Three activators of protein kinase C, bryostatins, dioleins, and phorbol esters, show differing specificities of action on GH₄ pituitary cells

Phorbol ester tumor promoters such as 12-O-tetradecanoylphorbol acetate (TPA) activate the calcium- and phospholipid-dependent protein kinase C and enhance three biological responses (prolactin release, prolactin synthesis, and cell stretching) in GH₄C₅ rat pituitary cells. We have examined several actions on GH₄C₅ cells of TPA and two other classes of protein kinase C activators, synthetic cell permeant dioleins and bryostatins isolated from the marine bryozoan Bugula neritina. Bryostatins 1 and 2 (B1 and B2, respectively) competed for [³H]phorbol 12,13-dibutyate binding to the protein kinase C complex in intact cells nearly equipotently with TPA. B1 and B2, 1-oleoyl-2-acetylglycerol (OAG) and 1,2-dioctanoylglycerol (Di8) as well as TPA each activated partially purified protein kinase C from GH₄C₅ cells. B1, B2, and TPA each enhanced the acute release of prolactin from GH₄C₅ cells to a similar maximal extent. B1, B2, and TPA also enhanced prolactin synthesis. However, B1 and B2 were only partial agonists because they enhanced prolactin synthesis to a lesser maximal extent than did TPA and, given in combination, they reduced TPA-enhanced prolactin synthesis. OAG and Di8 stimulated prolactin release (to a lesser maximal extent than TPA) and did not stimulate prolactin synthesis. Pretreatment with OAG did not reduce TPA-stimulated prolactin release or synthesis. B2 and TPA induced cell stretching in GH₄C₅ cells, whereas B1, OAG, and Di8 induced little if any stretching. B1, but not B2, given in combination with TPA antagonized TPA-induced stretching but did not reduce thyrotropin-releasing hormone- or epidermal growth factor-induced stretching. We conclude that the bryostatins, phorbol esters, and dioleins bind to the same site on the protein kinase C complex to activate the enzyme, but they alter three biological responses in GH₄C₅ cells with selectivities and efficacies that differ. We propose that different activators of protein kinase C (such as bryostatins, dioleins, and phorbol esters) may elicit different cellular responses by altering the substrate specificity or activating multiple forms of the kinase.