Dephosphorylation of standard prolactin produces a more biologically active molecule: evidence for antagonism between nonphosphorylated and phosphorylated prolactin in the stimulation of Nb2 cell proliferation.

Charge isomers of monomer PRL have been described for all species thus far examined. For the rat, cow, and chicken, at least a major proportion of the more acidic isomers have been shown to be phosphorylated. The physiological relevance of this posttranslational phosphorylation, however, remains unclear. In this study we have compared the growth-promoting activities of dephosphorylated and standard rat PRL (partially phosphorylated) in the widely used Nb2 bioassay. Dephosphorylated PRL was produced by treatment of standard rat PRL with acid phosphatase, resulting in the conversion of a 91.9% nonphosphorylated/8.1% phosphorylated preparation to a 98.6% nonphosphorylated preparation. These two preparations were added separately or in combination to stationary Nb2 cells. Cell number was assessed 3 days later using a colorimetric assay. Dephosphorylated PRL showed significantly higher growth-promoting activity than standard PRL at concentrations up to 10 ng/ml. In the 1-5 ng/ml concentration range, dephosphorylated PRL was twice as active. Given that the conversion of 6.7% phosphorylated PRL to nonphosphorylated PRL resulted in a doubling of activity, one can deduce not only that phospho-PRL acted as an antagonist to nonphosphorylated PRL in this assay, but also that it did not do so on an equimolar basis. Titration of the two PRL preparations produced data confirming this latter deduction. These data are of importance in our understanding of PRL-promoted cell proliferation.