P. Tuohimaa et al., NUCLEAR PROGESTERONE-RECEPTOR IS MAINLY HEAT-SHOCK PROTEIN 90-FREE INVIVO, Proceedings of the National Academy of Sciences of the United Statesof America, 90(12), 1993, pp. 5848-5852
Heat shock protein 90 (hsp'') is associated with many steroid receptor
s in tissue homogenates. It is widely accepted that hsp90 regulates th
e binding of the receptor to the corresponding gene regulatory element
. However there is no unequivocal evidence that steroid receptor-hsp90
complexes are present in the intact cells. We demonstrate here the ab
sence of progesterone receptor (PR)-hsp90 complexes in intact target c
ell nuclei, using immunohistochemical and biochemical methods to deter
mine the location and composition of the nonliganded (aporeceptor) and
liganded (holoreceptor) PR complexes. In the chicken oviduct cells, b
oth apo- and holoreceptors were nuclear, while hsp90 was exclusively c
ytoplasmic. When expressed transiently in HeLa cells, hsp90 was detect
ed in the cytoplasm and PR was detected in the nucleus. Their location
or staining intensity was not affected when they were coexpressed in
the same cells. To confirm that the sensitivity of the immunohistochem
ical detection of hsp90 and PR did not differ significantly, a chimeri
c hsp90-PR was transiently expressed in HeLa cells. Both hsp90 and PR
antigens of the chimera were detected in nuclei with the same intensit
y. In homogenates of the same tissue samples that were used for immuno
histochemistry, the PR was complexed with hsp90. Hsp90-PR complexes we
re formed in vitro when immature bursa of Fabricius, known to contain
high levels of hsp90, was homogenized in the presence of hsp90-free ap
oreceptor, while holoreceptor did not associate with hsp90. Our data s
how that nuclear PR is not complexed with hsp90 in vivo and suggest th
at the 8S-PR may be an in vitro artifact generated during tissue proce
ssing.