Calcium Mobilisation and CCK Secretion Induced by Modified Fatty Acids and Latex Microspheres Reveal Dual Receptor Mechanisms for Lipid Stimulation of STC‐1 Cells

Abstract

How fatty acids stimulate enteroendocrine cells to release cholecystokinin (CCK) is largely unknown. Recently, we proposed that the murine enteroendocrine cell line, STC‐1, responds to insoluble fatty acid aggregates rather than fatty acid monomers in solution. This hypothesis led to two testable predictions. First, other insoluble particles of similar size but unrelated to fatty acid may be able to stimulate STC‐1 cells in a similar fashion to dodecanoic acid and second, fatty acid sensing in STC‐1 cells should be fairly insensitive to chemical modifications of the fatty acid as long as these modifications do not greatly alter the ability of the molecule to form insoluble aggregates. We used several analogues of dodecanoic acid and several varieties of latex microsphere (varying in size and surface charge) to see whether the predictions of our model hold. We found that while there was at least one latex microsphere that could induce CCK secretion and calcium mobilisation in STC‐1 cells, there was a very poor correlation between the presence of insoluble aggregates and a cellular response. Instead the most important property, determining the potency of fatty acid analogues as stimulants of CCK secretion, was their amphipathicity. Removal of either the polar head or lipophilic tail completely abolished the ability of a given fatty acid analogue to stimulate STC‐1 cells. These data suggested that while fatty acids can stimulate cells as aggregates, they may also be acting in monomeric form with the oil:water partitioning coefficient playing a crucial role. We finally resolved this issue with the observation that the sulfate ion greatly altered the response of STC‐1 cells to monomeric dodecanoic acid. In the presence of sulfate, STC‐1 cells will only respond to dodecanoic acid aggregates whereas when sulfate is replaced with chloride the cells clearly respond to dodecanoic acid monomers which are completely in solution. In summary, we propose that dodecanoic acid can stimulate STC‐1 cells via two separate pathways one involving fatty acid monomers in solution and one involving fatty acid aggregates. Which pathway dominates depends on the presence of sulfate in the extracellular medium.

Activation of Enteroendocrine Cells via TLRs Induces Hormone, Chemokine, and Defensin Secretion

“…Accordingly, STC-1 cells stimulated by fatty acids exhibit an increase in CCK production (24). Analysis of CCK production by STC-1 cells stimulated in vitro with the TLR agonists LPS, flagellin, and CpG-ODN revealed a progressive increase in CCK levels with dose-dependent responses for each agonist, going to plateau after 18 h of treatment (Fig.…”

Fatty acids do not stimulate enteroendocrine cells via particle sensing mechanisms

“…This evidence was based upon an elaborate series of experiments using microfilters and sonication to remove or disrupt particulate FA aggregates, and led to the suggestion that both STC-1 cells and an additional EEC line (GluTAg) were responding directly to the aggregates rather than to monomeric fatty acids in solution. Further evidence was sought to test this hypothesis by attempting to mimic the physical nature of FA aggregates with non-lipid particles of similar size (Kazmi et al, 2003). These experiments demonstrated that certain latex particles (LP) could indeed generate intracellular [Ca 2þ ] i transients and induce CCK release from STC-1 cells, in a manner seemingly comparable to fatty acids.…”

Multiple Fatty Acid Sensing Mechanisms Operate in Enteroendocrine Cells

“…The time elapsed after sonication affects solubility, and re-aggregation of fatty acid occurs, which in turn affects the cellular responses to C 12 (10). This has led to the suggestion that fatty acid effects may in part be exerted by the extracellular aggregates themselves (10,11). Removing Ca 2ϩ from the buffer renders C 12 far more soluble and stable in its physicochemical state, yet results in [Ca 2ϩ ] i responses that are rapid, reversible, and more reproducible than in the presence of extracellular Ca 2ϩ .…”

“…Our recent data have demonstrated that STC-1 cells are indeed able to respond to such unsolubilized particulate material, either lipid or nonlipid in origin, and this may underpin a component of the fatty acid response (10,11). However, this cannot be the sole mechanism because, under different physicochemical conditions (e.g.…”