Abstract
BackgroundThe 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)/PKCθ, a member of the novel PKC family, may have a distinct isoform-specific role in breast cancer. PKCθ is preferentially expressed in triple-negative breast cancer (TNBC) compared to other breast tumor subtypes. We hypothesized that PRKCQ/PKCθ critically regulates growth and survival of a subset of TNBC cells.MethodsTo elucidate the role of PRKCQ/PKCθ in regulating growth and anoikis resistance, we used both gain and loss of function to modulate expression of PRKCQ. We enhanced expression of PKCθ (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 PKCθ 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 PKCθ kinase activity in the growth of TNBC cells.ResultsPRKCQ/PKCθ can promote oncogenic phenotypes when expressed in non-transformed MCF-10A mammary epithelial cells; PRKCQ/PKCθ enhances anchorage-independent survival, growth-factor-independent proliferation, and migration. PKCθ 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 PKCθ-stimulated extracellular signal-related kinase (Erk)/mitogen-activated protein kinase (MAPK) activity. Enhanced Erk/MAPK activity is dependent on the kinase activity of PKCθ, as overexpression of kinase-inactive PKCθ does not stimulate Erk/MAPK or Rb phosphorylation or promote growth-factor-independent proliferation. Downregulation of PRKCQ/PKCθ in TNBC cells enhances anoikis, inhibits growth in 3-D MatrigelTM cultures, and impairs triple-negative tumor xenograft growth. AEB071, an inhibitor of PKCθ kinase activity, also inhibits growth and invasive branching of TNBC cells in 3-D cultures, further supporting a role for PKCθ kinase activity in triple-negative cancer cell growth.ConclusionsEnhanced PRKCQ/PKCθ expression can promote growth-factor-independent growth, anoikis resistance, and migration. PRKCQ critically regulates growth and survival of a subset of TNBC. Inhibition of PKCθ kinase activity may be an attractive therapeutic approach for TNBC, a subtype in need of improved targeted therapies.Electronic supplementary materialThe online version of this article (doi:10.1186/s13058-016-0749-6) contains supplementary material, which is available to authorized users.
Highlights
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)/PKCθ is a member of the novel protein kinase C (PKC) family characterized by a unique protein domain structure consisting of diacylglycerol binding sites, but lacking calcium (Ca+) binding sites typical of classical PKCs
PRKCQ is sufficient to promote these phenotypes in MCF-10A cells, but, we showed for the first time that it is required for in vitro and in vivo growth of a subset of triple-negative breast cancer (TNBC) cells
Summary
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)/PKCθ, a member of the novel PKC family, may have a distinct isoform-specific role in breast cancer. PKCθ is preferentially expressed in triple-negative breast cancer (TNBC) compared to other breast tumor subtypes. Triple-negative breast cancer (TNBC) represents approximately 15 % of all breast cancer and is a subtype for which therapeutic options are more limited due to lack of effective targeted therapies. Triple-negative breast cancer is defined by the lack of expression of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (Her2), but it is genetically and genomically diverse. We first identified protein tyrosine kinase C theta isoform (PRKCQ)/PKCθ as a candidate regulator of anchorage-independent survival of breast cancer cells in a functional kinome screen [1]. PRKCQ/PKCθ is widely expressed throughout the hematopoietic system, primarily in T cells, mast cells, natural killer (NK) cells and platelets, and in skeletal muscle, liver, thymus, and the nervous system [3,4,5,6]
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