Raised serum prolactin in rheumatoid arthritis: genuine or laboratory artefact?

S. Ram, D. Blumberg1, P. Newton1, N. R. Anderson and R. Gama

Departments of Clinical Chemistry and 1 Rheumatology, New Cross Hospital, Wolverhampton, West Midlands WV10 0QP, UK.

Correspondence to: R. Gama, Clinical Chemistry, New Cross Hospital, Wolverhampton, West Midlands. WV10 0QP, UK. E-mail: rousseau.gama{at}rwh-tr.nhs.uk


    Abstract
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 Abstract
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 Material and methods
 Results
 Discussion
 References
 
Objectives. Serum prolactin concentrations have been reported as higher, similar or lower in patients with rheumatoid arthritis (RA) compared with control subjects. We investigated whether low biological activity macroprolactin (a prolactin antibody complex), which is detected variably in different prolactin immunoassays, could account for the discrepant total prolactin results reported in RA.

Methods. We compared serum total prolactin and free prolactin in 60 women with RA and 31 female controls.

Results. No subject had hyperprolactinaemia or macroprolactinaemia. Serum concentrations of total and free (monomeric) prolactin were higher (P<0.05) in women with RA [mean (S.D.), 225.6 (104.6) and 201.6 (95.4) mU/l respectively] compared with controls [175.0 (68.5) and 154.0 (60.9) mU/l respectively].

Conclusions. We report higher serum free prolactin concentrations in women with RA compared with control subjects. This result indicates that the higher serum total prolactin levels in patients with RA are the consequence of increased free prolactin concentrations and are not due to macroprolactin.

KEY WORDS: Prolactin, Macroprolactin, Big prolactin, Big big prolactin, Rheumatoid arthritis


    Introduction
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 Abstract
 Introduction
 Material and methods
 Results
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It has been suggested that prolactin may have a pathogenetic role in rheumatoid arthritis (RA), since in vitro studies have shown that prolactin enhances inflammatory responses [1–4]. Indeed, increased serum prolactin concentrations have been reported in patients with RA [5–10]. Furthermore, dopamine agonists, which suppress pituitary secretion of prolactin, may be a useful adjunct to treatment in patients with RA [11–13]. Prolactin circulates in several different molecular forms, predominantly monomeric prolactin but also as small but variable amounts of ‘big prolactin’ and ‘big, big prolactin’ (macroprolactin). The binding of prolactin to an immunoglobulin forms macroprolactin. Since macroprolactin is less physiologically active and is less effectively cleared than free unbound prolactin, the total concentration of serum prolactin increases. Depending on the immunoassay used, macroprolactin may account for 4–5% of cases of hyperprolactinaemia [14, 15] and has also been reported in normoprolactinaemic samples [16].

Systemic lupus erythematosus (SLE), an autoimmune disease with an increased prevalence of serum autoantibodies, is associated with macroprolactinaemia [16, 17]. RA is also associated with an increased frequency of circulating antibodies and macroprolactin has been reported in patients with RA [18]. It is therefore possible that macroprolactin could account for the hyperprolactinaemia observed in RA, particularly since increased serum prolactin in RA has been reported in most [5–10] but not all [19–23] studies, possibly due to the use of different prolactin assays with varying macroprolactin cross-reactivity [24, 25].

This is the first report investigating the contribution of low biological activity macroprolactin to circulating prolactin in RA by comparing total and free (monomeric) prolactin concentrations in women with and without RA.


    Material and methods
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 Abstract
 Introduction
 Material and methods
 Results
 Discussion
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Patients
Sixty women with RA and 31 women with osteoarthritis, who served as controls, were studied. A blood sample for serum prolactin was collected at least 2 h after awakening, between 09.30 and 12.00 h. Exclusion criteria included endocrine and any other disease or medication known to affect serum prolactin concentrations. All women gave informed written consent according to the Declaration of Helsinki to participate in this study, which was approved by the Wolverhampton District Local Research Ethics Committee.

