Presence and characterization of prolactin receptors in human benign breast tumours

Prolactin--to be reconsidered in canine mammary tumourigenesis?

Mammary tumours represent the most common neoplastic disease of the female dog, and the incidence in female dogs is much higher than in women. Whereas the influence of sexual steroids on breast cancer (BC) development in dogs has been studied, very little is known about the role of prolactin (PRL). New studies show that until recently, the importance of PRL in human BC development and progression has been highly underestimated. PRL plays a role in promoting benign as well as malignant neoplastic cell growth in BC in vitro and in vivo. Sporadic publications proposed a tumour promotor role in the dog. The goal of this review is to summarize our knowledge about PRL and human BC as well as canine mammary tumourigenesis, and propose future research in this area.

Prolactin receptors in human pituitary adenomas

OBJECTIVE

In the rat, prolactin receptors (PRL-R) have been identified in normal pituitary cells and in anterior pituitary tumours induced by oestradiol. No published data are available concerning PRL-R in the human pituitary. The aim of our study was therefore to detect the presence of PRL-R in the normal human pituitary gland and human pituitary adenomas.

DESIGN

Evaluation of free and total PRL-R in the normal pituitary gland and different pituitary tumours characterized by immunocytochemical analysis.

PATIENTS

Twenty-six unselected patients (14 M, 12 F) who underwent surgery for pituitary adenoma (3 prolactinomas, 4 GH-PRL adenomas, 5 GH adenomas, 1 ACTH adenoma, 9 glycoprotein and/or alpha-subunit adenomas, 4 null cells adenomas) were studied. Nine pituitaries from subjects whose death was unrelated to brain and endocrine diseases, were also studied as a control group in the PRL binding studies.

MEASUREMENTS

Free PRL-R in microsomal membranes were determined by in-vitro radioreceptor assay using 125I-labelled human PRL as ligand. Total PRL-R were also measured in the same membrane fractions by removing endogenous PRL bound to its receptors using 4 M MgCl2. Serum PRL levels were also evaluated in all patients before surgery using an IRMA method.

RESULTS

Specific binding values for PRL (free PRL-R) were 0.39 +/- 0.03% (range 0-1.96%) in the pituitary adenomas. These binding values were identical to those observed in normal pituitaries (0.38 +/- 0.07%, range 0.1-0.78%). Elevated PRL binding (1.25% and 1.96%) was found in two patients with PRL secreting adenomas and very high serum PRL levels (5768 and 11240 mU/I. No PRL binding was shown in 4 patients. Treatment of membranes with 4 M MgCl2 increased the specific binding (total PRL-R) in both pituitary tumours (0.5 +/- 0.11%; P < 0.001) and normal pituitaries (0.47 +/- 0.07%; P < 0.02).

CONCLUSIONS

Our data have demonstrated the presence of prolactin receptors in normal cadaveric pituitary and in most pituitary adenomas, irrespective of histological classification. In particular, elevated prolactin receptor levels were shown in PRL-secreting tumours from patients with markedly increased serum PRL levels. Our study may support several lines of experimental evidence for a specific functional role for PRL in the growth of some pituitary adenomas.

Characterization of prolactin receptor in human brain and choroid plexus

We have studied the binding of 125I-labeled human prolactin (PRL) to membranes from various regions of the human brain (hypothalamus, cerebral cortex, cerebellum and choroid plexus) derived from autopsy specimens. Among the various regions studied, the choroid plexus of both male and female subjects showed the highest specific binding and a clearly detectable specific binding was also observed in the hypothalamus of both sexes, whereas it was very low in other brain regions. No significant sex differences in PRL binding to various brain regions were observed except for the hypothalamus where a higher binding was seen in female subjects. The binding did not vary with the age of the subjects. Moreover, the cause of death and the time elapsed from death to autopsy in this study did not affect the binding significantly. The binding of 125I-labeled human PRL to hypothalamus and choroid plexus membranes from female specimens was inhibited in a dose-dependent manner by both unlabeled human and ovine PRL and by human growth hormone (GH), but not by other polypeptide hormones. Scatchard analysis of the binding revealed the presence of saturable binding sites with low capacity and high affinity for human PRL ligand. These results provide strong preliminary evidence for the presence of PRL binding sites in the human brain.

The endocrine basis of benign breast disorders

There is sound clinical evidence that many benign breast conditions, especially pain, nodularity, and cysts, are likely to have their pathogenesis in hormonal events during reproductive life. Yet the classical theories of persisting hyperestrogenism and luteal progestogen deficiency have not stood up to detailed investigation. Recent work has demonstrated the importance of dynamic hormonal changes and of chronobiological rhythms--daily, menstrual, and seasonal. Recent developments and findings are reviewed to point to the directions in which the basic etiology of these conditions is likely to be discovered.

Presence and characterization of insulin-like growth factor 1 receptors in human benign breast disease

Insulin-like growth factor 1 binding sites were characterized in human benign breast disease. We demonstrated the presence of one high affinity binding site. Chemical cross-linking of [125I]IGF1 to benign breast disease membranes in reducing condition and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) revealed one band with an apparent Mr of 130,000. The specificity of the binding was studied: IGF 2 was a good competitor whereas insulin competed for binding with a potency lower than 1/100 that of IGF1. This IGF1 binding corresponded to the previously described type I IGF receptor (IGF1-R). IGF1-R was assayed in 35 cases of benign breast disease and two samples of normal breast tissue. Forty-three per cent of the lesions were IGF1-R positive. The mean geometric level of specific binding was 1.98% in the whole population, it was significantly lower in adenofibromas (1.55%) than in epithelial hyperplasia (2.5%); it was 2% in dystrophic disease. IGF1-R was undetectable in normal tissue. Considering our previous results showing that almost all the breast cancers contained IGF1-R, these data suggest that the increase in IGF1-R could be a marker of malignant tumor development.