Abstract 1634: Cyclin D1 induces Akt1 activity in RB deficient prostate cancer

Abstract Background: Prostate cancer (PCa), the second leading cause of death in American men, is a genetically heterogeneous disease. The aberrant activation of AKT is one of the most frequent alterations in human cancers, and an elevated level of phosphorylated AKT at Ser473 site (pSer473) is associated with metastatic cancers (1). In prostate cancer (PCa), PI3K/AKT activation induces tumorigenesis (2), androgen receptor (AR) -mediated signaling, enzalutamide resistance and stemness, synergizing with other pathways including the notch intracellular domain (NICD). The cyclin D1 gene (CCND1), which is overexpressed in PCa, conveys canonical function by encoding the regulatory subunit of a holoenzyme that phosphorylates the retinoblastoma gene (RB) product, and non-canonical functions through phosphorylating other substrates and transcriptional activities. Cyclin D1 expression augments growth of some castrate-resistant PCa (CRPC) and clinical trials are targeting the cyclin D1 related CDK kinase activity in pRB+ PCa. Several Rb-independent mechanisms of action have been described (3). In recent studies the effect of CDK inhibitors was similar in patients with Rb+ vs. Rb− PCa (4). Whether Rb expression is required for the antiproliferative effects of CKD4/6 inhibition in PCa remains to be established. Methods: The kinetics of Akt1 induction by mitogens was assessed using the phosphorylation of the dichromic fluorescent (DCF) dye substrate LS456. We used cyclin D1 shRNA to show endogenous cyclin D1 augmented AKT1 signaling in response to mitogens. Results: Cyclin D1 reintroduction into cyclin D1−/− cells augmented Akt1 phosphorylation, requiring the cyclin D1 K112 residue. CDK inhibitors reduced Akt activity in RB+ and RB− human prostate cancer cell lines. Prostate-specific epithelial cell deletion of cyclin D1 (probasin-CRE-cyclin D1fl/fl) reduced prostate epithelial cell Akt1 activity assessed by immunohistochemical (IHC) staining. pAktSer473 was reduced in cell nuclei and the cytoplasm of both epithelial cells and stromal fibroblasts without a significant change in the abundance of Akt1. The downstream targets of Akt signaling (pTSC2Ser939, and pFKHRSer319) and ARSer213P were reduced in both epithelial cells and stromal fibroblasts. Genetic deletion of both epithelial cell and stromal cyclin D1 further reduced Akt1 activity, prostate epithelial cell proliferation, and markers of cancer stem cells. Proteomic analysis of cyclin D1-expressing stroma identified heterotypic chemokine signals that augment PCa AKT activity. Conclusions: Cyclin D1 induces Akt1 activity in both RB+ and RB− PCa cells which may contribute to tumor progression and therapy resistance. Reference: (1) Chen YL, Law PY, Loh HH. Curr Med Chem Anticancer Agents 2005;5: 575-892. (2) Stoyanova T, et al. Proc Natl Acad Sci U S A 2013;110: 20111-63. (3) Chen K, et al. Cell Rep 2020;32: 1081514. (4) de Kouchkovsky I, et al. Clin Cancer Res 2022;28: 1531-9 Citation Format: Xuanmao Jiao, Anthony W. Ashton, Zhiping Li, Ritika Harish, Danni Li, Gordon Robertson, Duanwen Shen, Xiaoming Ju, Ke Chen, Wei Zhang, Samuel Archilefu, Kenneth A. Iczkowski, Hallgeir Rui, Beatrice Knudsen, Paul Timpson, Max Nobis, Richard G. Pestell. Cyclin D1 induces Akt1 activity in RB deficient prostate cancer abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 1634.