Gq. Jiang et al., EXCLUSIVE HOMODIMERIZATION OF THE ORPHAN RECEPTOR HEPATOCYTE NUCLEAR FACTOR-4 DEFINES A NEW SUBCLASS OF NUCLEAR RECEPTORS, Molecular and cellular biology, 15(9), 1995, pp. 5131-5143
Hepatocyte nuclear factor 4 (HNF-4), a highly conserved member of the
steroid hormone receptor superfamily critical for development and five
r-specific gene expression, is very similar to another superfamily mem
ber, retinoid X receptor alpha (RXR alpha), in overall amino acid sequ
ence and DNA binding specificity. Since RXR alpha is known to heterodi
merize with many other nuclear receptors, the formation of heterodimer
s between HNF-4 and RXR alpha was examined. With the electrophoretic m
obility shift assay, coimmunoprecipitation, and transient transfection
assays, it is shown that, unlike other nuclear receptors, HNF-4 does
not form heterodimers with RXR alpha either in the presence or in the
absence of DNA. We also show that in vitro-translated HNF-4 does not f
orm heterodimeric complexes on DNA with a number of other receptors, i
ncluding RXR beta, RXR gamma, retinoic acid receptor alpha, or thyroid
hormone receptor alpha. To investigate the hypothesis that the lack o
f heterodimerization between HNF-4 and RXR alpha is due to a strong ho
modimerization activity of HNF-4, glycerol gradient sedimentation and
kinetic analysis were used to Show that HNF-4 is in fact a stable homo
dimer in solution. Finally, immunohistochemistry is used to show that
the HNF-4 protein is found exclusively in the nuclei in both HepG2 cel
ls, which express endogenous HNF-4, and transfected COS cells, which o
verexpress HNF-4. These findings lead us to propose that HNF-4 defines
a new subclass of nuclear receptors which reside primarily in the nuc
leus and which bind DNA and regulate transcription as homodimers.