Increasing evidences indicate that PKCε is overexpressed in various tumor tissues and functions as a transforming oncogene [
14‐
20]. To explore the oncogenic potential of PKCε, Mischak et al. [
31] overexpressed PKCε in NIH 3T3 fibroblasts and observed accelerated growth of cells with PKCε overexpression. In addition, tumors were developed in all mice injected with PKCε-overexpressing NIH 3T3 cells. In the same year, Cacace et al. [
32] confirmed the oncogenic role of PKCε in fibroblasts. Similarly, Perletti
et al. [
33] found that PKCε overexpression in colonic epithelial cells led to a metastatic phenotype, including morphological changes, increased anchorage-independent growth and tumorigenesis in a xenograft model. We also found that PKCε was overexpressed in RCC tissues as compared with that in normal renal tissues and that PKCε was closely related to higher grades of clear cell RCC. PKCε was also expressed in all five human RCC cell lines used in our study.
PKCε has been shown to regulate many cellular processes, including cell proliferation, migration, invasion, chemo-resistance, apoptosis, and differentiation [
9‐
12]. Multiple mechanisms are involved in PKCε-regulated tumorigenesis. For example, PKCε promotes cell proliferation and survival by regulating the Ras signaling pathway, which is a well characterized signaling pathway in cancer biology [
10,
34]. PKCε expression is related to the activation of cyclin D1 promoter, a downstream effects of Ras signaling, and to enhanced cell growth [
9‐
11]. In addition, PKCε plays a role in anti-apoptotic signaling pathways through interacting with caspases and Bcl-2 family members [
35,
36], and exerts its pro-survival effects by activating Akt/PKB [
27,
37]. These mechanisms may explain the inhibited growth of RCC cells by PKCε knockdown in our study.
Like in other cancer types, relapse and metastasis are the main causes of failure of surgical operation in treating clear cell RCC. Patients with RCC response to postoperative adjuvant chemotherapy at various levels and usually cannot achieve expected outcomes [
3]. The phenotype of tumor metastasis presents with promotion of cell proliferation, escape from apoptosis, and dysregulation of cellular adhesion and migration. The invasion of tumor cells to surrounding tissues and spreading to distal sites rely on cell migration ability. Cell migration, a complex event, depends on the coordinated remodeling of the actin cytoskeleton, regulated assembly, and turnover of focal adhesion [
11]. Interestingly, PKCε contains an actin-binding domain [
12] and promotes F-actin assembly in a cell-free system, indicating that PKCε modulates cell migration via actin polymers. In addition, PKCε has been observed to translocate to the cell membrane during the formation of focal adhesions [
38] and to reverse the effect of non-signaling β1-integrin molecules in inhibiting cell spreading [
39]. PKCε-driven cell migration was shown to be mediated, at least in part, by activating downstream small Rho GTPases, especially RhoA and/or RhoC [
17]. We found that silencing PKCε by RNAi decreased migration and invasion of clear cell RCC cells
in vitro, suggesting that PKCε may be one of the potential treatment targets for this disease. Additionally, PKCε is also cleaved by caspases in response to several apoptotic stimuli including chemotherapeutic agents. PKCε is a substrate for caspase-3 as evidenced by caspase-3-caused PKCε cleavage and the inhibition of PKCε cleavage by a cell permeable inhibitor of caspase-3 [
40]. PKCε has been shown to regulate apoptosis mediated by either DNA damage or receptor [
10]. PKCε up-regulation was associated with chemoresistance of non-small cell lung cancer (NSCLC) cell lines, whereas chemosensitivity was proved in PKCε-knockdown SCLC cells [
41]. In addition, PKCε was reported to mediate with induction of the drug-resistance gene P-glycoprotein in LNCaP cells [
42]. In our study, PKCε knockdown enhanced the activity of pro-apoptotic gene caspase-3 and sensitized 769P cells to chemotherapy, indicating the association between PKCε and chemosensitivity of RCC.