Structure and expression of the murine thromboxane A2 receptor gene

The Role and Regulation of Thromboxane A2 Signaling in Cancer-Trojan Horses and Misdirection

Over the last two decades, there has been an increasing awareness of the role of eicosanoids in the development and progression of several types of cancer, including breast, prostate, lung, and colorectal cancers. Several processes involved in cancer development, such as cell growth, migration, and angiogenesis, are regulated by the arachidonic acid derivative thromboxane A₂ (TXA₂). Higher levels of circulating TXA₂ are observed in patients with multiple cancers, and this is accompanied by overexpression of TXA₂ synthase (TBXAS1, TXA₂S) and/or TXA₂ receptors (TBXA2R, TP). Overexpression of TXA₂S or TP in tumor cells is generally associated with poor prognosis, reduced survival, and metastatic disease. However, the role of TXA₂ signaling in the stroma during oncogenesis has been underappreciated. TXA₂ signaling regulates the tumor microenvironment by modulating angiogenic potential, tumor ECM stiffness, and host immune response. Moreover, the by-products of TXA₂S are highly mutagenic and oncogenic, adding to the overall phenotype where TXA₂ synthesis promotes tumor formation at various levels. The stability of synthetic enzymes and receptors in this pathway in most cancers (with few mutations reported) suggests that TXA₂ signaling is a viable target for adjunct therapy in various tumors to reduce immune evasion, primary tumor growth, and metastasis.

Thromboxane A2: physiology/pathophysiology, cellular signal transduction and pharmacology

Thromboxane A(2) (TXA(2)), an unstable arachidonic acid metabolite, elicits diverse physiological/pathophysiological actions, including platelet aggregation and smooth muscle contraction. TXA(2) has been shown to be involved in allergies, modulation of acquired immunity, atherogenesis, neovascularization, and metastasis of cancer cells. The TXA(2) receptor (TP) communicates mainly with G(q) and G(13), resulting in phospholipase C activation and RhoGEF activation, respectively. In addition, TP couples with G(11), G(12), G(13), G(14), G(15), G(16), G(i), G(s) and G(h). TP is widely distributed in the body, and is expressed at high levels in thymus and spleen. The second extracellular loop of TP is an important ligand-binding site, and Asp(193) is a key amino acid. There are two alternatively spliced isoforms of TP, TPalpha and TPbeta, which differ only in their C-terminals. TPalpha and TPbeta communicate with different G proteins, and undergo hetero-dimerization, resulting in changes in intracellular traffic and receptor protein conformations. TP cross-talks with receptor tyrosine kinases, such as EGF receptor, to induce cell proliferation and differentiation. TP is glycosylated in the N-terminal region for recruitment to plasma membranes. Furthermore, TP conformation is changed by coupling to G proteins, showing several states of agonist binding. Finally, several drugs modify TP-mediated events; these include cyclooxygenase inhibitors, TXA(2) synthase inhibitors and TP antagonists. Some flavonoids of natural origin also have TP receptor antagonistic activity. Recent advances in TP research have clarified TXA(2)-mediated events in detail, and further study will supply more beneficial information about TXA(2) pathophysiology.

The stress-associated acetylcholinesterase variant AChE-R is expressed in human CD34(+) hematopoietic progenitors and its C-terminal peptide ARP promotes their proliferation

OBJECTIVE

Hematopoietic stress responses involve increases in leukocyte and platelet counts, implying the existence of stress responsive factors that modulate hematopoiesis. Acetylcholinesterase (AChE) is expressed in mammalian neurons and hematopoietic cells. In brain, it responds to stress by mRNA overexpression and alternative splicing, yielding the rare stress-associated "readthrough" AChE-R variant protein. This led us to explore the hematopoietic involvement of AChE-R and its cleavable C-terminal peptide ARP.

MATERIALS AND METHODS

AChE mRNA variants were labeled in CD34(+) hematopoietic progenitor cells by in situ hybridization. ARP expression was detected by multicolor flow cytometry. Bromo-deoxyuracil incorporation and viable cell counts served to evaluate the proliferative effects of ARP and suppressive effects of the AChE antisense oligonucleotide AS1 on CD34(+) cells.

RESULTS

The distal enhancer, proximal promoter, and first intron of the human AChE gene include consensus binding sites for hematopoietically active and stress-induced transcription factors. CD34(+) cells from human cord blood were found to express all three variant AChE mRNAs, having different intracellular distributions. ARP was found in 5 to 15% of adult peripheral blood, bone marrow, and fetal CD34(+) cells (both committed CD38(+) and uncommitted CD38(-)) and in acute myeloid leukemia blasts. Externally supplied ARP by itself facilitated the proliferation of CD34(+) cells in an antisense suppressible manner. When combined with early-acting cytokines, ARP enhanced survival and expansion of CD34(+) cells up to 28 days in culture.

CONCLUSIONS

Our findings support ARP, the C-terminal peptide of AChE-R, as a new hematopoietic growth factor that may promote the myelopoietic expansion and thrombopoiesis characteristic of stress and may be used to enhance the efficiency of ex vivo expansion for bone marrow transplantation.

