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Primary central nervous system lymphoma (PCNSL) is a malignant neoplasm of the central nervous system that is refractory to treatment and has extremely poor prognosis. One factor hindering the development of therapeutic options for PCNSL is its molecular heterogeneity and the extreme difficulty in establishing in vitro cell lines that permit intensive research on this disease. In the present study, we developed a method to propagate PCNSL cells in vitro using a contacting transwell cell culture system involving brain vascular pericytes. The co-culture system was found to recapitulate the tumor microenvironment that is influenced by the biological activity of adjacent pericytes, and to sustain the survival and proliferation of PCNSL cells in vitro. We further delineated the underlying molecular mechanisms and found that the HGF–c-Met axis may be involved in the long-term in vitro culture of PCNSL cells. Moreover, the peptidylprolyl isomerase Pin1 was found to play a key role in PCNSL cell survival and it sustained proliferation through interactions with key transcription factors related to B-cell lymphomagenesis. These results suggest that our in vitro co-culture system is well suited to analyzing the biological and molecular characteristics of PCNSL, and may contribute to the discovery of new therapeutic interventions.

The dynamic interplay between virus and host proteins is critical for establishing efficient viral replication and virus-induced pathogenesis. Phosphorylation-dependent prolyl isomerization by Pin1 provides a unique mechanism of molecular switching to control both protein function and stability. We demonstrate here that Pin1 binds and stabilizes hepatitis B virus core protein (HBc) in a phosphorylation-dependent manner, and promotes the efficient viral propagation. Phos-tag gel electrophoresis with various site-directed mutants of HBc revealed that Thr160 and Ser162 residues within the C terminal arginine-rich domain are phosphorylated concomitantly. GST pull-down assay and co-immunoprecipitation analysis demonstrated that Pin1 associated with phosphorylated HBc at the Thr160-Pro and Ser162-Pro motifs. Chemical or genetic inhibition of Pin1 significantly accelerated the rapid degradation of HBc via a lysosome-dependent pathway. Furthermore, we found that the pyruvate dehydrogenase phosphatase catalytic subunit 2 (PDP2) could dephosphorylate HBc at the Pin1-binding sites, thereby suppressing Pin1-mediated HBc stabilization. Our findings reveal an important regulatory mechanism of HBc stability catalyzed by Pin1 and may facilitate the development of new antiviral therapeutics targeting Pin1 function.