Prolactin (PRL) and its receptor (PRLr) have been implicated in the development and progression of human breast cancer. PRL activates its receptor and induces activation of proximal Janus kinase 2 (Jak2). Jak2 associates with PRLr, phosphorylates c-terminus of the PRLr, which leads to Stat5 recruitment and activation. Cyclophilin A (CypA) is a peptidyl-prolyl isomerase (PPI), which is constitutively bound to the PRLr that catalyze the cis-trans interconversion of proline imide bonds. Treatment with Cyclosporine (CsA) inhibited CypA binding to the PRLr and blocked PRLr-driven activation of Jak2/Stat5. Recently, Waters et al., utilizing Fluorescence Resonance Energy Transfer (FRET) with transfectants expressing CFP and YFP-tagged forms of the growth hormone receptor (GHR) showed that GHR activation induced a rotational movement in C-terminus of the GHR, resulting in a loss of baseline FRET signal. Like the GHR, PRL stimulation of transfectants expressing CFP/YFP-tagged PRLr constructs resulted in a loss of FRET efficiency. In contrast, treatment with NIM811 (a non-immunosuppressive form of CsA) or siRNA knockdown of CypA resulted in a return of FRET signal in the presence of PRL. These studies reveal that ligand stimulation of the PRLr results in a conformational change as measured by FRET signal of the receptor that is reversed by CypA inhibition or knockdown, implicating CypA as the mediator of this conformational change and ligand-induced signaling. To further assess the consequences of CypA inhibition or knockdown on the PRLr/Jak2 mediated signaling/functions, analyses of phospho-tyrosine residues that are believed to be important for interactions/signaling were investigated in breast cancer cells. It was found that NIM811 inhibition or CypA shRNA knockdown significantly reduced prolactin-stimulated phosphorylation of PRLr/Jak2 intermediates in ER+/PR+ T47D cells in a time dependent manner. A microarray analysis revealed that NIM811 inhibited approximately 66% of the top 50 PRL induced genes. NIM811 inhibited ER-/+, and HER2+ breast cancer cell proliferation, survival, migration and anchorage-independent growth. Subsequent pre-clinical testing of NIM811 in relevant mouse mammary cancer models has found that NIM811 treatment of a TNBC xenograft inhibited primary tumor growth, outgrowth of macro-metastasis and induced central tumor necrosis. Furthermore, loss of CypA (by constitutive genetic deletion) in the MMTV-PyMT mouse model demonstrated inhibition of tumorigenesis with significant reduction in lung and lymph node metastasis. Overall, CypA modulates conformational change in the C-terminus of the PRLr through its PPI activity, and alters PRLr/Jak2 complex signaling/functions in breast cancer and mammary epithelium, identifying this isomerase as a novel target for therapeutic intervention as a chemo-preventive and as an inhibitor of metastasis.

Citation Format: Shawn Hakim, Shannon E. Hedrick, Justin M. Craig, Charles V. Clevenger. Cyclophilin A as a molecular switch regulating PRLr-Jak2 complex-mediated signaling, mammary tumorigenesis, and metastasis [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 4508.