Cyclooxygenases and prostaglandins: shaping up the immune response

The relevance of cyclooxygenases (COX)-1 and -2 and their products to inflammation, thrombosis and gastroprotection are well known. Their importance in the immune response was first recognized more than 25 years ago, but has only gained widespread attention recently. In this review, we attempt to integrate information on prostanoids and both the innate and acquired immune responses, including effects on leukocytes, antigen presenting cells, dendritic cells, T and B lymphocytes. Prostanoids may be relevant to immunotolerance, autoimmune disorders, transplantation, immunologic defense against tumors, acquired immunodeficiencies and viral infections. Insight into the role of prostanoids in immune function may afford novel therapeutic opportunities.

G protein-coupled prostanoid receptors and the kidney

Renal cyclooxygenase 1 and 2 activity produces five primary prostanoids: prostaglandin E2, prostaglandin F2alpha, prostaglandin I2, thromboxane A2, and prostaglandin D2. These lipid mediators interact with a family of distinct G protein-coupled prostanoid receptors designated EP, FP, IP, TP, and DP, respectively, which exert important regulatory effects on renal function. The intrarenal distribution of these prostanoid receptors has been mapped, and the consequences of their activation have been partially characterized. FP, TP, and EP1 receptors preferentially couple to an increase in cell calcium. EP2, EP4, DP, and IP receptors stimulate cyclic AMP, whereas the EP3 receptor preferentially couples to Gi, inhibiting cyclic AMP generation. EP1 and EP3 mRNA expression predominates in the collecting duct and thick limb, respectively, where their stimulation reduces NaCl and water absorption, promoting natriuresis and diuresis. The FP receptor is highly expressed in the distal convoluted tubule, where it may have a distinct effect on renal salt transport. Although only low levels of EP2 receptor mRNA are detected in the kidney and its precise intrarenal localization is uncertain, mice with targeted disruption of the EP2 receptor exhibit salt-sensitive hypertension, suggesting that this receptor may also play an important role in salt excretion. In contrast, EP4 receptor mRNA is predominantly expressed in the glomerulus, where it may contribute to the regulation of glomerular hemodynamics and renin release. The IP receptor mRNA is highly expressed near the glomerulus, in the afferent arteriole, where it may also dilate renal arterioles and stimulate renin release. Conversely, TP receptors in the glomerulus may counteract the effects of these dilator prostanoids and increase glomerular resistance. At present there is little evidence for DP receptor expression in the kidney. These receptors act in a concerted fashion as physiological buffers, protecting the kidney from excessive functional changes during periods of physiological stress. Nonsteroidal anti-inflammatory drug (NSAID)-mediated cyclooxygenase inhibition results in the loss of these combined effects, which contributes to their renal effects. Selective prostanoid receptor antagonists may provide new therapeutic approaches for specific disease states.

Distribution and function of prostanoid receptors: studies from knockout mice

Recent developments in the molecular biology of the prostanoid receptors has allowed the investigation of the physiological roles of each individual receptor type and subtype. The following article reports the prostanoid receptor distributions deduced from Northern blot and in situ hybridization analyses, summarizes the phenotypes of each receptor knockout mice, and discusses recent studies investigating the effects of each receptor deficiency on the inflammatory response and female reproductive processes. The combination of expression pattern and knockout analyses enabled us to determine which receptor expressed in a particular cell is important for the maintenance of normal and/or pathological physiology. The results from these analyses may be useful in the development of novel therapeutics that can selectively manipulate prostanoid-mediated actions.

Prostaglandin receptors: their role in regulating renal function

Renal cyclooxygenase-1 and cyclooxygenase-2 actively metabolize arachidonate to metabolism five primary prostanoids: prostaglandin E2, prostaglandin F2a, prostaglandin I2, thromboxane A2, and prostaglandin D2. These lipid mediators interact with a family of distinct G-protein-coupled prostanoid receptors designated EP, FP, IP, TP, and DP, respectively, which exert important regulatory effects on renal function. The intrarenal distribution of these prostanoid receptors has been mapped and the consequences their activation are being characterized. The FP, TP, and EP1 receptors preferentially couple to increased cell Ca2+. EP2, EP4, DP, and IP receptors stimulate cyclic adenosine monophosphate, whereas the EP3 receptor preferentially couples to Gi, inhibiting cyclic adenosine monophosphate generation. EP1 and EP3 messenger RNA expression predominate in the collecting duct and thick limb, respectively, where their stimulation reduces sodium chloride and water absorption, promoting natriuresis and diuresis. Interestingly, only a mild change in renal water handling is seen in the EP3 receptor knockout mouse. Although only low levels EP2 receptor messenger RNA are detected in kidney and its precise intrarenal localization is uncertain, mice with targeted disruption of the EP2 receptor display salt-sensitive hypertension, suggesting it also plays an important role in salt excretion. In contrast, EP4 messenger RNA is readily detected in the glomerulus where it may contribute to the regulation of renin release and decrease glomerular resistance. TP receptors are also highly expressed in the glomerulus, where they may increase glomerular vascular resistance. The IP receptor messenger RNA is most highly expressed in the afferent arteriole and it may also modulate renal arterial resistance and renin release. At present there is little evidence for DP receptor expression in the kidney. Together these receptors act as physiologic buffers that protect the kidney from excessive functional changes during periods of physiologic stress. Loss of the combined effects of these receptors contributes to the side effects seen in the setting of nonsteroidal anti-inflammatory drug administration, whereas selective antagonists for these receptors may provide new therapeutic approaches in disease.