PRKCQ promotes oncogenic growth and anoikis resistance of a subset of triple-negative breast cancer cells

Abstract

Background

The protein kinase C (PKC) family comprises distinct classes of proteins, many of which are implicated in diverse cellular functions. Protein tyrosine kinase C theta isoform (PRKCQ)/PKC[theta], a member of the novel PKC family, may have a distinct isoform-specific role in breast cancer. PKC[theta] is preferentially expressed in triple-negative breast cancer (TNBC) compared to other breast tumor subtypes. We hypothesized that PRKCQ/PKC[theta] critically regulates growth and survival of a subset of TNBC cells.

Methods

To elucidate the role of PRKCQ/PKC[theta] in regulating growth and anoikis resistance, we used both gain and loss of function to modulate expression of PRKCQ. We enhanced expression of PKC[theta] (kinase-active or inactive) in non-transformed breast epithelial cells (MCF-10A) and assessed effects on epidermal growth factor (EGF)-independent growth, anoikis, and migration. We downregulated expression of PKC[theta] in TNBC cells, and determined effects on in vitro and in vivo growth and survival. TNBC cells were also treated with a small molecule inhibitor to assess requirement for PKC[theta] kinase activity in the growth of TNBC cells.

Results

PRKCQ/PKC[theta] can promote oncogenic phenotypes when expressed in non-transformed MCF-10A mammary epithelial cells; PRKCQ/PKC[theta] enhances anchorage-independent survival, growth-factor-independent proliferation, and migration. PKC[theta] expression promotes retinoblastoma (Rb) phosphorylation and cell-cycle progression under growth factor-deprived conditions that typically induce cell-cycle arrest of MCF-10A breast epithelial cells. Proliferation and Rb phosphorylation are dependent on PKC[theta]-stimulated extracellular signal-related kinase (Erk)/mitogen-activated protein kinase (MAPK) activity. Enhanced Erk/MAPK activity is dependent on the kinase activity of PKC[theta], as overexpression of kinase-inactive PKC[theta] does not stimulate Erk/MAPK or Rb phosphorylation or promote growth-factor-independent proliferation. Downregulation of PRKCQ/PKC[theta] in TNBC cells enhances anoikis, inhibits growth in 3-D MatrigelTM cultures, and impairs triple-negative tumor xenograft growth. AEB071, an inhibitor of PKC[theta] kinase activity, also inhibits growth and invasive branching of TNBC cells in 3-D cultures, further supporting a role for PKC[theta] kinase activity in triple-negative cancer cell growth.

Conclusions

Enhanced PRKCQ/PKC[theta] expression can promote growth-factor-independent growth, anoikis resistance, and migration. PRKCQ critically regulates growth and survival of a subset of TNBC. Inhibition of PKC[theta] kinase activity may be an attractive therapeutic approach for TNBC, a subtype in need of improved targeted therapies.