Activation of protein kinase C delta induces growth arrest in NPA thyroid cancer cells through extracellular signal-regulated kinase mitogen-activated protein kinase.

Protein kinase C (PKC) is a family of serine-threonine kinases that regulate many cell processes. To study the role of PKCdelta in thyroid cancer cells, we used a replication-deficient adenovirus (PKCdeltaAdV), to tightly control PKCdelta expression. In NPA cells, activation of wild-type (WT) PKCdelta with phorbol 12-myristate 13-acetate (PMA) induced an arrest in cell growth at G(1) phase, which was itself inhibited by the PKCdelta inhibitor rottlerin. Furthermore, overexpression of a dominant negative PKCdelta did not induce G(1) arrest. These findings strongly suggested that PKCdelta induced cell growth arrest in NPA cells. We investigated the mechanism of G1 arrest by examining G(1)-related proteins and mitogen-activated protein kinase (MAPK) by Western blotting. After activation of WTPKCdelta with PMA, cyclin E expression and retinoblastoma protein (Rb) phosphorylation decreased; the expression of p27(Kip1) increased and the phosphorylation of extracellular signal-regulated kinase (ERK) MAPK decreased. These results indicated that the activation of PKCdelta induced cell growth arrest in NPA cells, through an ERK MAPK-p27(Kip1)-cyclin E-pRb pathway. PKCdelta may therefore be an effective molecular target for novel therapy in thyroid cancer.

Antineoplaston induces G(1) arrest by PKCalpha and MAPK pathway in SKBR-3 breast cancer cells.

Antineoplastons such as A10 include naturally occurring peptides and amino acid derivatives that control the neoplastic growth of cells. The mechanism underlying this antitumor effect was investigated using the breast cancer cell line, SKRB-3. Cells treated with A10 were monitored for any changes in cell cycle, expression of protein kinase C (PKC), or intracellular signal transduction, particularly phos-phorylation of mitogen-activated protein kinase (MAPK). The A10 markedly inhibited SKBR-3 proliferation due to arrest in the G(1) phase. A10 down-regulated the expression of PKCalpha protein, resulting in inhibition of extracellular signal-regulated kinase (ERK) MAPK phosphorylation. This increased the expression of p16 and p21 protein, with resultant inhibition of Rb phosphorylation, leading to G(1) arrest. This study has defined a pathway in which A10 arrested SKBR-3 cells in the G(1) phase via PKCalpha and MAPK. Our findings indicate that the antineoplaston A10 antitumor effect could be utilized as an effective therapy for breast cancer patients.