Ds. Desai et Rm. Niles, EXPRESSION AND REGULATION OF RETINOID-X-RECEPTORS IN B16 MELANOMA-CELLS, Journal of cellular physiology, 165(2), 1995, pp. 349-357
Recently, a new subfamily of nuclear retinoid receptors that is distin
ct from that of RARs has been identified and named Retinoid X receptor
s (RXRs). These receptors specifically bind 9-cis-retinoic acid (9cisR
A), but not all-trans-retinoic acid (ATRA). We determined which RXR su
btypes were expressed in B16 mouse melanoma cells and then studied the
effect of ATRA, 8-bromo-cyclic AMP (8BrcA), and phorbol dibutyrate (P
DB) on RXR mRNA levels. ATRA induces differentiation in these cells wh
ile 8BrcA and PDB antagonize the RA-induced differentiation of B16 mel
anoma cells. Northern analysis demonstrated the expression of RXR alph
a and RXR beta mRNA in B16 cells, but RXR gamma was not detectable. Fu
rther analysis using RT-PCR also failed to detect RXR gamma in these c
ells. Longterm RA treatment decreased the expression of RXR alpha, but
not RXR beta mRNAs. PDB did not alter the expression of either RXR mR
NAs, however, 8BrcA treatment resulted in a time dependent decrease in
the amount of RXR beta, but not RXR alpha mRNA. Inhibition of protein
synthesis by cycloheximide resulted in a large increase in RXR alpha
and RXR beta mRNA levels. This effect of cycloheximide was time and co
ncentration dependent with maximal stimulation of RXR alpha and RXR be
ta mRNAs occurring at 4 h of treatment. Inhibition of transcription wi
th actinomycin D completely abolished the cycloheximide-induced increa
se of RXR beta. In contrast to its effect on other genes, such as imme
diate response genes, cycloheximide treatment did not increase the hal
f-life of RXR beta mRNA. Nuclear run-on assays showed that cycloheximi
de treatment of intact B16 melanoma cells stimulated the transcription
rate of RXRP, but not RXR alpha. These results suggest the presence o
f an unstable transcription factor that negatively regulates the expre
ssion of RXRP in B16 melanoma cells. In addition, since RXR beta is th
e predominant isotype in B16 cells, 8BrcA may, at least partially, inh
ibit RA-induced differentiation through down-regulation of this RXR. (
C) 1995 Wiley-Liss, Inc.