Adrenoceptor Desensitization: Current Understanding of Mechanisms.

G-protein coupled receptors (GPCRs) transduce a wide range of extracellular signals. They are key players in the majority of biologic functions including vision, olfaction, chemotaxis, and immunity. However, as essential as most of them are to body function and homeostasis, overactivation of GPCRs has been implicated in many pathologic diseases such as cancer, asthma, and heart failure (HF). Therefore, an important feature of G protein signaling systems is the ability to control GPCR responsiveness, and one key process to control overstimulation involves initiating receptor desensitization. A number of steps are appreciated in the desensitization process, including cell surface receptor phosphorylation, internalization, and downregulation. Rapid or short-term desensitization occurs within minutes and involves receptor phosphorylation via the action of intracellular protein kinases, the binding of ß-arrestins, and the consequent uncoupling of GPCRs from their cognate heterotrimeric G proteins. On the other hand, long-term desensitization occurs over hours to days and involves receptor downregulation or a decrease in cell surface receptor protein level. Of the proteins involved in this biologic phenomenon, ß-arrestins play a particularly significant role in both short- and long-term desensitization mechanisms. In addition, ß-arrestins are involved in the phenomenon of biased agonism, where the biased ligand preferentially activates one of several downstream signaling pathways, leading to altered cellular responses. In this context, this review discusses the different patterns of desensitization of the α 1-, α 2- and the ß adrenoceptors and highlights the role of ß-arrestins in regulating physiologic responsiveness through desensitization and biased agonism. SIGNIFICANCE STATEMENT: A sophisticated network of proteins orchestrates the molecular regulation of GPCR activity. Adrenoceptors are GPCRs that play vast roles in many physiological processes. Without tightly controlled desensitization of these receptors, homeostatic imbalance may ensue, thus precipitating various diseases. Here, we critically appraise the mechanisms implicated in adrenoceptor desensitization. A better understanding of these mechanisms helps identify new druggable targets within the GPCR desensitization machinery and opens exciting therapeutic fronts in the treatment of several pathologies.

Overcoming PD-1 Inhibitor Resistance with a Monoclonal Antibody to Secreted Frizzled-Related Protein 2 in Metastatic Osteosarcoma.

Secreted frizzled-related protein 2 (SFRP2) promotes the migration/invasion of metastatic osteosarcoma (OS) cells and tube formation by endothelial cells. However, its function on T-cells is unknown. We hypothesized that blocking SFRP2 with a humanized monoclonal antibody (hSFRP2 mAb) can restore immunity by reducing CD38 and PD-1 levels, ultimately overcoming resistance to PD-1 inhibitors. Treating two metastatic murine OS cell lines in vivo, RF420 and RF577, with hSFRP2 mAb alone led to a significant reduction in the number of lung metastases, compared to IgG1 control treatment. While PD-1 mAb alone had minimal effect, hSFRP2 mAb combination with PD-1 mAb had an additive antimetastatic effect. This effect was accompanied by lower SFRP2 levels in serum, lower CD38 levels in tumor-infiltrating lymphocytes and T-cells, and lower PD-1 levels in T-cells. In vitro data confirmed that SFRP2 promotes NFATc3, CD38 and PD-1 expression in T-cells, while hSFRP2 mAb treatment counteracts these effects and increases NAD+ levels. hSFRP2 mAb treatment further rescued the suppression of T-cell proliferation by tumor cells in a co-culture model. Finally, hSFRP2 mAb induced apoptosis in RF420 and RF577 OS cells but not in T-cells. Thus, hSFRP2 mAb therapy could potentially overcome PD-1 inhibitor resistance in metastatic osteosarcoma.

Vascular Cells Proteome Associated with Bradykinin and Leptin Inflammation and Oxidative Stress Signals.

Among the primary contributors to cardiovascular diseases are inflammation and oxidative imbalance within the vessel walls as well as the fibrosis of rat aortic smooth muscle cell (RASMC). Bradykinin (BK) and leptin are inflammatory modulators that are linked to vascular injury. In this study, we employed tandem LC-MS/MS to identify protein signatures that encompass protein abundance in RASMC treated with BK or leptin followed by systems biology analyses to gain insight into the biological pathways and processes linked to vascular remodeling. In the study, 1837 proteins were identified in control untreated RASMC. BK altered the expression of 72 (4%) and 120 (6.5%) proteins, whereas leptin altered the expression of 189 (10.2%) and 127 (6.5%) proteins after 24 and 48 h, respectively, compared to control RASMC. BK increased the protein abundance of leptin receptor, transforming growth factor-ß. On the other hand, leptin increased the protein abundance of plasminogen activator inhibitor 1 but decreased the protein abundance of cofilin. BK and leptin induced the expression of inflammatory cytokines such as tumor necrosis factor alpha (TNF-α) and interleukin-1ß (IL-1ß) and pathway analysis revealed the activation of mitogen-activated protein kinases (MAPKs) and AKT pathways. The proteome profile in response to BK and leptin revealed mechanistic interplay of multiple processes that modulate inflammation and oxidative stress signals in the vasculature.