No abstract

Metabotropic glutamate receptors (mGluRs) function as dimers. Recent work suggests that mGluR1 and mGluR5 may physically interact, but the nature and functional consequences of this relationship have not been addressed. In this study, the functional and pharmacological consequences of this interaction were investigated. Using heterologous expression of mGluR cDNA in rat sympathetic neurons from the superior cervical ganglion and inhibition of the native calcium currents as an assay for receptor activation, a functional interdependence between mGluR1 and mGluR5 was demonstrated. In neurons coexpressing these receptors, combining a selective mGluR1 competitive antagonist with either an mGluR1-or mGluR5-selective negative allosteric modulator[2-methyl-6-(phenylethynyl)pyridine hydrochloride], respectively, strongly occluded signaling by both receptors to an approximately equal degree. By contrast, in cells coexpressing mGluR1 and mGluR2, combining the same mGluR1 competitive inhibitor with an mGluR1 or mGluR2 NAM yielded partial and full inhibition of the response, respectively, as expected for independently acting receptors. In neurons expressing mGluR1 and mGluR5, the selective NAMs each strongly inhibited the response to glutamate, suggesting that these receptors do not interact as heterodimers, which would not be inhibited by selective NAMs. Finally, evidence for a similar mGluR1/mGluR5 functional dependence is shown in medium spiny striatal neurons.Together, these data demonstrate cooperative signaling between mGluR1 and mGluR5 in a manner inconsistent with heterodimerization, and thus suggest an interaction between homodimers.

Chitinase‐like proteins (CLP) are chitin‐binding proteins that lack chitin hydrolyzing activity, but possess cytokine‐like and growth factor‐like properties, and play crucial role in intercellular crosstalk. Both human and mice express two members of CLP family: YKL‐40 and stabilin‐1 interacting chitinase‐like protein (SI‐CLP). Despite numerous reports indicating the role of YKL‐40 in the support of angiogenesis, tumor cell proliferation, invasion and metastasis, the role of its structurally related protein SI‐CLP in cancer was not reported. Using gain‐of‐function approach, we demonstrate in the current study that the expression of recombinant SI‐CLP in mouse TS/A mammary adenocarcinoma cells results in significant and persistent inhibition of in vivo tumor growth. Using quantitative immunohistochemistry, we show that on the cellular level this phenomenon is associated with reduced infiltration of tumor‐associated macrophages (TAMs), CD4+ and FoxP3+ cells in SI‐CLP expressing tumors. Gene expression analysis in TAM isolated from SI‐CLP‐expressing and control tumors demonstrated that SI‐CLP does not affect macrophage phenotype. However, SI‐CLP significantly inhibited migration of murine bone‐marrow derived macrophages and human primary monocytes toward monocyte‐recruiting chemokine CCL2 produced in the tumor microenvironment (TME). Mechanistically, SI‐CLP did not affect CCL2/CCR2 interaction, but suppressed cytoskeletal rearrangements in response to CCL2. Altogether, our data indicate that SI‐CLP functions as a tumor growth inhibitor in mouse breast cancer by altering cellular composition of TME and blocking cytokine‐induced TAM recruitment. Taking into consideration weak to absent expression of SI‐CLP in human breast cancer, it can be considered as a therapeutic protein to block TAM‐mediated support of breast tumor growth.