The use of small molecule kinase inhibitors, which target specific enzymes that are overactive in cancer cells, has revolutionized cancer patient treatment. To treat some types of breast cancer, CDK4/6 inhibitors, such as palbociclib, have been developed that target the phosphorylation of the retinoblastoma tumor suppressor gene. Acquired resistance to CDK4/6 inhibitors may be due to activation of the AKT pro-survival signaling pathway that stimulates several processes, such as growth, metastasis and changes in metabolism that support rapid cell proliferation. The aim of the present study was to investigate whether targeting ATP citrate lyase (ACLY), a downstream target of AKT, may combine with CDK4/6 inhibition to inhibit tumorigenesis. The present study determined that ACLY is activated in breast and pancreatic cancer cells in response to palbociclib treatment and AKT mediates this effect. Inhibition of ACLY using bempedoic acid used in combination with palbociclib reduced cell viability in a panel of breast and pancreatic cancer cell lines. This effect was also observed using breast cancer cells grown in 3D cell culture. Mechanistically, palbociclib inhibited cell proliferation, whereas bempedoic acid stimulated apoptosis. Finally, using Transwell invasion assays and immunoblotting, the present study demonstrated that ACLY inhibition blocked cell invasion, when used alone or in combination with palbociclib. These data may yield useful information that could guide the development of future therapies aimed at the reduction of acquired resistance observed clinically.

The Retinoblastoma (Rb) tumor suppressor protein plays a role in the control of proliferation, apoptosis, differentiation, invasion and the maintenance of genome stability. Its activity is regulated by phosphorylation on several serine/threonine sites, which is thought to inactivate its tumor suppressive function. Cyclin dependent kinases (CDKs) are responsible for most of the Rb phosphorylation that occurs in cells. It is well established that alterations in the Rb pathway (consisting of CDK4/6, D-type cyclins, the CDK inhibitor p16INK4a, and Rb itself) that lead to excessive phosphorylation of Rb have been observed in almost all cancer types. Alterations that lead to increased CDK activity toward Rb are more common than mutations to the Rb gene itself, a finding that prompted investigations into the development of clinical treatments that inhibit the phosphorylation of Rb by CDKs. The CDK4/6 inhibitors palbociclib and abemaciclib were recently clinically approved for use in breast cancer. However, acquired resistance to CDK4/6 inhibition has been shown to limit clinical efficiency. Targeting Rb phosphorylation by CDK4/6 inhibition has been shown to activate alternate pro-survival pathways such as the AKT pathway. In our studies we developed a method of siRNA-mediated activation of Rb phosphatase that targets Rb phosphorylation in attempt to circumvent the development of resistance. Phosphatase Nuclear Targeting Subunit (PNUTS) associates with and regulates PP1 activity toward Rb. In several cancer cell types, such as HCT116 colon cancer, MIA PaCa-2 pancreatic cancer and MCF7 breast cancer, PNUTS depletion causes dephosphorylation of Rb without AKT activation. Furthermore, in Panc1 pancreatic cancer cells, activation of AKT is observed after 1-hour palbociclib (500nM) treatment, whereas PNUTS depletion does not induce AKT. In MD-MBA-231 breast cancer cells, 1-hour treatment of palbociclib or abemaciclib (20nM) induces the AKT pathway as well as the STAT pathways, however PNUTS depletion in these cells does not induce AKT activation. It has been shown that CDK4/6 inhibitors activate AKT by causing loss of Rb interaction with SIN1, a component the mTORC2 complex. Our studies reveal that PNUTS depletion causes dephosphorylation of Rb, without disrupting Rb-mediated inhibition of mTORC2. Thus, the stimulation of Rb phosphatase activity may target Rb phosphorylation without the development of resistance. Citation Format: Kathleen H. DiSalvo, Brian C. Velez, Rebecca L. Kravtsov, Nancy A. Krucher. Targeting Rb phosphatase activity to circumvent CDK4/6 inhibitor resistance [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 5295.

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