N. Genty et al., ENDOCYTOSIS AND DEGRADATION OF PROLACTIN AND ITS RECEPTOR IN CHINESE-HAMSTER OVARY CELLS STABLY TRANSFECTED WITH PROLACTIN RECEPTOR CDNA, Molecular and cellular endocrinology, 99(2), 1994, pp. 221-228
Molecular cloning of the prolactin (PRL) receptor cDNA has revealed di
fferent forms of the receptor: among them, the longest form encodes a
transmembrane protein of 592-598 amino acids and was originally found
in rabbit mammary gland as well as in human and rat tissues. It contai
ns a cytoplasmic domain of 358 amino acids. In CHO cells transfected w
ith the PRL receptor cDNA, PRL is able to induce the specific expressi
on of a reporter gene provided with the promoter of the milk protein g
ene beta-lactoglobulin. The cDNA encoding this long receptor form has
been expressed permanently after stable transfection of Chinese hamste
r ovary (CHO) cells. In these cells, we have determined the fate of th
e bound hormone and of the receptor. At 37 degrees C, transfected cell
s were able to endocytose I-125-labeled human growth hormone (hGH) or
ovine prolactin (oPRL) at an initial rate of about 1 fmol/h at 100 pM
labeled hormone and 10(6) cells/well. Lowering the temperature to 15 d
egrees C slowed the endocytosis of [I-125]hGH by a factor of 5. These
results were confirmed by electron microscopy with oPRL labeled with c
olloidal gold. At 37 degrees C, the receptor underwent rapid insertion
to the cell surface and constitutive endocytosis (half-life 80 min).
This rate of endocytosis was enhanced in the presence of 10 nM oPRL (h
alf-life 8 min), leading to down-regulation of the receptor by exhaust
ion of the intracellular receptor pool. After down-regulation, the cel
l surface was replenished with newly synthesized PRL receptor with a h
alf-time of 8-10 min. If cycloheximide was added, almost no receptors
could be found on the cell surface. These results indicate that in tra
nsfected cells the PRL receptor behaved largely as in classical target
cells. A ''conveyor belt'' endocytosis behavior was found, with degra
dation of the endocytosed receptors, and occupation by the hormone enh
ancing this process. Moreover, since the PRL receptor belongs to a fam
ily of receptors in which companion protein(s) seem to play important
roles, transfected CHO cells appear to provide the expressed receptors
with the necessary element(s) to function as in normal PRL target cel
ls.