigated the effects of ARRB1 on the phosphorylation of ERK1/2 and the expression of epithelial-mesenchymal transition (EMT) markers in HCC.

RESULTS

We reveal that ARRB1 accelerates metastasis in HCC and that ARRB1 deficiency inhibits hepatocarcinogenesis and reverses EMT in mice. ARRB1 regulates HCC cell migration and invasion and suppresses HCC metastasis in vivo. Furthermore, we show that ARRB1 promotes EMT through the phosphorylation of ERK1/2.

CONCLUSIONS

Our data suggest that ARRB1 promotes HCC invasion and metastasis through p-ERK1/2-mediated EMT and that suppression of ARRB1 or p-ERK1/2 may offer potential therapeutic targets for HCC therapy.

Phosphorylation barcode-dependent signal bias of the dopamine D1 receptor

Agonist-activated G protein-coupled receptors (GPCRs) must correctly select from hundreds of potential downstream signaling cascades and effectors. To accomplish this, GPCRs first bind to an intermediary signaling protein, such as G protein or arrestin. These intermediaries initiate signaling cascades that promote the activity of different effectors, including several protein kinases. The relative roles of G proteins versus arrestins in initiating and directing signaling is hotly debated, and it remains unclear how the correct final signaling pathway is chosen given the ready availability of protein partners. Here, we begin to deconvolute the process of signal bias from the dopamine D1 receptor (D1R) by exploring factors that promote the activation of ERK1/2 or Src, the kinases that lead to cell growth and proliferation. We found that ERK1/2 activation involves both arrestin and Gαs, while Src activation depends solely on arrestin. Interestingly, we found that the phosphorylation pattern influences both arrestin and Gαs coupling, suggesting an additional way the cells regulate G protein signaling. The phosphorylation sites in the D1R intracellular loop 3 are particularly important for directing the binding of G protein versus arrestin and for selecting between the activation of ERK1/2 and Src. Collectively, these studies correlate functional outcomes with a physical basis for signaling bias and provide fundamental information on how GPCR signaling is directed.

Central 5-HT receptors and their function; present and future

Since our review of central 5-HT receptors and their function twenty years ago, no new 5-HT receptor has been discovered and there is little evidence that this situation will change in the near future. Nevertheless, over this time significant progress has been made in our understanding of the properties of these receptors and in the clinical translation of this information, and some of these developments are highlighted herein. Such highlights include extensive mapping of 5-HT receptors in both animal and human brain, culminating in readily-accessible brain atlases of 5-HT receptor distribution, as well as emerging data on how 5-HT receptors are distributed within complex neural circuits. Also, a range of important pharmacological and genetic tools have been developed that allow selective 5-HT receptor manipulation, in cells through to whole organism models. Moreover, unexpected complexity in 5-HT receptor function has been identified including agonist-dependent signalling that goes beyond the pharmacology of canonical 5-HT receptor signalling pathways set down in the 1980s and 1990s. This new knowledge of 5-HT signalling has been extended by the discovery of combined signalling of 5-HT and co-released neurotransmitters, especially glutamate. Another important advance has been the progression of a large number of 5-HT ligands through to experimental medicine studies and clinical trials, and some such agents have already become prescribed therapeutic drugs. Much more needs to be discovered and understood by 5-HT neuropharmacologists, not least how the diverse signalling effects of so many 5-HT receptor types interact with complex neural circuits to generate neurophysiological changes which ultimately lead to altered cognitions and behaviour. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.