Biological Relevance of RGD-Integrin Subtype-Specific Ligands in Cancer.

Integrins are heterodimeric transmembrane proteins able to connect cells with the micro-environment. They represent a family of receptors involved in almost all the hallmarks of cancer. Integrins recognizing the Arg-Gly-Asp (RGD) peptide in their natural extracellular matrix ligands have been particularly investigated as tumoral therapeutic targets. In the last 30 years, intense research has been dedicated to designing specific RGD-like ligands able to discriminate selectively the different RGD-recognizing integrins. Chemists' efforts have led to the proposition of modified peptide or peptidomimetic libraries to be used for tumor targeting and/or tumor imaging. Here we review, from the biological point of view, the rationale underlying the need to clearly delineate each RGD-integrin subtype by selective tools. We describe the complex roles of RGD-integrins (mainly the most studied αvß3 and α5ß1 integrins) in tumors, the steps towards selective ligands and the current usefulness of such ligands. Although the impact of integrins in cancer is well acknowledged, the biological characteristics of each integrin subtype in a specific tumor are far from being completely resolved. Selective ligands might help us to reconsider integrins as therapeutic targets in specific clinical settings.

Convergent Signaling Pathways Controlled by LRP1 (Receptor-related Protein 1) Cytoplasmic and Extracellular Domains Limit Cellular Cholesterol Accumulation.

The low density lipoprotein receptor-related protein 1 (LRP1) is a ubiquitously expressed cell surface receptor that protects from intracellular cholesterol accumulation. However, the underlying mechanisms are unknown. Here we show that the extracellular (α) chain of LRP1 mediates TGFß-induced enhancement of Wnt5a, which limits intracellular cholesterol accumulation by inhibiting cholesterol biosynthesis and by promoting cholesterol export. Moreover, we demonstrate that the cytoplasmic (ß) chain of LRP1 suffices to limit cholesterol accumulation in LRP1(-/-) cells. Through binding of Erk2 to the second of its carboxyl-terminal NPXY motifs, LRP1 ß-chain positively regulates the expression of ATP binding cassette transporter A1 (ABCA1) and of neutral cholesterol ester hydrolase (NCEH1). These results highlight the unexpected functions of LRP1 and the canonical Wnt5a pathway and new therapeutic potential in cholesterol-associated disorders including cardiovascular diseases.

The Src homology and collagen A (ShcA) adaptor protein is required for the spatial organization of the costamere/Z-disk network during heart development.

Src homology and collagen A (ShcA) is an adaptor protein that binds to tyrosine kinase receptors. Its germ line deletion is embryonic lethal with abnormal cardiovascular system formation, and its role in cardiovascular development is unknown. To investigate its functional role in cardiovascular development in mice, ShcA was deleted in cardiomyocytes and vascular smooth muscle cells by crossing ShcA flox mice with SM22a-Cre transgenic mice. Conditional mutant mice developed signs of severe dilated cardiomyopathy, myocardial infarctions, and premature death. No evidence of a vascular contribution to the phenotype was observed. Histological analysis of the heart revealed aberrant sarcomeric Z-disk and M-band structures, and misalignments of T-tubules with Z-disks. We find that not only the ErbB3/Neuregulin signaling pathway but also the baroreceptor reflex response, which have been functionally associated, are altered in the mutant mice. We further demonstrate that ShcA interacts with Caveolin-1 and the costameric protein plasma membrane Ca(2+)/calmodulin-dependent ATPase (PMCA), and that its deletion leads to abnormal dystrophin signaling. Collectively, these results demonstrate that ShcA interacts with crucial proteins and pathways that link Z-disk and costamere.

Differential signaling by adaptor molecules LRP1 and ShcA regulates adipogenesis by the insulin-like growth factor-1 receptor.

The low density lipoprotein receptor-related protein (LRP1) is a transmembrane receptor that integrates multiple signaling pathways. Its cytoplasmic domain serves as docking sites for several adaptor proteins such as the Src homology 2/α-collagen (ShcA), which also binds to several tyrosine kinase receptors such as the insulin-like growth factor 1 (IGF-1) receptor. However, the physiological significance of the physical interaction between LRP1 and ShcA, and whether this interaction modifies tyrosine kinase receptor signaling, are still unknown. Here we report that LRP1 forms a complex with the IGF-1 receptor, and that LRP1 is required for ShcA to become sensitive to IGF-1 stimulation. Upon IGF-1 treatment, ShcA is tyrosine phosphorylated and translocates to the plasma membrane only in the presence of LRP1. This leads to the recruitment of the growth factor receptor-bound protein 2 (Grb2) to ShcA, and activation of the Ras/MAP kinase pathway. Conversely, in the absence of ShcA, IGF-1 signaling bifurcates toward the Akt/mammalian target of rapamycin pathway and accelerates adipocyte differentiation when cells are stimulated for adipogenesis. These results establish the LRP1-ShcA complex as an essential component in the IGF-1-regulated pathway for MAP kinase and Akt/mammalian target of rapamycin activation, and may help to understand the IGF-1 signaling shift from clonal expansion to growth-arrested cells and differentiation during adipogenesis.

