Loss of PKCθ-GADD45a axis drives triple-negative breast cancer cells into p53-independent senescence

PKCθ is a serine/threonine kinase that is well known for its role in the immune system. However, increasing evidence implicates PKCθ in the pathology of breast cancer. PKCθ is highly expressed in triple negative breast cancer (TNBC) cells in which it controls cell migration and invasion, while its implication in cell proliferation remains poorly understood.

Methods

To determine the function of PKCθ in cell proliferation, siRNAs were used to modulate the expression of PKCθ in TNBC cells (MDA-MB-231, MDA-MB-436, HCC1937) and cell growth was examined by clonogenic and EdU assays. β-galactosidase assay, RT-qPCR and western blot were used to characterize the senescence features. PCR microarrays and rescue experiments were conducted to investigate the underlying mechanism.

Results

We show that PKCθ inhibition leads to a growth arrest in TNBC cells harboring a p53 loss-of-function mutation. This p53-independent growth arrest is accompanied by an increased activity of senescence-associated β-galactosidase, the presence of a senescence-associated secretory phenotype, and the striking expression change of various genes implicated in cell proliferation and senescence. Thus, our data show that PKCθ silencing drives TNBC cells into a senescence-like phenotype. Mechanistically, we demonstrate that p27 is the main CDK inhibitor controlling the PKCθ loss-induced senescence. The accumulation of p27 is due to a surprising strong reduction in GADD45a expression. Indeed, similar to PKCθ silencing phenotype, GADD45a knockdown drives TNBC cells into a senescence-like phenotype.

Conclusions

Altogether, our study highlights that the loss of PKCθ-GADD45a axis triggers a p27-dependent senescence response in TNBC cells and further supports strategies targeting PKCθ as treatment for this type of aggressive breast cancer.