Thiazides attenuate insulin secretion through inhibition of mitochondrial carbonic anhydrase 5b in β-islet cells in mice

Background: Thiazides are associated with glucose intolerance and new
onset diabetes mellitus. Previous studies demonstrated that thiazides
attenuate insulin secretion, but the molecular mechanisms remain elusive.
We hypothesized that thiazides attenuate insulin secretion via one of the
known molecular thiazide targets in β-cells. Methods: We performed static
insulin secretion experiments with islets of wild-type, NCC (SLC12A3) and
NDCBE (SLC4A8) knock-out (KO) mice and with murine Min6 cells with
individual knock-down of carbonic anhydrase (CA) isoforms to identify the
molecular target of thiazides in β-cells. CA5b KO mice were then used to
assess the role of the putative thiazide target CA5b in β-cell function
and in mediating thiazide sensitivity in vitro and in vivo. Results:
Thiazides inhibited glucose- and sulfonylurea-stimulated insulin secretion
in islets and Min6 cells at pharmacologically relevant concentrations.
Inhibition of insulin secretion by thiazides was CO2/HCO3--dependent, not
additive to unselective CA inhibition with acetazolamide and independent
of extracellular potassium. In contrast, insulin secretion was unaltered
in islets of mice lacking the known molecular thiazide targets NCC or
NDCBE. CA expression profiling with subsequent knock-down of individual CA
isoforms suggested mitochondrial CA5b as molecular target. In support of
these findings, thiazides significantly attenuated Krebs cycle anaplerosis
through reduction of mitochondrial oxaloacetate synthesis. CA5b KO mice
were resistant to thiazide-induced glucose intolerance, and thiazides did
not alter insulin secretion in CA5b KO islets. Conclusions: In summary,
our study reveals that thiazides attenuate insulin secretion via
inhibition of the mitochondrial CA5b isoform in β-cells.