Rheumatology Advance Access originally published online on October 25, 2005
Rheumatology 2006 45(1):97-101; doi:10.1093/rheumatology/kei115
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Serum free prolactin concentrations in patients with systemic lupus erythematosus are associated with lupus activity
Research Unit in Reproductive Medicine, Hospital de Ginecología y Obstetricia ‘Luis Castelazo Ayala’, Instituto Mexicano del Seguro Social, México City, México.
Correspondence to: A. Leaños-Miranda, Research Unit in Reproductive Medicine. Don Luis #111, Col. Nativitas, México 03500, D.F., México. E-mail: alfredolm{at}yahoo.com
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Abstract |
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Objective. To determine in patients with systemic lupus erythematosus (SLE) the relationships among serum total and free prolactin (PRL) levels, isoforms of PRL and lupus activity.
Methods. In a cross-sectional study, 259 patients with SLE were tested for serum total PRL, serum free PRL, and PRL in fractions obtained after gel filtration chromatography (in 70 sera taken at random) by immunoradiometric assay based on disease activity.
Results. A significant correlation between direct PRL and free PRL levels was found in patients with and without lupus activity (r = 0.475, P<0.001); however, this was less so for non-active patients than for active patients (r=0.433, P<0.001 and r=0.909, P<0.001, respectively). SLE Disease Activity Index (SLEDAI) scores correlated positively with serum free PRL levels (r=0.314, P<0.001) and percentage of little PRL (r=0.33, P=0.005) and negatively with percentage of big big PRL (r=–0.3, P=0.012). Patients with active disease had higher serum free PRL levels (median 12.6 vs 9.3 ng/ml, P<0.001), higher percentages of little PRL (83.1 ± 21.2 vs 63.6 ± 24.8%, P=0.011) and lower percentages of big big PRL (9.4 ± 18.0 vs 25.2 ± 24.3%, P=0.031). Different clinical manifestations and serological parameters of lupus disease activity were more frequent in patients with free hyperprolactinaemia than in patients with normal serum free PRL levels (such as neurological manifestations, renal involvement, serositis, haematological manifestations, anti-double-stranded DNA and hypocomplementaemia; P<0.021).
Conclusion. An increase in serum free PRL levels, higher percentages of little PRL and lower percentages of big big PRL proved to be factors related to lupus activity in a subset of patients with SLE. These novel data must be taken into account in future studies aiming to establish a relationship between PRL and disease activity in SLE.
KEY WORDS: Prolactin, Free prolactin, Isoforms of prolactin, Systemic lupus erythematosus, Disease activity
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Introduction |
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Immune response abnormalities in systemic lupus erythematosus (SLE) may result from the interplay of genetic, environmental and hormonal factors. Elevated serum prolactin (PRL) levels have been reported in SLE patients, and hyperprolactinaemia (HPRL) occurs in approximately 15–31% of SLE patients [1–3]. In the majority of SLE patients (66%), the causes are unknown and this status is classified as idiopathic HPRL [4]. In murine models of lupus, there is clear evidence that PRL has a deleterious effect on disease activity [5–7]. In contrast, clinical reports have led to equivocal data on the relationship between serum PRL levels and clinical and/or serological activity [1, 3]. Serum PRL from normal subjects and most patients with HPRL circulates in several isoforms, including predominantly free PRL [monomeric (little PRL), molecular weight 23 kDa] and lesser amounts of big PRL and big big PRL (45–50 and >100 kDa respectively) [8–12]. SLE patients with HPRL have a high prevalence of big big PRL or macroprolactinaemia (31.7%), detected as PRL (monomeric)–IgG complex with a molecular weight of
150 kDa [4]. Patients with HPRL associated with the presence of macroprolactinaemia, independently of the nature of big big PRL (i.e. due or not due to anti-PRL autoantibodies), frequently do not present clinical symptoms of HPRL, such as amenorrhoea and galactorrhoea in women and impotence in men [4, 12–16]. Interestingly, SLE patients with idiopathic HPRL associated with macroprolactinaemia due to anti-PRL autoantibodies had less clinical and serological activity than those with idiopathic HPRL who were macroprolactin-negative [4, 9]. These findings suggest that high molecular weight PRL isoforms have low biological activity in vivo compared with little PRL. To date, it is well recognized that the molecular heterogeneity of PRL is present in sera from SLE patients [4, 9, 16–18]. Independently of the nature of big big PRL, its presence in SLE patients has been associated with lower clinical and serological activity of disease [9, 17]. However, very active disease has also been reported [18]. All these data, taken together, suggest that not only the serum PRL level or the presence of macroprolactinaemia, but also the concentration and proportion of little or monomeric PRL could be associated with lupus activity.
