Rat Liver Prolactin Receptor Research Articles

The effect of chloroquine on lysosomal prolactin receptors in rat liver.

PRL receptors have been previously identified in purified rat liver plasma membrane and Golgi vesicle preparations. In this study, we report on PRL receptors located in highly purified lysosome preparations. These lysosomal PRL receptors were characterized using Scatchard analysis and compared to other intracellular and cell surface receptors. We have identified two classes of lysosomes. Lighter lysosome-like vesicles, which are greatly enriched in acid phosphatase activity (the marker enzyme of lysosomes), contain a great deal of binding activity. This PRL binding was only slightly increased by pretreatment of animals with the lysosomotropic agent chloroquine. In contrast, mature lysosomes showed very little binding activity in control animals, but chloroquine treatment increased binding 7- to 8-fold in these mature lysosomes. We suggest that the lysosome-like structures are immature lysosomes (namely prelysosomes) toward which the hormone-receptor complex is internalized: they appear to bear little proteolytic activity. These structures could play a role in PRL receptor recycling. Lysosomal PRL receptors showed curvilinear Scatchard plots, in contrast to plasma membrane and Golgi counterparts, which were linear over the same range of hormone concentrations. The high affinity site in lysosomes had a Kd comparable to the cell surface and Golgi receptors. The number of binding sites per mg protein in prelysosomes and lysosomes was 3 times greater than that in the homogenate, but Golgi preparations were 3 times as rich as lysosomes. The great number of PRL receptors in prelysosomes could be attributed, in large part, to the low affinity sites. The internalization of PRL into rat liver was examined after in vivo injection of [125I]iodoovine PRL. The labeled hormone was found initially in the plasma membrane fraction, after which it localized preferentially in the Golgi fraction, with maximum incorporation 15 min postinjection. Substantial radioactivity was observed in both classes of lysosomes (L-1 and L-2). In contrast to the Golgi fraction, maximum incorporation of [125I]iodoovine PRL in lysosomes occurred at 30 min. This suggests either that during internalization, PRL first reaches Golgi elements and is then transferred to the lysosomal compartment, or that there are two independent pathways of internalization, one rapid toward the Golgi complex (may be a path of receptor recycling) and the other toward lysosomes (probably leading to receptor degradation).

Rat growth hormone (GH) but not prolactin (PRL) induces both GH and PRL receptors in female rat liver.

This study was designed to elucidate which hormone is responsible for the induction of GH and PRL receptors in rat liver. Intact female rats were implanted with osmotic minipumps delivering rat GH (rGH) or ovine GH (oGH) or PRL at various rates from 75 to 800 micrograms/day for 7 days, and binding of radioiodinated bovine GH or ovine PRL (oPRL) tracer was measured on liver microsomal membranes. MgCl2 treatment was used to remove bound hormones from receptors before tracer binding. Infusion of rGH resulted in a significant increase (P less than 0.001) in both GH and PRL binding, the effect being maximal (2.5- to 3-fold for both ligands) at rGH infusion rates from 150 to 400 micrograms/day. Serum rGH levels were elevated 3- to 5-fold in these animals, but somatomedin-C concentrations were not higher than in controls. MgCl2 treatment showed that GH, but not PRL, binding sites in rGH-treated animals were significantly occupied by administered hormone. Analysis of competitive binding curves indicated that receptors for both GH and PRL increased in concentration without changes in binding affinity. In contrast to the rGH effect, oGH infusion from 75 to 400 micrograms/day failed in two experiments to consistently alter either bovine GH or oPRL binding sites. This was not explained by the potency of the preparation at the somatogenic receptor; oGH was in fact more potent than rGH. The effects of rat PRL and oPRL infusion on receptor levels were also assessed. In contrast to previous reports, neither preparation caused induction of either PRL or GH binding sites. oPRL decreased PRL binding by 30-40% when infused between 200 and 400 micrograms/day, whereas rat PRL had a less consistent effect. MgCl2 stripping of membranes suggested that administered PRL preparations did not significantly occupy PRL receptors. GH receptors were unaffected in any PRL-treated group. It is concluded that in intact female rats, rGH regulates the concentration of both GH and PRL receptors. The slight down-regulation of PRL receptors resulting from PRL infusion casts further doubt on the concept that PRL induces its own hepatic receptors.

