Upregulation of PKCη by PKCε and PDK1 involves two distinct mechanisms and promotes breast cancer cell survival

Abstract

Background Protein kinase C (PKC) serves as the receptor for tumor-promoting phorbol esters, which are potent activators of conventional (c) and novel (n) PKCs. We recently showed that these activators induced selective upregulation of PKCη in breast cancer cells. The objective of this study is to understand unique regulation of PKCη and its importance in breast cancer. Methods The levels of PKC isozymes were monitored in breast cancer cells following treatment with inhibitors of kinases, proteasome and proteases by Western blotting. PKCε was introduced by adenoviral delivery. PKCη and PDK1 were depleted by siRNA silencing. Cell growth was determined by the MTT or clonal assay. Results The general PKC inhibitors Gö 6983 and bisindolylmaleimide but not cPKC inhibitor Gö 6976 led to substantial PKCη downregulation, which was partly rescued by the introduction of nPKCε. Inhibition of phosphoinositide-dependent kinase-1 (PDK1) by Ly294002 or knockdown of PDK1 also led to downregulation of basal PKCη but had no effect on PKC activator-induced upregulation of PKCη. Proteasome inhibitors blocked PKCη downregulation triggered by PDK1 inhibition/depletion but not by Gö 6983. PKCη level increased in malignant but not in non-tumorigenic or pre-malignant cells in the progressive MCF-10A series associated with activated PDK1, and knockdown of PKCη inhibited breast cancer cell growth and clonogenic survival. Conclusion Upregulation of PKCη contributes to breast cancer cell growth and targeting either PKCε or PDK1 triggers PKCη downregulation but involves two distinct mechanisms. General significance The status of PKCη may serve as a potential biomarker for breast cancer malignancy.

Overexpression of sigma‑1 receptor in MCF‑7 cells enhances proliferation via the classic protein kinase C subtype signaling pathway

“…In the present study, the data revealed that GF109203x inhibited MCF-41 and MCF-7 cell proliferation, consistently with previous studies reporting that overexpression of PKCα may contribute to increased anchorage-independent growth, tumorigenicity and metastasis (39). The PKCβ isoform has been implicated in mammary tumorigenesis inhuman and rodent models, and is generally considered to be a growth-promoting kinase; in addition, the PKCβ-specific inhibitor LY379196 significantly reduces the growth of MCF-7, MDA-MB-231and BT-474 breast cancer cells (23). All these previous studies support that classic PKCs are key enzymes in tumor cell proliferation.…”

“…Stimulating PKCβI and βII in MCF-7 cells enhance their growth by upregulating the expression of cyclin D1 (22). PKCη upregulation in malignant breast cells was found to be associated with cancer cell growth and survival via hormone-dependent cell growth pathways (23,24). PKCζ stimulates estrogen-mediated breast cancer cell growth by stabilizing steroid receptor coactivator-3 (25,26).…”

Phosphorylation at Ser-181 of Oncogenic KRAS Is Required for Tumor Growth

“…We found that at doses reported to inhibit PKC (22,29,39,41), cell growth was compromised. Most importantly, after 12 hours of treatment, band corresponding to phospho-KRAS was lost, thus reinforcing the idea that PKC inhibition is able to revert growth in a KRAS S181-dependent manner (Fig.…”

“…Next, we wanted to determine whether BIM and G€ o6983, two PKC inhibitors (39,40) were able to affect DLD-1 cells in a KRAS S181-specific manner. To this aim, dose response to these PKC inhibitors was evaluated in DLD1…”

Protein Kinase C (PKC) Isozymes and Cancer

“…Further, PKCinduced polymeric fibronectin assembly is required for the pulmonary metastasis of breast cancer cells [67]. Upregulation of PKC in malignant breast cells is also associated with cancer cell growth and survival [68], likely through hormone-dependent cell growth pathways [69]. PKC expression in the membrane of breast cancer cells after chemotherapy tends to predict a poor patient prognosis [70].…”