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The rat placental PRL family consists of molecules structurally similar
to PRL and GH, and to date nine members have been identified. In the
course of investigating late stage specific placental PRL family
expression by differential display, we have isolated a complementary
DNA encoding a new molecule that is highly homologous to PRL-like
protein C (PLP-C) and PLP-D, and named this molecule PLP-H. The
complementary DNA encoded a mature protein of 239 amino acids,
including a 31-amino acid signal sequence. Sequence comparison between
PLP-H and other members of the placental PRL family showed that PLP-H
is highly homologous to PLP-C and PLP-D (78% and 67% homology at the
amino acid level, respectively). Expression of PLP-H was similar to
that of PLP-C and PLP-D; PLP-H messenger RNA (mRNA) first appeared on
day 14 of pregnancy, and its expression increased until term. RT-PCR
analysis showed that PLP-H as well as PLP-C and PLP-D are expressed in
all rat strains examined, confirming that PLP diversity is not due to
strain differences. In situ hybridization analysis
indicated that PLP-H mRNA is specifically expressed in
spongiotrophoblast cells and in trophoblast giant cells of the
placental junctional zone. Differentiated Rcho-1 cells
also expressed PLP-H mRNA, whereas undifferentiated
Rcho-1 cells did not. PLP-H seems to exist as a
secretory protein because its N-terminal sequence is identical to that
of GH/PRL-like molecule secreted from placental explants. PLP-H
contains two putative N-glycosylation sites and eight
cysteine residues, of which six are highly conserved in the placental
PRL family. We prepared a recombinant protein for PLP-H together with
PLP-D using a COS7 transfection system. Purified PLP-H showed two bands
with molecular masses of 27 and 29 kDa. Only the 27-kDa protein
was detected after N-glycosidase treatment, indicating
that PLP-H is a glycoprotein. PLP-H and PLP-D did not stimulate the
proliferation of Nb2 lymphoma cells or the phosphorylation of Janus
kinase-2 and signal transducer and activator of transcription-5. These
data indicate that PLP-H and PLP-D are nonlactogenic hormones. Thus, we
have cloned a new member of the PLP subfamily, PLP-H, which has
features in common with PLP-C and PLP-D.