Tyrosine kinase inhibition affects type 1 angiotensin II receptor internalization

Effects of Genistein on Common Kidney Diseases

Genistein is a naturally occurring phytoestrogen (soy or soybean products) that is classified as an isoflavone, and its structure is similar to that of endogenous estrogens; therefore, genistein can exert an estrogen-like effect via estrogen receptors. Additionally, genistein is a tyrosine kinase inhibitor, which enables it to block abnormal cell growth and proliferation signals through the inhibition of tyrosine kinase. Genistein is also an angiogenesis inhibitor and an antioxidant. Genistein has effects on kidney cells, some of the kidney’s physiological functions, and a variety of kidney diseases. First, genistein exerts a protective effect on normal cells by reducing the inflammatory response, inhibiting apoptosis, inhibiting oxidative stress, inhibiting remodeling, etc., but after cell injury, the protective effect of genistein decreases or even has the opposite effect. Second, genistein can regulate renin intake to maintain blood pressure balance, regulate calcium uptake to regulate Ca²⁺ and Pi balances, and reduce vasodilation to promote diuresis. Third, genistein has beneficial effects on a variety of kidney diseases (including acute kidney disease, kidney cancer, and different chronic kidney diseases), such as reducing symptoms, delaying disease progression, and improving prognosis. Therefore, this paper reviews animal and human studies on the protective effects of genistein on the kidney in vivo and in vitro to provide a reference for clinical research in the future.

[Angiotensin-2 type 1 receptors (AT1R) and cancers]

Recently, several meta-analysis suggested an increased risk of cancers linked to the use of antagonists of angiotensin-2 receptors or inhibitors of angiotensinogen converting enzyme. The results of epidemiological studies are conflicting. Meta-analysis as well as retrospective studies are not reliable and biased, since they have never been designed to explore any pro- or antitumoral effect. We lack of prospective studies that could take off the doubt on these drugs. Nevertheless, all experimental researches pointed out potent antitumoral properties. Indeed, direct antiproliferative and neo-angiogenic inhibition have been described on tumor cell cultures as well as on animal models. Moreover, we are convinced that the use of antagonists of angiotensin-2 receptors and inhibitors of angiotensinogen converting enzyme may be then of clinical use in the near future in association with classical antitumor drugs. In this review, we proposed to explore these data by a thorough analysis of recent literature associating epidemiological and experimental studies.

The Type 1 Angiotensin II Receptor Tail Affects Receptor Targeting, Internalization, and Membrane Fusion Properties

Endocytosis modulates cell responses by removing and recycling receptors from the cell surface. Type I angiotensin II receptors (AT1R) are somewhat unique in that they are expressed at apical (AP) and basolateral (BL) membranes in proximal tubule cells and both receptor sites undergo endocytosis. We analyzed AT1R cytoplasmic (-COOH) tail deletion mutants to determine whether classic AT1R endocytosis motifs functioned similarly in polarized cells and simultaneously altered receptor properties. Serially truncating the AT1R tail had little effect on AP/BL AT1R distribution as determined by 125I-angiotensin II binding in LLCPK(Cl4) cells transfected with an AT1R transcript. AP AT1R expression required the proximal 12 amino acids in the AT1R-COOH tail. Deleting all but the proximal 12 aa of the AT1R-COOH tail (T316L mutant) decreased AP AT1R internalization at 20 min (17 +/- 6%; p < 0.05 versus full-length; n = 5) and inhibited AP AT1R-stimulated arachidonic acid release (counts released per milligram of protein at 20 min: full-length, 18,762 +/- 4018; T316L, 2430 +/- 1711; n = 4; p < 0.02). Endosomal fusion assays were performed using peptide sequences of regions in the AT1R tail involved in endocytosis (YFLQLLKYIPP [LL] and LSTKMSTLSY [STL]). Peptide STL significantly inhibited endosomal fusion (22 +/- 10% of control; n = 5; p < 0.05 versus positive control). Peptide LL had no significant inhibitory effect. AT(1)R in polarized cells contain dominant endocytosis signals but these motifs do not correlate with AP or BL AT1R expression. Moreover, peptide sequences within the AT1R-COOH tail necessary for endocytosis also modulate endosomal fusion properties.

Recycling ability of the mouse and the human neurotensin type 2 receptors depends on a single tyrosine residue

Receptor recycling plays a key role in the modulation of cellular responses to extracellular signals. The purpose of this work was to identify residues in G-protein coupled neurotensin receptors that are directly involved in recycling. Both the high affinity receptor-1 (NTR1) and the levocabastine-sensitive NTR2 are internalized after neurotensin binding. Here, we show that only the mouse NTR2 recycled to the plasma membrane, whereas the rat NTR1 and the human NTR2 did not. Using site-directed mutagenesis, we demonstrate that tyrosine 237 in the third intracellular loop is crucial for recycling of the mouse NTR2. We show that the mouse NTR2 is phosphorylated on tyrosine residues by NT. This phosphorylation is essential for receptor recycling since the tyrosine kinase inhibitor genistein blocks this process. The absence of recycling observed with the human NTR2 could be completely explained by the presence of a cysteine instead of a tyrosine in position 237. Indeed, substitution of this cysteine by a tyrosine gave a mutant receptor that has acquired the ability to recycle to the cell surface after neurotensin-induced internalization. This work demonstrates that a single tyrosine residue in the third intracellular loop of a G-protein-coupled receptor is responsible for receptor phosphorylation and represents an essential structural element for receptor recycling.