Methylation of imidazoline related compounds leads to loss of α2-adrenoceptor affinity. Synthesis and biological evaluation of selective I1 imidazoline receptor ligands.

Methylated analogues of imidazoline related compounds (IRC) were prepared; their abilities to bind I(1) imidazoline receptors (I(1)Rs), I(2) imidazoline binding sites (I(2)BS) and α(2)-adrenoceptor subtypes (α(2)ARs) were assessed. Methylation of the heterocyclic moiety of IRC resulted in a significant loss of α(2)AR affinity. Amongst the selective ligands obtained, LNP 630 (4) constitutes the first highly selective I(1)R agent showing hypotensive activity after intravenous administration.

Cav1/EREG/YAP Axis in the Treatment Resistance of Cav1-Expressing Head and Neck Squamous Cell Carcinoma.

The EGFR-targeting antibody cetuximab (CTX) combined with radiotherapy is the only targeted therapy that has been proven effective for the treatment of locally advanced head and neck squamous cell carcinoma (LA-HNSCC). Recurrence arises in 50% of patients with HNSCC in the years following treatment. In clinicopathological practice, it is difficult to assign patients to classes of risk because no reliable biomarkers are available to predict the outcome of HPV-unrelated HNSCC. In the present study, we investigated the role of Caveolin-1 (Cav1) in the sensitivity of HNSCC cell lines to CTX-radiotherapy that might predict HNSCC relapse. Ctrl- and Cav-1-overexpressing HNSCC cell lines were exposed to solvent, CTX, or irradiation, or exposed to CTX before irradiation. Growth, clonogenicity, cell cycle progression, apoptosis, metabolism and signaling pathways were analyzed. Cav1 expression was analyzed in 173 tumor samples and correlated to locoregional recurrence and overall survival. We showed that Cav1-overexpressing cells demonstrate better survival capacities and remain proliferative and motile when exposed to CTX-radiotherapy. Resistance is mediated by the Cav1/EREG/YAP axis. Patients whose tumors overexpressed Cav1 experienced regional recurrence a few years after adjuvant radiotherapy ± chemotherapy. Together, our observations suggest that a high expression of Cav1 might be predictive of locoregional relapse of LA-HNSCC.

Evidence for synergy between alpha(2)-adrenergic and nonadrenergic mechanisms in central blood pressure regulation.

BACKGROUND: Both alpha(2)-adrenergic and non--alpha(2)-adrenergic mechanisms seem to be involved in the hypotensive effect of imidazoline-like drugs. This study aimed at investigating how these 2 mechanisms work together to modify blood pressure (BP). METHODS AND RESULTS: LNP 509, which appeared in this study to be devoid of alpha(2A)-adrenergic activity, was administered to anesthetized rabbits and wild-type (WT) mice into the cisterna magna and into the fourth ventricle, respectively. Mean arterial pressure decreased by a maximum of 46 +/- 4% and 16 +/- 2%, respectively. In D79N mice, which lack functional alpha(2A)-adrenergic receptors, LNP 509 also reduced mean arterial pressure by 17 +/- 2%. The hypotension induced by LNP 509 (100 microg/kg intracisternally) was prevented by S23757 (1 mg/kg intracisternally), an antagonist highly selective for I(1)-imidazoline binding sites (I(1)BS). A synergy between LNP 509 and the alpha(2)-adrenergic agonist alpha-methylnoradrenaline (alpha-MNA) was observed in rabbits (cisterna magna injection) and in WT mice (fourth ventricle injection) but not, as expected, in D79N mice. Similar to LNP 509 alone, rilmenidine (fourth ventricle injection), which binds both to alpha(2)-adrenergic receptors and to I(1)BS, decreased BP in D79N mice. In WT animals, rilmenidine had a significantly greater effect. Microinjections performed in rabbits showed that the synergism occurred at least in part in the nucleus reticularis lateralis of the brain stem. CONCLUSIONS: These results demonstrate that a central imidazoline-sensitive, but non--alpha(2)-adrenergic, mechanism can modify BP by itself. This mechanism, which may involve I(1)BS, interacts synergistically with an alpha(2)-adrenergic mechanism to decrease BP.

Nitric oxide discriminates the sites and mechanisms of action of centrally acting anti-hypertensive drugs in rabbits.

