FFA2-Elicited Glucagon Like Peptide-1 Secretion: Roles of GGαi and GGαq

Introduction: The incretin hormone glucagon like peptide-1 (GLP-1) is secreted from the L-cells of the colon in response to short chain free fatty acids (SCFA) which are generated by the fermentation of non-digestible carbohydrates by the gut microbiota. This effect is mediated at least in part by signalling through the free fatty acid receptor 2 (FFA2) (1). FFA2 is reported to couple to both Gαi and Gαq (2). This study aims determine the signalling pathways which regulate GLP-1secretion in the L-cells of the colon.

Methods: Colons from adult C57BL6/NTac mice were digested with collagenase into cell structures resembling colonic crypts as previously described (3). After overnight incubation, the crypts were incubated for two hours in physiological saline, containing 1 mM acetate, 1 mM propionate, or 1 mM butyrate, in the presence or absence of 100 nM PTX (overnight) or 100 nM FR900359 (30 min). Active GLP-1 was assayed by ELISA in the culture supernatant and cell lysates. Amount released was expressed as a percentage of total GLP-1 content and was normalised to baseline secretion per mouse. Data are means ± SEM, statistical analyses are one-way or two-way ANOVA with post hoc Dunnett’s or Sidak’s, where appropriate.

Results: Propionate significantly increased GLP-1 secretion, while acetate and butyrate showed similar trends. Propionate-elicited secretion was unaffected by concomitant PTX incubation, but was abolished by the Gαq inhibitor FR900359 (see table 1). In colonic crypts from Ffar2 -/- mice, no increases in GLP-1 secretion were observed with acetate, butyrate or propionate.

Table 1. Fold increases in GLP-1 secretion over baseline (n=3).

Conclusions: GLP-1 secretion in response to SCFAs appears to be downstream of FFAR2 coupling to Gαq, whereas Gαi appears not to be involved. Potentially, FFA2 may be entirely responsible for GLP-1 secretion, rather than FFA3, which is believed to be expressed on the same cells. This study is the first time the new Gαq inhibitor FR900359 has been used to demonstrate the involvement of Gαq downstream of FFA2, or indeed any GPCR in the L-cell.

References

1. Tolhurst G. et al. (2012) Diabetes 61: 364–371.

2. Brown et al. (2003) JBC 278: 11312–11319.

3. Reimann F. et al. (2008) Cell Metabolism 8: 532–9.