Basal receptor activation by locally produced glucagon-like peptide-1 contributes to maintaining beta-cell function.

Glucagon-like peptide 1 (GLP-1) is a physiological stimulus of pancreatic beta-cell function. This enteroendocrine hormone is produced by intestinal L cells, and is delivered via the bloodstream to GLP-1 receptors (GLP-1Rs) on pancreatic beta-cells. In addition, there is evidence that beta-cell GLP-1Rs maintain sustained basal activity even in the absence of intestinal peptide, an observation that has raised the question whether these receptors have some degree of ligand-independent function. Here, we provide an alternative explanation for basal receptor activity based on our finding that biologically relevant amounts of fully processed GLP-1 are locally generated by insulinoma cell lines, as well as by alpha-cells of isolated rat islets in primary culture. Presence of GLP-1 was established by immunocytochemistry, as well as by selective ELISAs and bioassays of cell supernatants. A GLP-1R antagonist significantly reduced insulin secretion/production in beta-TC-6 insulinoma cells and isolated rat islets, suggesting a functionally important loop between locally produced GLP-1 and its cognate receptor. Treatment with this antagonist also inhibited the growth of beta-TC-6 cells. These observations provide novel insight into the function of insulin-producing cell lines and native beta-cells during in vitro culture, and they support the idea that locally produced GLP-1 may play a role in intra-islet regulation.

Excentric cleavage products of beta-carotene inhibit estrogen receptor positive and negative breast tumor cell growth in vitro and inhibit activator protein-1-mediated transcriptional activation.

Both retinoids and carotenoids are potentially useful chemopreventive agents. In this study we tested the effect of synthetic excentric cleavage products of beta-carotene on the growth of the MCF-7, Hs578T and MDA-MB-231 human breast cancer cells. The apo-beta-carotenoic acids (beta-apo-CA) beta-apo-14'-, beta-apo-12'-, beta-apo-10'- and beta-apo-8'-CA are structurally similar to all-trans-retinoic acid (atRA) but have different side chain lengths. Nine days of treatment with atRA inhibited MCF-7 and Hs578T cell proliferation in a dose-dependent manner. beta-apo-14'-CA and beta-apo-12'-CA significantly inhibited MCF-7 growth, whereas only beta-apo-14'-CA inhibited Hs578T growth. None of these treatments inhibited the growth of MDA-MB-231 cells. Potential mechanisms of growth inhibition, i.e., regulation of the cell cycle control proteins E2F1 and retinoblastoma protein (RB), and effect on activator protein-1 (AP-1)-mediated gene regulation were examined. beta-apo-14'-CA and atRA inhibited the expression of E2F1 protein in MCF-7 and Hs578T cells. beta-apo-14'-CA, beta-apo-12'-CA and atRA down-regulated RB protein expression in MCF-7 but not in Hs578T cells. The effect of phorbol ester-induced transcriptional activation of a collagenase promoter-reporter gene construct was strongly inhibited by 1 micromol/L beta-apo-14'-CA, atRA (MCF-7, Hs578T) or beta-apo-12'-CA (MCF-7). These effects were due neither to cellular conversion of beta-apo-CA to atRA nor to high affinity binding to the retinoid acid receptors. Thus, beta-apo-CAs were effective inhibitors of breast tumor cell proliferation, possibly mediated through down-regulation of cell cycle regulatory proteins and/or inhibition of AP-1 transcriptional activity. The ability of beta-apo-CA to regulate breast tumor cell growth independently of conversion to atRA suggests that these compounds may have fewer side effects than retinoids and, therefore, have a potential chemotherapeutic value that deserves further examination.