Glandular Kallikrein in Estrogen-induced Pituitary Tumors: Time Course of Induction and Correlation with Prolactin

Glandular kallikrein (a trypsin-like serine protease) is an estrogen-induced and dopamine-repressed protein in the rat anterior pituitary which appears to be associated with lactotrophs. This study examined glandular kallikrein levels in diethylstilbestrol (DES)-induced pituitary tumors in F344 rats and compared it to plasma and pituitary prolactin, and pituitary wet weight. Ovariectomized F344 rats were implanted with Silastic tubes containing 0 or 5 mg DES for 1, 3, 5, 7, or 9 weeks. Glandular kallikrein was measured by microenzymatic assay using d-valylleucylarginyl-p-nitroanilide following trypsin treatment of extracts to activate latent forms of glandular kallikrein. Prolactin was measured by radioimmunoassay. DES induced steady time-dependent increases in pituitary wet weight with 7- and 16-fold increases observed by 5 and 9 weeks, respectively. Growth rates averaged 11.4 mg/week during the first 5 weeks of DES exposure, and then increased to 23.2 mg/week between weeks 5 and 9. Glandular kallikrein total activity (nmol/min/pituitary) increased 130- and 240-fold after 3 and 5 weeks of DES exposure, respectively, and then abruptly plateaued. The specific activity (nmol/min/mg protein) of glandular kallikrein peaked at 3–5 weeks (36-fold increase compared to controls) and then declined as pituitary protein but not glandular kallikrein continued to increase. Total pituitary prolactin constantly rose during DES exposure with 12- and 26-fold increases after 5 and 9 weeks, respectively. Plasma prolactin levels also continuously rose during exposure to DES with 130- and 290-fold increases after 5 and 9 weeks, respectively. No major strain differences were found with regard to sensitivity to the acute effects of estrogen or dopaminergic stimulation on glandular kallikrein induction. DES-induced pituitary tumors in F344 rats are well known to arise via lactotroph proliferation, and the striking elevation in glandular kallikrein and prolactin during the early phases of tumor growth provide further support for a localization of glandular kallikrein in lactotrophs. However, the abrupt stabilization in glandular kallikrein levels by week 5 was unexpected and may signal a biochemical transformation of the tissue during tumor progression.