The aim of the present study was to further investigate the mechanisms of the central hypotensive action of catecholamines and imidazolines, in particular the role of nitric oxide (NO). Microinjections into the nucleus reticularis lateralis (NRL/RVLM) located in the rostroventrolateral part of the medulla (RVLM) and/or into the nucleus tractus solitarii (NTS) were performed in pentobarbital-anesthetized rabbits. Microinjections of brimonidine (1 ng/kg), which binds both alpha(2)-adrenergic receptors (alpha(2)-ARs) and I(1) imidazoline receptors (I(1)Rs), into the NRL/RVLM induced hypotension (69+/-2 vs. 88+/-2 mm Hg) (p<0.05). Microinjections of S23757 (1 microg/kg), an antagonist highly selective for I(1)Rs, into the same site, prevented the hypotensive effect of brimonidine. These data show that the hypotensive effects of low doses of brimonidine involve the I(1)Rs of the NRL/RVLM. Alpha-methylnoradrenaline (alpha-MNA) (0.5 microg/kg) microinjected into the NTS induced hypotension (76+/-4 vs. 91+/-4 mm Hg) (p<0.05). Microinjections of a low dose of brimonidine (1 ng/kg) into the NTS had no blood pressure (BP) effect at all. In contrast, a higher dose (10 ng/kg) acting on alpha(2)-ARs induced hypotension (72+/-3 vs. 96+/-2 mm Hg) (p<0.05). Nomega-Nitro-L-arginine (L-NNA) (1.5 microg/kg) injected into the NRL/RVLM prevented the hypotensive effect of both alpha-MNA and the higher dose of brimonidine injected into the NTS. Bicuculline (1.5 microg/kg) injected into the NRL/RVLM prevented the hypotensive effect of alpha-MNA injected into the NTS. It is demonstrated that (i) the activation of alpha(2)-ARs of NTS triggers a neuronal GABAergic pathway projecting to the NRL/RVLM region which is NO dependent (ii) both alpha(2)-adrenergic (NTS) and non-adrenergic I(1)R (NRL/RVLM) mechanisms account for the very powerful hypotensive effect of brimonidine, a compound with high affinities at both types of receptors.

The nuclear hormone receptor PPARγ counteracts vascular calcification by inhibiting Wnt5a signalling in vascular smooth muscle cells.

Vascular calcification is a hallmark of advanced atherosclerosis. Here we show that deletion of the nuclear receptor PPARγ in vascular smooth muscle cells of low density lipoprotein receptor (LDLr)-deficient mice fed an atherogenic diet high in cholesterol, accelerates vascular calcification with chondrogenic metaplasia within the lesions. Vascular calcification in the absence of PPARγ requires expression of the transmembrane receptor LDLr-related protein-1 in vascular smooth muscle cells. LDLr-related protein-1 promotes a previously unknown Wnt5a-dependent prochondrogenic pathway. We show that PPARγ protects against vascular calcification by inducing the expression of secreted frizzled-related protein-2, which functions as a Wnt5a antagonist. Targeting this signalling pathway may have clinical implications in the context of common complications of atherosclerosis, including coronary artery calcification and valvular sclerosis.

[125I]2-(2-chloro-4-iodo-phenylamino)-5-methyl-pyrroline (LNP 911), a high-affinity radioligand selective for I1 imidazoline receptors.

The I1 subtype of imidazoline receptors (I1R) is a plasma membrane protein that is involved in diverse physiological functions. Available radioligands used so far to characterize the I(1)R were able to bind with similar affinities to alpha2-adrenergic receptors (alpha2-ARs) and to I1R. This feature was a major drawback for an adequate characterization of this receptor subtype. New imidazoline analogs were therefore synthesized and the present study describes one of these compounds, 2-(2-chloro-4-iodo-phenylamino)-5-methyl-pyrroline (LNP 911), which was of high affinity and selectivity for the I1R. LNP 911 was radioiodinated and its binding properties characterized in different membrane preparations. Saturation experiments with [125I]LNP 911 revealed a single high affinity binding site in PC-12 cell membranes (K(D) = 1.4 nM; B(max) = 398 fmol/mg protein) with low nonspecific binding. [125I]LNP 911 specific binding was inhibited by various imidazolines and analogs but was insensitive to guanosine-5'-O-(3-thio)triphosphate. The rank order of potency of some competing ligands [LNP 911, PIC, rilmenidine, 4-chloro-2-(imidazolin-2-ylamino)-isoindoline (BDF 6143), lofexidine, and clonidine] was consistent with the definition of [125I]LNP 911 binding sites as I1R. However, other high-affinity I1R ligands (moxonidine, efaroxan, and benazoline) exhibited low affinities for these binding sites in standard binding assays. In contrast, when [125I]LNP 911 was preincubated at 4 degrees C, competition curves of moxonidine became biphasic. In this case, moxonidine exhibited similar high affinities on [125I]LNP 911 binding sites as on I1R defined with [125I]PIC. Moxonidine proved also able to accelerate the dissociation of [125I]LNP 911 from its binding sites. These results suggest the existence of an allosteric modulation at the level of the I1R, which seems to be corroborated by the dose-dependent enhancement by LNP 911 of the agonist effects on the adenylate cyclase pathway associated to I1R. Because [125I]LNP 911 was unable to bind to the I2 binding site and alpha2AR, our data indicate that [125I]LNP 911 is the first highly selective radioiodinated probe for I1R with a nanomolar affinity. This new tool should facilitate the molecular characterization of the I1 imidazoline receptor.