Abstract
Active phorbol esters such as TPA (12-0-tetra-decanoylphorbol-13-acetate) inhibited growth of mammary carcinoma cells (MCF-7 > BT-20 > MDA-MB-231 > = ZR-75-1 > HBL-100) with the exception of T-47-D cells presumably by interacting with the phospholipid Ca 2+ -dependent protein kinase (PKC). The non-responsive T-47-D cells exhibited the lowest PKC activity. A rapid (30 min) TPA-dependent translocation of cytosolic PKC to membranes was found in the five TPA-sensitive cell without affecting cell growth. However, TPA-treatment of more than 10 hours inhibited reversibly the growth of TPA-responsive cells. This effect coincided with the complete loss of cellular PKC activity due to the proteolysis of the translocated membrane-bound PKC holoenzyme (75K) into 60K and 50K PKC fragments. Resumption of cell growth after TPA-removal was closely related to the specific reappearance of the PKC holoenzyme activity (75K) in the TPA-responsive human mammary tumor cell lines suggesting an involvement of PKC in growth regulation.
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