Identification and characterization of distinct C-terminal domains of the human hydroxycarboxylic acid receptor-2 that are essential for receptor export, constitutive activity, desensitization, and internalization

Rare and potentially pathogenic variants in hydroxycarboxylic acid receptor genes identified in breast cancer cases

BACKGROUND

Three genes clustered together on chromosome 12 comprise a group of hydroxycarboxylic acid receptors (HCARs): HCAR1, HCAR2, and HCAR3. These paralogous genes encode different G-protein coupled receptors responsible for detecting glycolytic metabolites and controlling fatty acid oxidation. Though better known for regulating lipid metabolism in adipocytes, more recently, HCARs have been functionally associated with breast cancer proliferation/survival; HCAR2 has been described as a tumor suppressor and HCAR1 and HCAR3 as oncogenes. Thus, we sought to identify germline variants in HCAR1, HCAR2, and HCAR3 that could potentially be associated with breast cancer risk.

METHODS

Two different cohorts of breast cancer cases were investigated, the Alabama Hereditary Cancer Cohort and The Cancer Genome Atlas, which were analyzed through nested PCRs/Sanger sequencing and whole-exome sequencing, respectively. All datasets were screened for rare, non-synonymous coding variants.

RESULTS

Variants were identified in both breast cancer cohorts, some of which appeared to be associated with breast cancer BC risk, including HCAR1 c.58C > G (p.P20A), HCAR2 c.424C > T (p.R142W), HCAR2 c.517_518delinsAC (p.G173T), HCAR2 c.1036A > G (p.M346V), HCAR2 c.1086_1090del (p.P363Nfs*26), HCAR3 c.560G > A (p.R187Q), and HCAR3 c.1117delC (p.Q373Kfs*82). Additionally, HCAR2 c.515C > T (p.S172L), a previously identified loss-of-function variant, was identified.

CONCLUSIONS

Due to the important role of HCARs in breast cancer, it is vital to understand how these genetic variants play a role in breast cancer risk and proliferation and their consequences on treatment strategies. Additional studies will be needed to validate these findings. Nevertheless, the identification of these potentially pathogenic variants supports the need to investigate their functional consequences.

A novel splice variant of Gαq-coupled Bombyx CAPA-PVK receptor 1 functions as a specific Gαi/o-linked receptor for CAPA-PK

Alternative splicing enables G protein-coupled receptor (GPCR) genes to greatly increase the number of structurally and functionally distinct receptor isoforms. However, the functional role and relevance of the individual GPCR splice variants in regulating physiological processes are still to be assessed. A naturally occurring alternative splice variant of Bombyx CAPA-PVK receptor, BomCAPA-PVK-R1-Δ341, has been shown to act as a dominant-negative protein to regulate cell surface expression and function of the canonical CAPA-PVK receptor. Herein, using functional assays, we identify the splice variant Δ341 as a specific receptor for neuropeptide CAPA-PK, and upon activation, Δ341 signals to ERK1/2 pathway. Further characterization demonstrates that Δ341 couples to Gαi/o, distinct from the Gαq-coupled canonical CAPA-PVK receptor, triggering ERK1/2 phosphorylation through Gβγ-PI3K-PKCζ signaling cascade. Moreover, our ELISA data show that the ligand-dependent internalization of the splice variant Δ341 is significantly impaired due to lack of GRKs-mediated phosphorylation sites. Our findings highlight the potential of this knowledge for molecular, pharmacological and physiological studies on GPCR splice variants in the future.

Genetic coding variants in the niacin receptor, hydroxyl-carboxylic acid receptor 2, and response to niacin therapy

OBJECTIVE

Niacin has been used for seven decades to modulate plasma lipids, but its mechanism of action is still unclear. We sought to determine whether variants in the niacin receptor gene, hydroxyl-carboxylic receptor 2 (HCAR2), are associated with lipid response to treatment.

PARTICIPANTS AND METHODS

Coding variants, rs7314976 (p.R311C) and rs2454727 (p.M317I), were genotyped in 2067 participants from the Atherothrombosis Intervention in Metabolic Syndrome with Low HDL/High Triglycerides and Impact on Global Health Outcomes (AIM-HIGH) trial. AIM-HIGH was a randomized, placebo-controlled trial that was conducted to assess the effect of extended-release niacin in patients with cardiovascular disease aggressively treated with low-density lipoprotein cholesterol-lowering therapy.

RESULTS

There was no association of p.R311C or p.M317I with changes in low-density lipoprotein cholesterol, triglycerides, or high-density lipoprotein cholesterol at 1 year in groups receiving placebo or extended-release niacin. In White patients, the reduction in lipoprotein (a) [Lp(a)] in response to niacin was greater in homozygous carriers of the major 317M allele (-22.7%; P=0.005) compared with minor allele carriers (-15.3%). This was directionally consistent in the Black participants. Upon combining both groups, the reduction in Lp(a) in response to niacin was significantly greater in the homozygous major allele carriers (-23.0%; P=0.003) compared with minor allele carriers (-15.2%).

CONCLUSION

Understanding the genetic contribution toward variation in response to niacin therapy, including Lp(a) reduction, could uncover mechanisms by which niacin decreases Lp(a), an important independent risk factor for cardiovascular disease.

