Therapeutically activating RB: reestablishing cell cycle control in endocrine therapy-resistant breast cancer

Abstract

The majority of estrogen receptor (ER)-positive breast cancers are treated with endocrine therapy. While this is effective, acquired resistance to therapies targeted against ER is a major clinical challenge. Here, model systems of ER-positive breast cancers with differential susceptibility to endocrine therapy were employed to define common nodes for new therapeutic interventions. These analyses revealed that cell cycle progression is effectively uncoupled from the activity and functional state of ER in these models. In this context, cyclin D1 expression and retinoblastoma tumor suppressor protein (RB) phosphorylation are maintained even with efficient ablation of ER with pure antagonists. These therapy-resistant models recapitulate a key feature of deregulated RB/E2F transcriptional control. Correspondingly, a gene expression signature of RB-dysfunction is associated with luminal B breast cancer, which exhibits a relatively poor response to endocrine therapy. These collective findings suggest that suppression of cyclin D-supported kinase activity and restoration of RB-mediated transcriptional repression could represent a viable therapeutic option in tumors that fail to respond to hormone-based therapies. Consistent with this hypothesis, a highly selective CDK4/6 inhibitor, PD-0332991, was effective at suppressing the proliferation of all hormone refractory models analyzed. Importantly, PD-0332991 led to a stable cell cycle arrest that was fundamentally distinct from those elicited by ER antagonists, and was capable of inducing aspects of cellular senescence in hormone therapy refractory cell populations. These findings underscore the clinical utility of downstream cytostatic therapies in treating tumors that have experienced failure of endocrine therapy.

Therapeutic response to CDK4/6 inhibition in breast cancer defined by ex vivo analyses of human tumors

“…To specifically interrogate this concept, explants were treated with the CDK4/6 inhibitor PD-0332991, a potent cytostatic agent that is in phase II clinical trials for multiple malignancies, including breast cancer. [11][12][13] Treatment with PD-0332991 revealed highly profound suppression of Ki67 staining. This effect was specific to the drug and was not observed in tissues treated with DMSO (vehicle) for the same time period (Fig.…”

“…11,12,14 RB is the proto-typical tumor suppressor that functions downstream of CDK4/6 to modulate cell cycle progression. [15][16][17] Tumors that are RB-deficient express exceedingly high levels of p16ink4a.…”

RB1: a prototype tumor suppressor and an enigma

“…Cdk4/6 inhibitors also have pRB-independent effects, particularly when used at high concentrations. In contexts where deregulation of Cyclin D:Cdk4/6 kinases drives tumorigenesis, inhibition of these kinases can trigger cellular senescence or apoptosis (Fry et al 2004;Thangavel et al 2011;Choi et al 2012;Sawai et al 2012). Cdk4/6 inhibitors had modest effects when tested as a monotherapy in solid tumors (Flaherty et al 2012;Dickson et al 2013;Cadoo et al 2014;DeMichele et al 2015;Vaughn et al 2015).…”

The Role of CDK4/6 Inhibition in Breast Cancer

“…Furthermore, cyclin D1 augments estrogendependent gene expression through ERa signaling [35]. Meanwhile, the development of endocrine resistance in ER1 breast cancer cells is associated with persistent cyclin D1 expression and Rb phosphorylation [36]. Direct evidence of a particular role for CDK4/6 inhibition in ER1 cells comes from the evaluation of the CDK4/6 inhibitor palbociclib in vitro in a panel of molecularly characterized breast cancer cell lines that demonstrated most activity in luminal cancers (including those with conditioned estrogen resistance), whereas nonluminal and basal types were most resistant [37].…”