Bioactivity and glycosylation of circulating prolactin in various physiological and pathological conditions

Multiple posttranslational processes modify native PRL and result in the secretion of several PRL isoforms with different bioactivity. Since we observed that serum samples contain non-lactogenic substances able to interfere in Nb2 cell bioassay, in this study we extracted PRL molecules from sera of pregnant and non-pregnant normal adults, fetuses and patients with prolactinoma and evaluated the ability of partially purified PRL to stimulate Nb2 cell proliferation. The preliminary immunopurification of PRL samples, conferred good sensitivity and specificity to PRL biological assay. Whenever possible, bioactivity values were correlated with glycosylated-PRL levels (G-PRL), the major posttranslational modification known to reduce PRL bioactivity. The ratio of bioactive (B-) vs immunoreactive PRL (I-PRL) (B/I) in normal subjects was 0.9 +/- 0.1 (mean +/- SD), and not affected by TRH and sulpiride administration. PRL B/I ratio did not change during pregnancy, both in maternal (0.8 +/- 0.1) and fetal circulation (1.0 +/- 0.01). In patients with prolactinoma PRL B/I ratios (0.8 +/- 0.18) were in the normal range. However, in 2 women with microprolactinoma, with a clear discrepancy between high I-PRL levels and mild clinical features, a significantly reduced PRL B/I ratio was observed (0.51 +/- 0.08 and 0.52 +/- 0.1 respectively). Conversely, a woman with clear clinical features of hyperprolactinemia, but border-line elevated I-PRL levels had a PRL B/I ratio in the upper limit of normal range. No variation in G-PRL vs NG-PRL percentages was observed in all the cases studied. In conclusion, our data show that physiological and pathological conditions of hyperprolactinemia, including fetal life, are associated in the majority of cases, with the secretion of PRL molecules with unchanged mitogenic activity on Nb2 cells. Nb2 PRL bioassay may be an useful tool to explain the discrepancies between clinical features and immunoreactive PRL levels in some particular cases.