α(1A)-adrenergic receptor differentially regulates STAT3 phosphorylation through PKCϵ and PKCδ in myocytes

STAT3 pathway in cancers: Past, present, and future

Signal transducer and activator of transcription 3 (STAT3), a member of the STAT family, discovered in the cytoplasm of almost all types of mammalian cells, plays a significant role in biological functions. The duration of STAT3 activation in normal tissues is a transient event and is strictly regulated. However, in cancer tissues, STAT3 is activated in an aberrant manner and is induced by certain cytokines. The continuous activation of STAT3 regulates the expression of downstream proteins associated with the formation, progression, and metastasis of cancers. Thus, elucidating the mechanisms of STAT3 regulation and designing inhibitors targeting the STAT3 pathway are considered promising strategies for cancer treatment. This review aims to introduce the history, research advances, and prospects concerning the STAT3 pathway in cancer. We review the mechanisms of STAT3 pathway regulation and the consequent cancer hallmarks associated with tumor biology that are induced by the STAT3 pathway. Moreover, we summarize the emerging development of inhibitors that target the STAT3 pathway and novel drug delivery systems for delivering these inhibitors. The barriers against targeting the STAT3 pathway, the focus of future research on promising targets in the STAT3 pathway, and our perspective on the overall utility of STAT3 pathway inhibitors in cancer treatment are also discussed.

Pivotal Importance of STAT3 in Protecting the Heart from Acute and Chronic Stress: New Advancement and Unresolved Issues

The transcription factor, signal transducer and activator of transcription 3 (STAT3), has been implicated in protecting the heart from acute ischemic injury under both basal conditions and as a crucial component of pre- and post-conditioning protocols. A number of anti-oxidant and antiapoptotic genes are upregulated by STAT3 via canonical means involving phosphorylation on Y705 and S727, although other incompletely defined posttranslational modifications are involved. In addition, STAT3 is now known to be present in cardiac mitochondria and to exert actions that regulate the electron transport chain, reactive oxygen species production, and mitochondrial permeability transition pore opening. These non-canonical actions of STAT3 are enhanced by S727 phosphorylation. The molecular basis for the mitochondrial actions of STAT3 is poorly understood, but STAT3 is known to interact with a critical subunit of complex I and to regulate complex I function. Dysfunctional complex I has been implicated in ischemic injury, heart failure, and the aging process. Evidence also indicates that STAT3 is protective to the heart under chronic stress conditions, including hypertension, pregnancy, and advanced age. Paradoxically, the accumulation of unphosphorylated STAT3 (U-STAT3) in the nucleus has been suggested to drive pathological cardiac hypertrophy and inflammation via non-canonical gene expression, perhaps involving a distinct acetylation profile. U-STAT3 may also regulate chromatin stability. Our understanding of how the non-canonical genomic and mitochondrial actions of STAT3 in the heart are regulated and coordinated with the canonical actions of STAT3 is rudimentary. Here, we present an overview of what is currently known about the pleotropic actions of STAT3 in the heart in order to highlight controversies and unresolved issues.

α1A-Adrenergic receptor prevents cardiac ischemic damage through PKCδ/GLUT1/4-mediated glucose uptake

While α(1)-adrenergic receptors (ARs) have been previously shown to limit ischemic cardiac damage, the mechanisms remain unclear. Most previous studies utilized low oxygen conditions in addition to ischemic buffers with glucose deficiencies, but we discovered profound differences if the two conditions are separated. We assessed both mouse neonatal and adult myocytes and HL-1 cells in a series of assays assessing ischemic damage under hypoxic or low glucose conditions. We found that α(1)-AR stimulation protected against increased lactate dehydrogenase release or Annexin V(+) apoptosis under conditions that were due to low glucose concentration not to hypoxia. The α(1)-AR antagonist prazosin or nonselective protein kinase C (PKC) inhibitors blocked the protective effect. α(1)-AR stimulation increased (3)H-deoxyglucose uptake that was blocked with either an inhibitor to glucose transporter 1 or 4 (GLUT1 or GLUT4) or small interfering RNA (siRNA) against PKCδ. GLUT1/4 inhibition also blocked α(1)-AR-mediated protection from apoptosis. The PKC inhibitor rottlerin or siRNA against PKCδ blocked α(1)-AR stimulated GLUT1 or GLUT4 plasma membrane translocation. α(1)-AR stimulation increased plasma membrane concentration of either GLUT1 or GLUT4 in a time-dependent fashion. Transgenic mice overexpressing the α(1A)-AR but not α(1B)-AR mice displayed increased glucose uptake and increased GLUT1 and GLUT4 plasma membrane translocation in the adult heart while α(1A)-AR but not α(1B)-AR knockout mice displayed lowered glucose uptake and GLUT translocation. Our results suggest that α(1)-AR activation is anti-apoptotic and protective during cardiac ischemia due to glucose deprivation and not hypoxia by enhancing glucose uptake into the heart via PKCδ-mediated GLUT translocation that may be specific to the α(1A)-AR subtype.

