Endocrine Abstracts

Background: The prolactin/vasoinhibin axis constitutes a novel endocrine axis in which the generation, secretion, and actions of the pituitary hormones prolactin and vasoinhibins are under control of the hypothalamus, the pituitary gland, and of regulators in the target tissue microenvironment. Vasoinhibins are generated in the pituitary gland and in multiple target tissues through proteolytic cleavage of prolactin; it is not known wether they are also produced in the circulation.

Methods: Human serum or plasma and recombinant human prolactin (corresponding to protein data bank identification P01236) were incubated at 37 °C, alone and in combination, with a variation of pH, time, and buffer composition. The samples were resolved on sodium dodecyl sulfate polyacrylamide gel electrophoresis and Western Blotting was performed using polyclonal and monoclonal, epitope-mapped anti-prolactin and anti-vasoinhibin antibodies.

Results: Sodium dodecyl sulfate polyacrylamide gel electrophoresis and Western Blotting analyses demonstrated an anti-prolactin antibody immunoreactive band with an apparent molecular mass of 23 kDa, consistent with the presence of the recombinant prolactin-protein. Samples incubated throughout a time period ranging from 10 minutes to 24 hours at pH 7.4 did not demonstrate immunoreactive bands lower than 23 kDa. An anti-vasoinhibin immunoreactive band featuring an apparent molecular mass of 17 kDa was intermittently observed in samples incubated at pH 3.4.

Conclusion: Vasoinhibin generation by enzymatic cleavage of prolactin does not occur in human serum under physiological conditions. Thus, the generation of vasoinhibins seems to be restricted to the pituitary gland and the target tissue level. A limited proteolysis of prolactin, resulting in the generation of a prolactin-fragment with an apparent molecular weight of 17 kDa seems to occur at acid pH.