Analytical methods
Polyethylene glycol (PEG) was used to precipitate and remove big prolactin and macroprolactin from serum samples [26, 27]. Serum prolactin was therefore measured before (total prolactin) and after (free or monomeric prolactin) precipitation with PEG 6000 [27] using the Architect prolactin assay (Abbott Laboratories, Diagnostics Division, IL, USA). Respective inter-assay and intra-assay coefficients of variation for serum prolactin were 4.7 and 3.8%. Prolactin recovery <40% is indicative of macroprolactin [27].

Statistical analysis
Data were normally distributed. Unpaired and paired Student's t tests were therefore used to assess differences in variables between groups and within groups respectively. Results are expressed as mean (S.D.). Pearson's linear correlation was used to measure the significance of association between variables.


    Results
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 Material and methods
 Results
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Demographic, clinical and biochemical data are shown in Table 1. No subjects had hyperprolactinaemia or macroprolactinaemia. Serum concentrations of total and free (monomeric) prolactin were higher (P<0.05) in women with RA [225.6 (104.6) and 201.6 (95.4) mU/l respectively] than in controls [175.0 (68.5) and 154.0 (60.9) mU/l respectively]. Although serum prolactin fell following PEG precipitation in women with RA and controls, this was not significant (P = 0.1936 and P = 0.2877 respectively).


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TABLE 1. Clinical and biochemical characteristics of women with rheumatoid arthritis and their controls

 
In women with RA, there were no correlations between ESR and total prolactin (r = 0.1163; P = 0.3762) or free prolactin (r = 0.07725; P = 0.5574), but ESR inversely correlated with the percentage of free prolactin of the total prolactin (r = –0.3278; P = 0.0106).


    Discussion
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 Abstract
 Introduction
 Material and methods
 Results
 Discussion
 References
 
Although no subjects in this study had hyperprolactinaemia, we report higher serum prolactin concentrations in women with RA compared with controls. This result is consistent with other studies similarly reporting higher total prolactin concentrations in patients with RA than in controls [5–10], but differs from studies reporting similar [19, 20] or lower [21, 22] prolactin concentrations in patients with RA. In addition we report for the first time higher free prolactin concentrations in subjects with RA. It is therefore unlikely that the different total prolactin concentrations reported in various studies are solely due to varying macroprolactin cross-reactivity in different prolactin assays. The difference in these studies, however, could be due to selection of patient groups and type 1 and 2 statistical errors.

Macroprolactinaemia is well recognized in SLE [16, 17] and the difference in the prevalence of macroprolactinaemia between SLE and RA may be related to the type of circulating immunoglobulin (Ig). Macroprolactin is usually formed by the binding of prolactin to an IgG antibody. In RA the predominant circulating immunoglobulin is an IgM antibody, which may not bind to prolactin.

Prolactin has a role in immunomodulation and it has been proposed that prolactin is a risk factor for the development of autoimmunity [1–4]. However, it remains unclear whether the higher prolactin concentrations are the cause or consequence of RA. There is increased risk of developing RA post-partum and this further increases five-fold if breastfeeding [28–30] and disease activity in RA improve with dopamine agonists [11–13]. This suggests that prolactin may have a role in the pathogenesis, or at least the modulation, of disease activity in RA. However, stress increases serum prolactin concentration. It is therefore possible that stress associated with RA may be responsible for the increased prolactin concentrations in RA, but this is unlikely because cortisol, which also increases with stress, has been reported to be similar or lower in subjects with RA than in controls [10, 20, 21].

In summary, we confirm higher free prolactin concentrations in subjects with RA. We have found no evidence to support the notion that low biological activity macroprolactin contributes to the elevated prolactin concentrations observed in RA.

The authors have declared no conflicts of interest.


    References
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 Abstract
 Introduction
 Material and methods
 Results
 Discussion
 References
 

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Submitted 8 March 2004; Accepted 15 June 2004





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