α₁A-adrenergic receptors regulate cardiac hypertrophy in vivo through interleukin-6 secretion

The role of α₁-adrenergic receptors (ARs) in the regulation of cardiac hypertrophy is still unclear, because transgenic mice demonstrated hypertrophy or the lack of it despite high receptor overexpression. To further address the role of the α₁-ARs in cardiac hypertrophy, we analyzed unique transgenic mice that overexpress constitutively active mutation (CAM) α₁A-ARs or CAM α₁B-ARs under the regulation of large fragments of their native promoters. These constitutively active receptors are expressed in all tissues that endogenously express their wild-type counterparts as opposed to only myocyte-targeted transgenic mice. In this study, we discovered that CAM α₁A-AR mice in vivo have cardiac hypertrophy independent of changes in blood pressure, corroborating earlier studies, but in contrast to myocyte-targeted α₁A-AR mice. We also found cardiac hypertrophy in CAM α₁B-AR mice, in agreement with previous studies, but hypertrophy only developed in older mice. We also discovered unique α₁-AR-mediated hypertrophic signaling that was AR subtype-specific with CAM α₁A-AR mice secreting atrial naturietic factor and interleukin-6 (IL-6), whereas CAM α₁B-AR mice expressed activated nuclear factor-κB (NF-κB). These particular hypertrophic signals were blocked when the other AR subtype was coactivated. We also discovered that crossbreeding the two CAM models (double CAM α₁A/B-AR) inhibited the development of hypertrophy and was reversible with single receptor activation, suggesting that coactivation of the receptors can lead to novel antagonistic signal transduction. This was confirmed by demonstrating antagonistic signals that were even lower than normal controls in the double CAM α₁A/B-AR mice for p38, NF-κB, and the IL-6/glycoprotein 130/signal transducer and activator of transcription 3 pathway. Because α₁A/B double knockout mice fail to develop hypertrophy in response to IL-6, our results suggest that IL-6 is a major mediator of α₁A-AR cardiac hypertrophy.

Novel proteins associated with human dilated cardiomyopathy: selective reduction in α(1A)-adrenergic receptors and increased desensitization proteins

Abstract Therapeutics to treat human heart failure (HF) and the identification of proteins associated with HF are still limited. We analyzed α(1)-adrenergic receptor (AR) subtypes in human HF and performed proteomic analysis on more uniform samples to identify novel proteins associated with human HF. Six failing hearts with end-stage dilated cardiomyopathy (DCM) and four non-failing heart controls were subjected to proteomic analysis. Out of 48 identified proteins, 26 proteins were redundant between samples. Ten of these 26 proteins were previously reported to be associated with HF. Of the newly identified proteins, we found several muscle proteins and mitochondrial/electron transport proteins, while novel were functionally similar to previous reports. However, we also found novel proteins involved in functional classes such as β-oxidation and G-protein coupled receptor signaling and desensitization not previously associated with HF. We also performed radioligand-binding studies on the heart samples and not only confirmed a large loss of β(1)-ARs in end-stage DCM, but also found a selective decrease in the α(1A)-AR subtype not previously reported. We have identified new proteins and functional categories associated with end-stage DCM. We also report that similar to the previously characterized loss of β(1)-AR in HF, there is also a concomitant loss of α(1A)-ARs, which are considered cardioprotective proteins.

ATP stimulates PGE(2)/cyclin D1-dependent VSMCs proliferation via STAT3 activation: role of PKCs-dependent NADPH oxidase/ROS generation

Vascular smooth muscle cells (VSMCs) that function as synthetic units play important roles in cardiovascular diseases. Extracellular nucleotides, such as ATP, have been shown to act via activation of P2 purinoceptors implicated in various inflammatory diseases, we hypothesized that extracellular nucleotides contribute to vascular diseases via up-regulation of inflammatory proteins, including cyclooxygenase-2 (COX-2) and cytosolic phospholipase A2 (cPLA2) in VSMCs. However, the mechanisms of ATP-induced cPLA2 and COX-2 expression and PGE2 synthesis remain largely unclear. We showed that pretreatment with the inhibitors of STAT3 (CBE), NADPH oxidase [diphenyleneiodonium chloride (DPI) or apocynin (APO)], ROS [N-acetyl-l-cysteine (NAC)], and PKC (Ro-318220, Gö6983, or Rottlerin) or transfection with siRNAs of STAT3 and p47(phox) markedly inhibited ATPγS-induced cPLA2 and COX-2 mRNA/protein expression and promoter activity and PGE2 secretion. ATPγS further stimulated PKC, p47(phox), and STAT3 translocation. Moreover, ATPγS-induced STAT3 phosphorylation and translocation was inhibited by pretreatment with the inhibitors of PKC, NADPH oxidase, and ROS. ATPγS enhanced NADPH oxidase activity and ROS generation in VSMCs, which were reduced by pretreatment with Ro-318220, Gö6983, or Rottlerin. Finally, we found that ATPγS significantly induced cyclin D1 expression and VSMCs proliferation, which were inhibited by pretreatment with NAC, APO, DPI, Ro-318220, Gö6983, Rottlerin, or CBE or transfection with siRNAs of COX-2 and cyclin D1. We also demonstrated that ATPγS induced cyclin D1 expression via a PGE2-dependent pathway. These results suggested that ATPγS-induced cPLA2/COX-2 expression and PGE2 secretion is mediated through a PKC/NADPH oxidase/ROS/STAT3-dependent pathway in VSMCs.