Nicotinic acid timed to feeding reverses tissue lipid accumulation and improves glucose control in obese Zucker rats[S]

Nicotinic acid (NiAc) is a potent inhibitor of lipolysis, acutely reducing plasma free fatty acid (FFA) concentrations. However, a major FFA rebound is seen during rapid NiAc washout, and sustained exposure is associated with tolerance development, with FFAs returning to pretreatment levels. Our aim was to find a rational NiAc dosing regimen that preserves FFA lowering, sufficient to reverse nonadipose tissue lipid accumulation and improve metabolic control, in obese Zucker rats. We compared feeding-period versus fasting-period NiAc dosing for 5 days: 12 h subcutaneous infusion (programmable, implantable mini-pumps) terminated by gradual withdrawal. It was found that NiAc timed to feeding decreased triglycerides in liver (-47%; P < 0.01) and heart (-38%; P < 0.05) and reduced plasma fructosamine versus vehicle. During oral glucose tolerance test, plasma FFA levels were reduced with amelioration of hyperglycemia and hypertriglyceridemia. Furthermore, timing NiAc to feeding resulted in a general downregulation of de novo lipogenesis (DNL) genes in liver. By contrast, NiAc timed to fasting did not reduce tissue lipids, ameliorate glucose intolerance or dyslipidemia, or alter hepatic DNL genes. In conclusion, NiAc dosing regimen has a major impact on metabolic control in obese Zucker rats. Specifically, a well-defined NiAc exposure, timed to feeding periods, profoundly improves the metabolic phenotype of this animal model.

Agonist-Activated Bombyx Corazonin Receptor Is Internalized via an Arrestin-Dependent and Clathrin-Independent Pathway

Agonist-induced internalization plays a key role in the tight regulation of the extent and duration of G protein-coupled receptor signaling. Previously, we have shown that the Bombyx corazonin receptor (BmCrzR) activates both Gαq- and Gαs-dependent signaling cascades. However, the molecular mechanisms involved in the regulation of the internalization and desensitization of BmCrzR remain to be elucidated. Here, vectors for expressing BmCrzR fused with enhanced green fluorescent protein (EGFP) at the C-terminal end were used to further characterize BmCrzR internalization. We found that the BmCrzR heterologously expressed in HEK-293 and BmN cells was rapidly internalized from the plasma membrane into the cytoplasm in a concentration- and time-dependent manner via a β-arrestin (Kurtz)-dependent and clathrin-independent pathway in response to agonist challenge. While most of the internalized receptors were recycled to the cell surface via early endosomes, some others were transported to lysosomes for degradation. Assays using RNA interference revealed that both GRK2 and GRK5 were essentially involved in the regulation of BmCrzR phosphorylation and internalization. Further investigations indicated that the identified cluster of Ser/Thr residues ((411)TSS(413)) was responsible for GRK-mediated phosphorylation and internalization. This is the first detailed investigation of the internalization and trafficking of Bombyx corazonin receptors.

Distinct regions in the C-Terminus required for GLP-1R cell surface expression, activity and internalisation

The glucagon-like peptide-1 (GLP-1) receptor (GLP-1R), an important drug target in the treatment of type 2 diabetes, is a G-protein coupled receptor (GPCR) that mediates insulin secretion by GLP-1. The N-terminus controls GLP-1R biosynthetic trafficking to the cell surface but the C-terminus involvement in that trafficking is unknown. The aim of this study was to identify distinct regions within the C-terminal domain required for human GLP-1R (hGLP-1R) cell surface expression, activity and internalisation using a number of C-terminal deletions and site-directed mutations. The results of this study revealed that the residues 411-418 within the C-terminal domain of the hGLP-1R are critical in targeting the newly synthesised receptor to the plasma membrane. The residues 419-430 are important for cAMP producing activity of the receptor, most likely by coupling to Gαs. However, the residues 431-450 within the C-terminus are essential for agonist-induced hGLP-1R internalisation. In conclusion, these findings demonstrate the hGLP-1R has distinct regions within the C-terminal domain required for its cell surface expression, activity and agonist-induced internalisation.

Dosing profile profoundly influences nicotinic acid's ability to improve metabolic control in rats

Acute nicotinic acid (NiAc) administration results in rapid reduction of plasma FFA concentrations. However, sustained NiAc exposure is associated with tolerance development resulting in return of FFA to pretreatment levels. The aim of this study was to determine whether a 12 h rectangular exposure profile (intermittent dose group) could avoid tolerance development and thereby reverse insulin resistance induced by lipid overload. FFA lowering was assessed in male Sprague Dawley (lean) and obese Zucker rats (obese) in response to a 5 h NiAc infusion, in either NiAc-naïve animals or after 5 days of continuous (24 h/day) or intermittent (12 h/day) NiAc dosing (via implantable, programmable minipump). We found that intermittent dosing over 5 days preserved NiAc-induced FFA lowering, comparable to dosing in NiAc-naïve animals. By contrast, following 5 days continuous administration, NiAc-induced FFA lowering was lost. The effect of intermittent NiAc infusion on insulin sensitivity was assessed in obese Zucker rats using hyperinsulinemic-isoglycemic clamps. The acute effect of NiAc to elevate glucose infusion rate (vs. saline control) was indeed preserved with intermittent dosing, while being lost upon continuous infusion. In conclusion, an intermittent but not continuous NiAc dosing strategy succeeded in retaining NiAc's ability to lower FFA and improve insulin sensitivity in obese Zucker rats.-Kroon, T., A. Kjellstedt, P. Thalén, J. Gabrielsson, and N. D. Oakes.