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Protein kinase C activity and messenger RNA modulation by estrogen in normal and neoplastic rat pituitary tissue
Background:
Estrogens stimulate pituitary cell growth by prolactin cell hyperplasia in normal rat pituitaries while inhibiting growth of transplantable rat pituitary tumors in vivo and in vitro. The mechanisms by which estrogens stimulate proliferation of normal and hyperplastic pituitary tissues while inhibiting growth of established transplantable tumors is unknown. Protein kinase C (PKC) is involved in proliferation and differentiation of many cells including those of the anterior pituitary gland. PKC has a major role in regulating pituitary hormone synthesis and secretion. Analysis of PKC activity in normal, hyperplastic and transplantable pituitary tumors may provide insight into the mechanism of estrogen action on tumor growth and differentiation in pituitary tissues.
Experimental Design:
Normal female rats and female rats with transplantable pituitary tumors were treated with estradiol 17 beta (E2) in vivo and subsequently analyzed for PKC activity. Rats with GH3 pituitary tumors were treated with E2 in vivo and a GH3 pituitary cell line maintained in vitro were also analyzed for PKC activity after estrogen treatment. The calcium-dependent and calcium-independent PKC isozymes were analyzed by in situ and Northern hybridization in normal and neoplastic pituitary tissue using specific oligonucleotide probes to characterize the subtypes of PKCs present in pituitary tissues. The calcium-dependent PKCs were also analyzed by immunohistochemistry using specific antibodies to PKC alpha, beta and gamma.
Results:
Estrogen treatment increased total PKC activity significantly in normal pituitaries, but did not change total PKC activity in transplantable MtT/F4 and MtT/W15 tumors or in GH3 cells in vivo or in vitro. The mRNAs for PKC alpha and beta were detected in normal and neoplastic pituitary tissues and were increased after estrogen treatment in normal pituitary and MtT/F4 tumors, but not in MtT/W15 tumors. The calcium-independent PKC delta, epsilon and zeta were also detected in normal pituitary and in transplantable pituitary tumors and in GH3 cells by Northern hydridization and the levels of these PKCs were also regulated by estrogens.
Conclusions:
Estrogen modulates PKC levels in pituitary cells with an increase in total PKC activity in hyperplastic pituitaries but not in transplantable pituitary tumors. Estrogen also regulates the mRNA levels of the calcium-dependent and calcium-independent PKC isozymes.