Human β1-adrenergic receptor is subject to constitutive and regulated N-terminal cleavage

Abstract The β₁-adrenergic receptor (β₁AR) is the predominant βAR in the heart, mediating the catecholamine-stimulated increase in cardiac rate and force of contraction. Regulation of this important G protein-coupled receptor is nevertheless poorly understood. We describe here the biosynthetic profile of the human β₁AR and reveal novel features relevant to its regulation using an inducible heterologous expression system in HEK293i cells. Metabolic pulse-chase labeling and cell surface biotinylation assays showed that the synthesized receptors are efficiently and rapidly transported to the cell surface. The N terminus of the mature receptor is extensively modified by sialylated mucin-type O-glycosylation in addition to one N-glycan attached to Asn15. Furthermore, the N terminus was found to be subject to limited proteolysis, resulting in two membrane-bound C-terminal fragments. N-terminal sequencing of the fragments identified two cleavage sites between Arg³¹ and Leu³² and Pro⁵² and Leu⁵³, which were confirmed by cleavage site and truncation mutants. Metalloproteinase inhibitors were able to inhibit the cleavage, suggesting that it is mediated by a matrix metalloproteinase or a disintegrin and metalloproteinase (ADAM) family member. Most importantly, the N-terminal cleavage was found to occur not only in vitro but also in vivo. Receptor activation mediated by the βAR agonist isoproterenol enhanced the cleavage in a concentration- and time-dependent manner, and it was also enhanced by direct stimulation of protein kinase C and adenylyl cyclase. Mutation of the Arg³¹–Leu³² cleavage site stabilized the mature receptor. We hypothesize that the N-terminal cleavage represents a novel regulatory mechanism of cell surface β₁ARs

Inefficient maturation of the rat luteinizing hormone receptor:a putative way to regulate receptor numbers at the cell surface

Abstract Increasing evidence suggests that the folding and maturation of monomeric proteins and assembly of multimeric protein complexes in the endoplasmic reticulum (ER) may be inefficient not only for mutants that carry changes in the primary structure but also for wild type proteins. In the present study, we demonstrate that the rat luteinizing hormone receptor, a G protein-coupled receptor, is one of these proteins that matures inefficiently and appears to be very prone to premature degradation. A substantial portion of the receptors in stably transfected human embryonic kidney 293 cells existed in immature form of Mr 73,000, containing high mannose-type N-linked glycans. In metabolic pulse-chase studies, only ∼20% of these receptor precursors were found to gain hormone binding ability and matured to a form of Mr 90,000, containing bi- and multiantennary sialylated N-linked glycans. The rest had a propensity to form disulfide-bonded complexes with a Mr 120,000 protein in the ER membrane and were eventually targeted for degradation in proteasomes. The number of membrane-bound receptor precursors increased when proteasomal degradation was inhibited, and no cytosolic receptor forms were detected, suggesting that retrotranslocation of the misfolded/incompletely folded receptors is tightly coupled to proteasomal function. Furthermore, a proteasomal blockade was found to increase the number of receptors that were capable of hormone binding. Thus, these results raise the interesting possibility that luteinizing hormone receptor expression at the cell surface may be controlled at the ER level by regulating the number of newly synthesized proteins that will mature and escape the ER quality control and premature degradation

Peroxiredoxin 6 serum levels and risk of neutropenic infections in diffuse large B-cell lymphoma

Abstract Background/Aim: Diffuse large B‐cell lymphoma (DLBCL) is an aggressive malignancy where antioxidant enzyme peroxiredoxin 6 (Prx6) has previously been associated with adverse outcomes. Its systemic effects in DLBCL are unknown. Materials and methods: This study included 53 patients with DLBCL, five patients with primary central nervous system lymphoma (PCNSL) and 20 healthy controls. The expression of Prx6 was evaluated immunohistochemically in DLBCL tissue samples and compared to its expression in blood serum. Results: Prx6 expression was the highest in healthy controls, followed by DLBCL patients and PCNSL patients. Febrile neutropenic infection after the first treatment course was associated with low pre-treatment Prx6 serum levels (<14 ng/ml) (p=0.025, OR=8.615, 95% confidence interval=1.032‐71.933). Serum levels of Prx6 recovered after treatment (p=0.006). Conclusion: Patients with low Prx6 levels might be more prone to treatment‐related adverse effects through elevated levels of oxidative stress

Thioredoxin‐1 as a biological predictive marker for selecting diffuse large B‐cell lymphoma patients for etoposide‐containing treatment

Abstract Objective: In diffuse large B‐cell lymphoma (DLBCL), there is an unmet medical need to select patients who would benefit from intensified frontline treatments such as adding etoposide to an R‐CHOP regimen. Methods: The present work included a retrospective clinical analysis of two patient cohorts and an in vitro study. Primary biopsy samples from DLBCL patients treated with an etoposide‐containing high‐dose regimen (n = 37) and etoposide‐containing frontline treatment (n = 69, R‐CHOEP) were studied using immunohistochemical thioredoxin‐1 (Trx1) staining. Two DLBCL cell lines expressing Trx1 were cultured, and their expression was silenced using the small interfering RNA knockdown technique. Chemoresistance was tested with doxorubicin, etoposide, vincristine, prednisolone and carboplatin. Results: Thioredoxin‐1 knockdown sensitised DLBCL cells to doxorubicin (P < 0.0001) but decreased etoposide‐induced cell death (P < 0.00001). In DLBCL patients who received etoposide‐containing frontline treatment, low cytoplasmic Trx1 expression was associated with inferior 5‐year overall survival (46% vs 76%, P = 0.026) and disease‐specific survival (68% vs 90%, P = 0.026). Conclusions: Strong Trx1 expression appears to increase drug resistance to doxorubicin but sensitises cells to etoposide. This implies that Trx1 expression might be the first predictive biological marker to select the patients who might benefit from adding etoposide to R‐CHOP immunochemotherapy