C. Zhao et al., Expression and distribution of lactate/monocarboxylate transporter isoforms in pancreatic islets and the exocrine pancreas, DIABETES, 50(2), 2001, pp. 361-366
Transport of lactate across the plasma membrane of pancreatic islet beta -c
ells is slow, as described by Sekine et al. (J Biol Chem 269:4895-4902, 199
4), which is a feature that may be important for normal nutrient-induced in
sulin secretion. Although eight members of the monocarboxylate transporter
(MCT) family have now been identified, the expression of these isoforms wit
hin the exocrine and endocrine pancreas has not been explored in detail. Us
ing immunocytochemical analysis of pancreatic sections fixed in situ, we de
monstrated three phenomena. First, immunoreactivity of the commonly express
ed lactate transporter isoform MCT1 is near zero in both alpha- and beta -c
ells but is abundant in the pancreatic acinar cell plasma membrane. No MCT2
or MCT4 was detected in any pancreatic cell type. Second, Western analysis
of purified beta- and non-beta -cell membranes revealed undetectable level
s of MCT1 and MCT4. In derived beta -cell lines, MCT1 was absent from MING
cells and present in low amounts in INS-1 cell membranes and at high levels
in RINm5F cells. MCT4 was weakly expressed in MIN6 beta -cells. Third, CD1
47, an MCT-associated chaperone protein, which is closely colocalized with
MCT1 on acinar cell membranes, was absent from islet cell membranes. CD147
was also largely absent from MIN6 and INS-1 cells but abundant in RINm5F ce
lls. Low expression of MCT1, MCT2, and MCT4 contributes to the enzymatic co
nfiguration of beta -cells, which is poised to ensure glucose oxidation and
the generation of metabolic signals and may also be important for glucose
sensing in islet non-beta -cells. MCT overexpression throughout the islet c
ould contribute to deranged hormone secretion in some forms of type 2 diabe
tes.