DETECTION OF A KALLIKREIN IN THE MOUSE LACTATING MAMMARY-GLAND - A POSSIBLE PROCESSING ENZYME FOR THE EPIDERMAL GROWTH-FACTOR PRECURSOR

Kallikreins are a multigene subfamily of serine proteases that may hav e a role in processing precursors of polypeptide hormones and growth f actors. The epidermal growth factor (EGF) immunoreactivity in mouse mi lk is derived from the membrane-bound EGF precursor located on the lum enal border of the alveolar cells in the mammary gland. Release of EGF into the milk requires the hydrolysis of the EGF precursor at Arg-X c leavage sites. We report the presence of a candidate EGF precursor-pro cessing enzyme in the lactating mouse mammary gland. Kallikrein transc ripts in the mouse lactating mammary gland were detected by primer-dir ected enzyme amplification of complementary DNA (cDNA). Primers to sel ected consented regions of the kallikrein cDNA resulted in an amplifie d product of the predicted size (573 basepairs). Sequence analysis of the product over three nonconserved regions identified mGK-6 (mouse re nal kallikrein) as the primary kallikrein in BALB/c mouse lactating ma mmary gland. Transcription products for the EGF-binding protein (mGK-9 ), mGK-1, mGK-3, and mGK-4 were not detected by enzyme amplification w ith specific primers corresponding to these kallikrein cDNAs, Positive immunohistochemical staining of the apical membrane of mammary alveol ar cells was detected with a polyclonal antiserum to mouse kallikrein. Incubation of cell membranes isolated from lactating mammary glands r eleased soluble EGF-immunoreactive material. Aprotinin partially inhib ited the release of this material, whereas other protease inhibitors, such as leupeptin, benzamidine, and limabean trypsin inhibitor, had no detectable effect. These results support the hypothesis that the rele ase of EGF-immunoreactive material into the milk is in part dependent upon a kallikrein enzyme (mGK-6) in the BALB/c mouse lactating mammary gland.