okadaic-acid and Ovarian-Neoplasms

Intracellular signaling governed by serine/threonine kinases comprises the molecular interface between cell surface receptors and the nuclear transcriptional machinery. The protein kinase C (PKC) family members are involved in the control of many signaling processes directing cell proliferation, motility, and survival. Here, we examined a role of different PKC isoenzymes in protein phosphatase 2A (PP2A) and HRSL3 tumor suppressor-dependent cell death induction in the ovarian carcinoma cell line OVCAR-3. Phosphorylation and activity of PKC isoenzymes were measured in response to PP2A or phosphoinositide 3-kinase inhibition or HRSL3 overexpression. These experiments indicated a regulation of PKC, epsilon, zeta, and iota through PP2A and/or HRSL3, but not of PKCalpha and beta. Using isoform-specific peptide inhibitors and overexpression approaches, we verified a contribution to PP2A- and HRLS3-dependent apoptosis only for PKCzeta, suggesting a proapoptotic function of this kinase. We observed a significant proportion of human ovarian carcinomas expressing high levels of PKCzeta, which correlated with poor prognosis. Primary ovarian carcinoma cells isolated from patients also responded to okadaic acid treatment with increased phosphorylation of PKCzeta and apoptosis induction. Thus, our data indicate a contribution of PKCzeta in survival control in ovarian carcinoma cells and suggest that upregulation or activation of tyrosine kinase receptors in this tumor might impinge onto apoptosis control through the negative regulation of the atypical PKCzeta.

Topics: Apoptosis; Apoptosis Regulatory Proteins; Biomarkers, Tumor; Cell Death; Cell Line, Tumor; Cell Survival; Female; Humans; Intracellular Signaling Peptides and Proteins; Isoenzymes; Okadaic Acid; Ovarian Neoplasms; Phospholipases A2, Calcium-Independent; Phosphorylation; Protein Kinase C; Tumor Suppressor Proteins

High-fidelity maintenance of genomic integrity in eukaryotes is ensured by cell cycle checkpoints and DNA repair. The checkpoint kinase, Chk2, has been implicated in both of these responses. In response to DNA damage, Chk2 is initially phosphorylated at Thr-68, which leads to its full activation. The fully activated Chk2 then phosphorylates downstream substrates of cell cycle control. However, the mechanism of inactivation of Chk2 is still unknown. Protein phosphatase type 2A (PP2A) plays an essential role in cell cycle regulation and induction of G2 arrest by a mechanism of phosphorylation/dephosphorylation with a variety of protein kinases. Data from our investigation provide evidence that, in response to cisplatin exposure, PP2A associates with Chk2 as a complex in cells and functions as a negative regulator of Chk2 activation by dephosphorylating p-Chk2. Results from immunostaining and coimmunoprecipitation demonstrate that Chk2 and PP2A can colocalize in cells, and the holoenzyme of PP2A (subunits A, B and C) coimmunoprecipitates with p-Chk2. Further, inhibition of PP2A by okadaic acid, an inhibitor of PP2A, and by small interfering RNA (siRNA) to PP2A results in enhanced Chk2 phosphorylation, implicating a direct enzyme-substrate relationship. An in vitro PP2A dephosphorylation assay shows that PP2A dephosphorylates p-Chk2 in a cell-free system. These findings suggest that the protein serine/threonine kinase, Chk2, is activated after cisplatin exposure and negatively regulated by a tightly associated protein serine/threonine phosphatase, PP2A.

Topics: Antineoplastic Agents; Blotting, Western; Cell Line, Tumor; Checkpoint Kinase 2; Cisplatin; Enzyme Inhibitors; Female; Humans; Immunoprecipitation; Okadaic Acid; Ovarian Neoplasms; Phosphoprotein Phosphatases; Phosphorylation; Protein Binding; Protein Phosphatase 2; Protein Serine-Threonine Kinases; RNA Interference; RNA, Small Interfering; Threonine; Time Factors; Tumor Suppressor Protein p53