Possible role of somatostatin in the regulation of the sexually differentiated steroid metabolism and prolactin receptor in rat liver.

The regulation of the sexually differentiated metabolism of 4-[4-14C]androstene-3,17-dione and the presence of PRL receptors in rat liver were studied. Electrolytic lesions in male rats placed in a restricted area in the anterior hypothalamic periventricular area caused a feminization of hepatic steroid metabolism (i.e. increased the 5 alpha-reductase and decreased the 6 beta- and 16 alpha-hydroxylase activities) and of the levels of PRL receptors (increased binding of [125I]-labeled human PRL). After periventricular lesions, histochemical analysis revealed a decrease in somatostatin-like immunoreactive cell bodies in the periventricular area. Also the number of immunoreactive somatostatin fibers in the median eminence was dramatically reduced. Somatostatin levels in the median eminence, as measured by RIA, were reduced to approximately 2-10% of control values after periventricular lesions. Large lesions in the amygdaloid complex in male rats caused a partial feminization of hepatic steroid metabolism and PRL receptors. Passive immunization during 4 days by multiple injections of an antiserum generated against somatostatin resulted in a partial feminization of the male rat liver. When somatostatin was injected into female rats, the PRL receptors were reduced to approximately 60% of the control female receptor levels. The present study indicates that the anterior periventricular hypothalamic area is important in the control of the sexually differentiated steroid metabolism and PRL receptors in the liver and that the amygdaloid complex also may have regulatory influences on this system. A possible central neuro-endocrine mediator of these sex differences in the liver could be somatostatin or a related compound.

Induction of prolactin receptors in rat liver after the administration of growth hormone.

The pituitary regulation of the sexually differentiated PRL receptor in rat liver was studied. PRL receptors were measured in a crude membrane fraction (105,000 × g pellet) using [125I]iodohuman PRL as tracer. Human GH (hGH), continuously administered by Alzet osmotic minipumps with an infusion rate of 5 μg/h for 1 week, was shown to induce PRL receptors in livers from male and female hypophysectomized-gonadectomized rats. The PRL receptors were increased to a level found in control female rat livers. This inductive effect of hGH was also seen in adrenalectomized and thyroidectomized male rats. In intact male rats given hGH, PRL receptors were increased to a female level in 4–7 days. hGH was effective in doses of 2.5 and 5 Μg/μl in inducing a female receptor pattern. The induced PRL receptors in male rats had characteristics similar to those of hepatic PRL receptors in female rats when data were calculated according to Scatchard (Kd = 0.13 × 10-9 vs. 0.15 × 10-9 M; number of binding sites 88 vs. 57 fmol/m...

Prolactin stimulation of prolactin receptors in rat liver

Hypophysectomy of the female rat results in a loss of prolactin receptors from the liver. Seventy percent of specific prolactin binding is lost within 24 hours; by 48 hours receptor levels are less than 5% of those found in livers from intact rats. A single dose of ovine prolactin to hypophysectomized rats causes a partial restoration of prolactin receptors between 12 and 18 hours after injection. As little as 0.5 mg prolactin significantly increases receptors, while doses above the 2 mg optimum are apparently less effective. These restored receptor sites are unaltered in their affinity for prolactin. Estradiol (E 2), progesterone, hydrocortisone, triiodothyronine (T 3) or E 2 plus T 3 could not mimic the prolactin effect. Neither combinations of prolactin with E 2 or T 3 nor repeated daily injections of prolactin alone increase receptors more than does a single prolactin injection. It appears that prolactin modulates the level of its own receptor in rat liver.

Prolactin Receptors in Rat Liver: Possible Induction by Prolactin

A prolactin receptor, present in adult female rat liver, can be induced in males by estrogen. Hypophysectomy diminished receptor levels in the female and rendered males unresponsive to estrogen. A renal pituitary implant blunted the decrease in hypophysectomized females and induced the receptor in hyophysectomized males. The increased receptor level in hypophysectomized males with a renal pituitary implant was preceded by a sustained elevation of circulating prolactin. Our observations suggest that prolactin induces its own receptor.