Effects of all-trans-retinoic acid (ATRA) and retinoic acid receptor (RAR) expression on secretion, growth, and apoptosis of insulin-secreting RINm5F cells

To define the functions of retinoids and their receptors in insulin secretion, we tested the effects of all-trans-retinoic acid (ATRA) and retinoic acid receptor (RAR) expression on cell growth, differentiation, and secretion using insulin-secreting RINm5F cells. Wild-type cells with a low abundance of mRNA for RAR beta were transfected with RAR beta or chloramphenicol acetyltransferase (CAT control). Cells were cultured for 2-7 days in media without (A-def) or with ATRA, 1, 10, 100, and 1,000 nM. At day 2 of culture, ATRA stimulated insulin release in wild-type and transfected cells, and this effect was dose dependent. At 7 days, ATRA stimulated insulin secretion from wild-type cells twofold at glucose concentrations of 0.5 mM (A-def, 5.1 +/- 0.27; ATRA, 1,000 nM, 10.5 +/- 1.43 ng/10(6) cells) and at 11.0 mM (A-def. 6.9 +/- 0.24; ATRA, 1,000 nM, 13.6 +/- 1.86 ng/l0(6) cells). The cellular insulin content was increased about threefold (A-def, 39.2 +/- 2.95; ATRA, 1,000 nM, 118 +/- 8.54 ng/10(6) cells). ATRA inhibited growth of wild-type cells as early as 3 days, and this effect was dose dependent. Whereas in the absence of ATRA, the cell number increased over fivefold between day 3 and day 5, ATRA, 1,000 nM, inhibited cell growth completely. ATRA, 1,000 nM, increased apoptotic RINm5F cells (day 3 A-def, 0.53 +/- 0.27% of total cells, and ATRA, 2.30 +/- 1.44: day 5 A-def, 0.38 +/- 0.23, and ATRA, 2.14 +/- 0.59; day 7 A-def, 0.90 +/- 0.29, and ATRA, 6.02 +/- 1.64). RAR beta-transfected cells showed overexpression of mRNA to RAR beta and dose-dependent inhibition of growth, with almost-complete inhibition at ATRA concentrations as low as 100 nM. Overexpression of RAR beta increased insulin secretion at ATRA, 100-1,000 nM. In summary, ATRA increased the insulin secretion and content of RINm5F cells, while inhibiting growth and increasing apoptosis. Increased expression of RAR beta facilitated these effects on growth and secretion. These findings may reflect the known effect of ATRA on differentiation of cells and mediation through RAR beta.