The previous data led us to re-evaluate the relationship between serum PRL and disease activity in a larger number of patients with SLE. We measured immunological PRL activity in total serum (total or direct PRL), in the serum supernatant precipitated with 12.5% polyethylene glycol (PEG) (free or monomeric PRL), and in fractions obtained after gel chromatography, to investigate these concentrations in relation to lupus activity.
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The study was approved by the local Human Ethics and Medical Research Committee of the Instituto Mexicano del Seguro Social, and written informed consent was obtained from all subjects, who participated voluntarily in this study.
A group of 259 consecutive Mexican mestizo patients who fulfilled four or more of the American College of Rheumatology revised criteria for SLE [19] were included. A venous blood sample was drawn between 9:00 and 11:00 a.m. under basal conditions without hormonal or drug stimulus. Sera were stored at –35°C until used. None of these patients had obvious causes of HPRL. Disease activity was classified according to a published index (SLE Disease Activity Index, SLEDAI) [20]. For this study, any value above 4 was considered active disease.
Determination of serum direct or total PRL levels
PRL concentration in serum was measured by immunoradiometric assay (RIA-gnost Prolactin; CIS Bio International, Gif-sur-Yvette, France) as previously described [21].
Determination of serum free or monomeric PRL levels
Free PRL was extracted from the serum using PEG, as previously described [4, 21, 22]. This procedure precipitates and removes the PRL isoforms with high molecular weight (big PRL and big big PRL) [22].
Gel filtration chromatography
Gel filtration was performed in 70 sera taken at random using a column (60 x 1 cm) of Sephadex G-100 Superfine (Pharmacia) as previously described [22], and PRL immunoreactivity from each fraction was determined by ultrasensitive enzyme immunoassay as previously described [23].
Statistical analysis
The significance of differences between continuous variables was determined with the non-paired Student's t-test (or the Mann–Whitney U-test for non-normal distribution variables). The significance of differences between categorical variables was determined with the 
2 test with Yates's continuity correction (or Fisher's exact test for small samples). The linear relationship between PRL concentrations and SLEDAI or isoforms of PRL was assessed with Pearson's correlation coefficient. A two-tailed P<0.05 was considered statistically significant.
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Results |
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General description
A total of 259 patients with SLE (243 women and 16 men) were studied. Mean age was 35.5 ± 11.2 yr and median disease duration was 73 months (range 1–927). The median serum total PRL was 13.8 ng/ml [range 1.2–281.7; P<0.001 when compared with median serum free PRL (9.8 ng/ml, range 1.3–105.6)]. Although a statistically significant correlation between direct PRL and free PRL levels was found (r = 0.475, P<0.001; Fig. 1A), its coefficient of determination was low (R2 = 0.23). Gel filtration in the sera of 70 patients showed that different PRL isoforms were present. The mean of the proportion of big big PRL was 22.3 ± 24.0% (range 0–89.8), big PRL 10.5 ± 5.2% (range 0–27.8), and little PRL 67.2 ± 25.2% (range 5.5–100). The relationship between the percentage of big big PRL and serum free PRL levels is shown in Fig. 1B. As can be seen, normal or elevated serum free PRL levels were found in sera from SLE patients with a predominant pattern of macroprolactinaemia (
50%) or with substantial quantities of macroprolactinaemia (20–50%).
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Serum PRL levels, PRL isoforms and lupus disease activity
On the basis of the SLEDAI (>4 points), 50/259 patients (19.3%) were identified with lupus activity. There was no significant difference in serum direct PRL levels between active and non-active patients (mean 22.7 and 20.1 ng/ml, respectively, P = 0.53). Similarly, the median of serum free PRL level was higher in active patients than in non-active patients; in contrast to direct PRL, it did display a significant difference (median 12.6 and 9.3 ng/ml, respectively; P<0.001). On the other hand, there was a significant difference in the proportion of different PRL isoforms between active and non-active patients [in 13/70 sera (18.6%) with a gel filtration done at random; this revealed a proportion of active patients similar to that in the total group (50/259, 19.3%)]. Therefore, it is assumed that it was a representative sample of all the patients. The percentages of big big PRL and big PRL were both lower in active patients than in non-active patients (mean 9.4 and 7.5% vs 25.2 and 11.2%, respectively; P<0.031). In contrast, the percentage of little or monomeric PRL was higher in active patients than in non-active patients (mean 83.1 and 63.3% respectively; P = 0.011). A similar finding was demonstrated when another higher SLEDAI cut-off score (10) was used to compare direct and free PRL levels and PRL isoforms.
