Prolactin decrease and shift to a normal-like isoform profile during treatment with quinagolide in a patient affected by an invasive prolactinoma

Macroprolactinemia: diagnostic, clinical, and pathogenic significance

Macroprolactinemia is characterized by a large molecular mass of PRL (macroprolactin) as the main molecular form of PRL in sera, the frequent elevation of serum PRL (hyperprolactinemia), and the lack of symptoms. Macroprolactin is largely a complex of PRL with immunoglobulin G (IgG), especially anti-PRL autoantibodies. The prevalence of macroprolactinemia is 10–25% in patients with hyperprolactinemia and 3.7% in general population. There is no gender difference and a long-term followup demonstrates that macroprolactinemia develops before middle age and is likely a chronic condition. Polyethylene-glycol- (PEG-) precipitation method is widely used for screening macroprolactinemia, and gel filtration chromatography, protein A/G column, and I125-PRL binding studies are performed to confirm and clarify its nature. The cross-reactivity of macroprolactin varies widely according to the immunoassay systems. The epitope on PRL molecule recognized by the autoantibodies is located close to the binding site for PRL receptors, which may explain that macroprolactin has a lower biological activity. Hyperprolactinemia frequently seen in macroprolactinemic patients is due to the delayed clearance of autoantibody-bound PRL. When rats are immunized with rat pituitary PRL, anti-PRL autoantibodies are produced and hyperprolactinemia develops, mimicking macroprolactinemia in humans. Screening of macroprolactinemia is important for the differential diagnosis of hyperprolactinemia to avoid unnecessary examinations and treatments.

Immunoglobulin G subclasses and prolactin (PRL) isoforms in macroprolactinemia due to anti-PRL autoantibodies

Although macroprolactinemia due to antiprolactin (anti-PRL) autoantibodies is not uncommon among hyperprolactinemic patients, the pathogenesis of such macroprolactinemia is still unknown. We examined IgG subclasses of anti-PRL autoantibodies by enzyme immunoassay, and PRL phosphorylation and isoforms by Western blotting, mass spectrometry, and two-dimensional electrophoresis in six patients with anti-PRL autoantibodies and in 29 controls. PRL-specific IgG subclasses in patients with anti-PRL autoantibodies were heterogeneous, but five of six patients showed IgG4 predominance, which is known to be produced by chronic antigen stimulation. Western blot and mass spectrometric analyses revealed that human pituitary PRL was phosphorylated at serine 194 and serine 163, whereas serine 163 in serum PRL was dephosphorylated. On two-dimensional electrophoresis, serum PRL mainly consisted of isoform with isoelectric point (pI) 6.58 in control hyperprolactinemic patients, whereas acidic isoforms (pIs 6.43 and 6.29) were also observed in patients with anti-PRL autoantibodies. Our data first demonstrate that human pituitary PRL is serine phosphorylated and partially dephosphorylated in serum, and suggest that the acidic isoforms may give rise to chronic antigen stimulation in patients with anti-PRL autoantibodies.

Hyperprolactinemia in asymptomatic patients is related to high molecular weight posttranslational variants or glycosylated forms

Circulating human Prolactin (PRL) exists in different variants related to posttranslational modifications, dimerization or association with other serum proteins. Compared to monomeric prolactin these variants usually have little or no biologic activity and include BigBig (BB PRL), Big (B PRL), and Glycosylated forms (G PRL). The aim of the present study was to assess levels of BB PRL, B PRL, little PRL (L PRL) and G PRL in hyperprolactinemic patients with no menstrual alterations or galactorrhea. L PRL, B PRL, and BB PRL were identified by gel filtration chromatography on Sephadex G-100; G PRL and NG PRL were identified by chromatography on Concanavalin A Sepharose. PRL was measured by IRMA DPC. Eleven women, aged 22-50 yrs, were studied for: breast dysplasia (1), controlled hypothyroidism (3), dysmenorrhea (3), microadenoma follow-up (2), and gynecological control (2). Pituitary MRI was normal in all but one patient, who had a microadenoma discovered by Magnetic Resonance Imaging. Six patients had normal L PRL levels, and their hyper PRL was due to excess BPRL or BB PRL. Five patients had increased L PRL levels, but excess G PRL. Patients harboring molecular PRL variants do not present the symptoms typical of the hyperprolactinemic syndrome. Furthermore in patients with clinically controlled prolactinomas the presence of PRL variants should be ruled out to avoid an unnecessary increase of dopamine agonist dosage.