Increased protein kinase C delta in mammary tumor cells: relationship to transformation and metastatic progression

Relatively little is known about the molecular mechanisms of tumor promotio n/progression in mammary carcinogenesis. Increased protein kinase C (PKC) a ctivity is known to promote tumor formation in several tissues; however, it s role in mammary carcinogenesis is not yet known. To determine if individu al PKCs may selectively regulate properties of mammary tumor cells, we comp ared PKC isozyme levels in mammary tumor cell lines,vith low, moderate and high metastatic potential. All three cell lines expressed alpha, delta, eps ilon and zeta PKCs; however, PKC delta levels were relatively increased in the highly metastatic cells. To determine if increased PKC delta could cont ribute to promotion/progression, we overexpressed PKC delta in the low and moderately metastatic cell lines, PKC delta overexpression had no significa nt effect on growth of adherent cells, but significantly increased anchorag e-independent growth. Conversely, expressing the regulatory domain of PKC d elta (RD delta), a putative PKC delta inhibitory fragment, inhibited anchor age-independent growth. The efficacy of RD delta as a PKC delta inhibitor w as demonstrated by showing that RD delta selectively interfered with PKC de lta subcellular location and significantly interfered with phosphorylation of the PKC cytoskeletal substrate, adducin, PKC-dependent phosphorylation o f cytoskeletal substrate proteins, such as adducin, provides a mechanistic link between increased PKC delta activity and phenotypic changes in cytoske letal-dependent processes such as migration and attachment, two processes t hat are relevant to metastatic potential. The reciprocal growth effects of expressing PKC delta and RD delta as gain and loss of function constructs, respectively, provide strong evidence that PKC delta regulates processes im portant for anchorage-independent growth in these mammary tumor cells.