Correlation among serum PRL levels, PRL isoforms and SLEDAI, C3, C4 and anti-double-stranded DNA
A significant positive correlation was observed between direct PRL levels and anti-double-stranded DNA (anti-dsDNA; r = 0.15, P = 0.015), but not with SLEDAI (r = 0.09, P = 0.15), C3 (r = –0.10, P = 0.1) or C4 (r = –0.06, P = 0.35). In contrast, there was a significant correlation between free PRL levels and SLEDAI (r = 0.31, P<0.001), anti-dsDNA (r = 0.2, P = 0.002), C3 (r = –0.33, P<0.001) and C4 (r = –0.16, P = 0.013). There was a significant negative correlation between the percentage of big big PRL and SLEDAI (r = –0.3, P = 0.012). In contrast, a significant positive correlation was observed between the percentage of little PRL and SLEDAI (r = 0.33, P = 0.005). In the same way, there was a significant positive correlation between the percentage of big big PRL and C3 (r = 0.33, P = 0.005), and the correlation between the percentage of little PRL and C3 was negative and significant (r = –0.37, P = 0.002). Additionally, the correlation between direct PRL and free PRL was significant in patients with or without activity lupus. However, this was less so for non-active patients vs active patients (r = 0.433, P<0.001 and r = 0.909, P<0.001, respectively) (Fig. 1A). Therefore, in some non-active patients with a high direct PRL level, the measurements of free PRL were substantially smaller. In contrast, the measurements of direct PRL were similar to the measurements of free PRL in all active patients.
Association between free HPRL and clinical and serological findings
For this analysis, the SLE patients were divided into two groups: without and with free HPRL. Free HPRL was defined as a free PRL value >13.9 ng/ml. This result represented the mean + 3 S.D. from 57 normal healthy women without direct HPRL (i.e. with serum direct PRL levels <20 ng/ml). Free HPRL was found in 56/259 patients (21.6%, 95% confidence interval 16.6–26.6%). The mean SLEDAI score among patients with free HPRL was 5.5 ± 7.7, higher than that in patients with a normal free PRL level (2.4 ± 4.4); the difference was statistically significant (Mann–Whitney U-test, P<0.001). The frequencies of patients with SLEDAI scores at three different cut-offs (>4, >10 and >15) was higher in patients with free HPRL than in patients with normal serum free PRL levels (P<0.028; Table 1). There was a significant difference in the frequency of several clinical manifestations and serological parameters between SLE patients with free HPRL and normal free PRL (neurological manifestations, renal involvement, serositis, haematological manifestations, anti-dsDNA and hypocomplementaemia; P<0.047) (Table 1). On the other hand, mucocutaneous manifestations appeared to be more common in the free HPRL group, although the difference did not reach statistical significance; the remaining variables (vasculitis, arthritis and myositis) appeared to be similar in the two groups. There was no association between specific organ involvement and the percentage of little or big big PRL.
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Discussion |
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Previous clinical studies in SLE patients have focused only on serum direct PRL levels and their association with disease activity, not taking into account the presence of different isoforms of PRL. In this study, different isoforms of PRL in serum from SLE patients with normal and high serum PRL levels were identified. The presence of these isoforms can be attributed to post-translational modifications (aggregates of little or monomeric PRL, and PRL bound to binding proteins). These modifications of PRL may affect biological properties differently from immunoreactivity [24, 26]. The major circulating isoform of PRL is a 23-kDa single-chain polypeptide (little PRL), which constitutes up to 80% of the total PRL in the serum of normal subjects and most patients with HPRL [25]. Several studies have reported that women with HPRL caused by the presence of macroprolactinaemia do not manifest the clinical symptoms commonly associated with HPRL (such as galactorrhoea and menstrual disturbances) and that they are even fertile. In contrast, women with HPRL whose major circulating isoform is little PRL present galactorrhoea and amenorrhoea and are infertile [13–16, 26]. The lack of clinical symptoms of HPRL cannot be attributed to low biological activity of big big PRL; the biological activity (Nb2 cell assay) of samples containing big big PRL (independently of its nature) was similar to that of samples containing little PRL [9, 14]. However, in vivo big big PRL may not exert enough action, because it does not easily cross the capillary walls to reach their target tissues due to its high molecular weight.
