Induction of a less aggressive breast cancer phenotype by protein kinase Cα and -β overexpression

To address the isoenzyme-specific involvement of protein kinase C (PKC) in breast cancer biology, hormone-responsive MCF-7 breast cancer cells were infected with either PKC-α or -β₁ cDNAs subcloned in the retroviral expression vector pMV7. Several stable clones of PKC-overexpressing cells were generated. Western analysis revealed cross-regulation between the α and β isoforms, because induction of overexpression of one up-regulated the other. Overexpression of the α and β isoenzymes, on the other hand, did not affect the already high endogenous expression of the novel δ, ε, η, and ζ isoforms. Compared with control clones, PKC-α- and -β-overexpressing MCF-7 cells exhibited more drastic morphological changes in response to phorbol 12- myristate 13-acetate administration characterized by cellular flattening and vacuolization. More importantly, induction of PKC-α and -β overexpression induced a less aggressive biological behavior, which was characterized by reduced in vitro invasiveness and markedly diminished tumor formation and growth in nude mice. These in vivo findings can probably best be explained by the dramatic down-regulation of estrogen receptor levels observed in tumors derived from PKC-α-infected MCF-7 cells. Our data clearly show that it is possible to induce a less aggressive breast cancer phenotype by altering PKC isoenzyme expression.