Angiotensin IV induces tyrosine phosphorylation of focal adhesion kinase and paxillin in proximal tubule cells

Angiotensin IV (ANG IV), the COOH-terminal hexapeptide fragment of angiotensin II (ANG II), binds to specific sites in the kidney, distinct from type 1 (AT(1)) and type 2 (AT(2)) receptors and designated type 4 (AT(4)) receptors. We determined signaling pathways for ANG IV in a proximal tubular cell line, LLC-PK(1)/Cl(4). In these cells, we found no specific binding of [(125)I]-ANG II. In contrast, ANG IV dose dependently competed for [(125)I]-labeled ANG IV binding, with no displacement by either ANG II, the AT(1) receptor antagonist losartan, or the AT(2) antagonist PD-123319. Saturation binding indicated the presence of AT(4) receptors of high affinity [dissociation constant (K(d)) = 1.4 nM]. ANG IV did not affect cAMP or cGMP production and did not increase cytosolic calcium concentration in these cells. In contrast, immunoprecipitation and immunoblotting studies revealed that ANG IV caused dose-dependent tyrosine phosphorylation of p125-focal adhesion kinase (p125-FAK) and p68-paxillin within 2 min, with maximal stimulation at 30 min. ANG IV-stimulated tyrosine phosphorylation of p125-FAK and paxillin was not affected by pretreatment with either losartan or PD-123319, and ANG II (10(-7) M) did not induce protein tyrosine phosphorylation. Our results indicate that LLC-PK(1)/Cl(4) cells express ANG IV receptors, which we demonstrate for the first time are linked to tyrosine phosphorylation of focal adhesion-associated proteins. This suggests that ANG IV, a product of ANG II metabolism, may regulate function of the focal adhesion complex in proximal tubule cells.

The plant isoflavenoid genistein activates p53 and Chk2 in an ATM-dependent manner

Genistein is an isoflavenoid that is abundant in soy beans. Genistein has been reported to have a wide range of biological activities and to play a role in the diminished incidence of breast cancer in populations that consume a soy-rich diet. Genistein was originally identified as an inhibitor of tyrosine kinases; however, it also inhibits topoisomerase II by stabilizing the covalent DNA cleavage complex, an event predicted to cause DNA damage. The topoisomerase II inhibitor etoposide acts in a similar manner. Here we show that genistein induces the up-regulation of p53 protein, phosphorylation of p53 at serine 15, activation of the sequence-specific DNA binding properties of p53, and phosphorylation of the hCds1/Chk2 protein kinase at threonine 68. Phosphorylation and activation of p53 and phosphorylation of Chk2 were not observed in ATM-deficient cells. In contrast, the topoisomerase II inhibitor etoposide induced phosphorylation of p53 and Chk2 in ATM-positive and ATM-deficient cells. In addition, genistein-treated ATM-deficient cells were significantly more susceptible to genistein-induced killing than were ATM-positive cells. Together our data suggest that ATM is required for activation of a DNA damage-induced pathway that activates p53 and Chk2 in response to genistein.

Mechanisms and functions of AT(1) angiotensin receptor internalization

The type 1 (AT(1)) angiotensin receptor, which mediates the known physiological and pharmacological actions of angiotensin II, activates numerous intracellular signaling pathways and undergoes rapid internalization upon agonist binding. Morphological and biochemical studies have shown that agonist-induced endocytosis of the AT(1) receptor occurs via clathrin-coated pits, and is dependent on two regions in the cytoplasmic tail of the receptor. However, it is independent of G protein activation and signaling, and does not require the conserved NPXXY motif in the seventh transmembrane helix. The dependence of internalization of the AT(1) receptor on a cytoplasmic serine-threonine-rich region that is phosphorylated during agonist stimulation suggests that endocytosis is regulated by phosphorylation of the AT(1) receptor tail. beta-Arrestins have been implicated in the desensitization and endocytosis of several G protein-coupled receptors, but the exact nature of the adaptor protein required for association of the AT(1) receptor with clathrin-coated pits, and the role of dynamin in the internalization process, are still controversial. There is increasing evidence for a role of internalization in sustained signal generation from the AT(1) receptor. Several aspects of the mechanisms and specific function of AT(1) receptor internalization, including its precise mode and route of endocytosis, and the potential roles of cytoplasmic and nuclear receptors, remain to be elucidated.