In this study, it was found that serum free PRL level was higher in patients with active disease (evaluated by SLEDAI score >4, >10 and >15) than in patients with non-active disease. Moreover, there was a significant correlation between serum free PRL level and SLEDAI, anti-dsDNA, C3 and C4. In contrast, although the serum direct PRL level was higher in patients with active disease, no clinical and serological significance was observed in lupus activity. When SLE patients were analysed according to the presence or absence of free HPRL, it was demonstrated that free HPRL was associated with particular clinical manifestations (such as neurological manifestations, renal involvement, serositis, haematological manifestations and serological disease markers, namely complement and anti-dsDNA). On the other hand, when the percentages of PRL isoforms were analysed, it was found that percentage of big big PRL was higher in non-active patients than in patients with active disease, and there was a significant negative correlation between big big PRL and SLEDAI. In contrast, the opposite, with little PRL isoform, was found: the percentage of little PRL was higher in patients with active disease than in patients with non-active disease and a significant positive correlation between little PRL and SLEDAI. These findings support those previously obtained by researchers in our group [9] as well as by Cruz et al. [17], showing that patients with significant quantities of macroprolactin, despite having high serum total PRL levels, have less lupus activity than patients in whom the predominant circulating PRL isoform is little PRL. In the same manner, García et al. [18] found very active disease in three of four patients with macroprolactinaemia; they focused their interpretation on the presence of macroprolactin. However, all three patients in whom active disease was reported [18], despite having macroprolactinaemia, also had free HPRL, ranging from 14.4 to 36.4 ng/ml. In contrast, the non-active patient with a predominant pattern of big big PRL had a serum free PRL level of 6.4 ng/ml; this may explain the lupus activity in the patients described. Therefore, our results, as well as those of García et al., show that high serum free PRL levels are associated with lupus activity independently of the presence of macroprolactinaemia.
The inconsistencies in the clinical findings reported thus far in studies that have examined the association between PRL and lupus activity can be explained by the findings of this study, which have not been taken into account in previous reports. One finding of this study suggests that the presence of PRL isoforms of high molecular weight in combination with the serum free or monomeric PRL level is associated with lupus activity. The major source of circulating monomeric PRL is the pituitary gland, and bromocriptine, a dopaminergic agonist, lowers serum monomeric PRL levels by inhibiting PRL secretion from the anterior pituitary. This mechanism may explain the beneficial effects of bromocriptine on disease activity in SLE [5, 27, 28]. Another important fact arising from the present study is that SLE patients with a serum free PRL level above 10 ng/ml (a value below 10 ng/ml was present in the non-active patients studied) can benefit from drugs that decrease PRL secretion. Additionally, in a recent study, Peeva et al. [29] demonstrated in mice that treatment with PRL, which causes mild to moderate HPRL, similar to that present in SLE patients, breaks tolerance and induces a lupus-like disease in non-spontaneously autoimmune mice with a susceptible genetic background, suggesting that only a subset of SLE patients are likely to have PRL-responsive disease. Further studies on the role of PRL in SLE are needed, even if its importance is limited to a minority subgroup of SLE with PRL-modulated disease that can benefit from drugs that decrease PRL secretion or block its biological action at PRL receptor level.
In conclusion, the presence of different isoforms of PRL in SLE patients was detected. Elevated serum direct or total PRL levels were not associated with disease activity. In contrast, elevated serum free PRL levels and free HPRL were associated with disease activity, as evaluated by SLEDAI score, as well as specific organ involvement. On the other hand, patients with higher percentages of big big PRL or lower percentages of little PRL had less lupus activity. However, neither normal nor higher serum free PRL levels are associated with the presence of an exclusive pattern of PRL. Taken together, the data suggest that both serum free PRL levels and the detection of individual and/or combinations of PRL isoforms in sera of patients with SLE might prove of value in understanding the clinical or pathological relevance of PRL in autoimmune diseases. Also, all these data may explain why some clinical studies have failed to find an association between PRL and disease activity in SLE patients.
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Acknowledgments |
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This work was supported by grants from the Fondo para el Fomento de la Investigación Médica (FOFOI)-IMSS FP-2003/078 and FP-2004/462 (A.L.-M).
The authors have declared no conflicts of interest.
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