Elevated serum bioactive prolactin concentrations in patients with systemic lupus erythematosus are associated with disease activity as disclosed by homologous receptor bioassays.

OBJECTIVE

To assess the bioactivity of circulating prolactin (PRL) in serum samples from patients with systemic lupus erythematosus (SLE) using 2 novel homologous in vitro bioassays, and to correlate PRL bioactivity with lupus activity.

METHODS

Serum samples from 98 SLE patients with and without disease activity were tested for immunoreactive and bioactive concentrations of PRL.

RESULTS

Patients with active disease exhibited higher bioactive serum PRL levels in homologous bioassays (p <or= 0.013). In contrast, bioactivity in Nb2 cells was similar between patients with and without activity. The bioactive/immunoreactive PRL ratio (BA/IA) in homologous bioassays was significantly higher in patients with both clinical manifestations and serological indicators of lupus disease activity. SLE patients with idiopathic hyperprolactinemia (HPRL) and macroprolactinemia (MPRL) had low SLEDAI scores, and the BA/IA ratio in homologous bioassays was significantly lower compared to those with idiopathic HPRL and no MPRL. There was a negative but significant correlation between MPRL and BA/IA in homologous bioassays (p < 0.001), but not when the heterologous bioassay was employed.

CONCLUSION

Elevated serum bioactive PRL levels revealed by homologous bioassays were associated with disease activity, as well as with specific organ involvement. Big big PRL or macroprolactin is a PRL variant with reduced bioactivity towards its homologous receptor, and this altered bioactivity may contribute to the lower disease activity and absence of symptoms related to HPRL in SLE patients. These novel data must be considered in future studies to establish a relationship between PRL and disease activity in SLE.