KSHV viral cyclin binds to p27KIP1 in primary effusion lymphomas

Abstract

Primary effusion lymphomas (PELs) represent a unique non-Hodgkin lymphoma that is consistently infected by Kaposi sarcoma herpesvirus (KSHV). PEL cells express high levels of the cell cycle inhibitor p27 KIP1 and yet proliferate actively. KSHV genome encodes a viral cyclin homolog, v-cyclin, which has previously been implicated in down-regulation of p27 KIP1 levels. To address how PEL cells can tolerate high p27 KIP1 levels, we investigated functional interactions between v-cyclin and p27 KIP1 using PEL-derived cell lines as a model system. Here we demonstrate that v-cyclin and p27 KIP1 stably associate in PEL cells in vivo suggesting an attractive model by which p27 KIP1 is inactivated in the actively proliferating PEL cells. Moreover, we show that vcyclin and cyclin-dependent kinase 6 (CDK6) form an active kinase without p27 KIP1 and that CDK6 is the in vivo catalytic subunit of v-cyclin in PEL cells. These findings suggest that KSHV may promote oncogenesis in PEL by expressing v-cyclin, which both overrides negative cell cycle controls present in the PEL precursor cells and induces a strong proliferative signal via CDK6 kinase activity.

Phosphorylation of the Cyclin-Dependent Kinase Inhibitor p21Cip1 on Serine 130 Is Essential for Viral Cyclin-Mediated Bypass of a p21Cip1-Imposed G1 Arrest

“…K cyclin expression was induced with 1 mM IPTG (isopropyl-␣-D-thiogalactopyranoside). Culture of U2-OS cells and transfection has been previously described (16,23). The U2-OS-K cyclin cell line (U2-OS-K) was a kind gift from Heike Laman (Division of Virology, Department of Pathology, University of Cambridge) and Flag-tagged K cyclin expression was induced with 5 mM IPTG.…”

“…For immunohistochemical analysis, transfected cells were fixed with 3.5% (wt/vol) paraformaldehyde and permeabilized with 0.1% Triton X-100 for 5 min. Immunofluorescence labeling was performed as previously described (16).…”

“…Even though K cyclin forms complexes with cdk2, cdk4, cdk5, and cdk6 in BC-3 cells, we have shown that cdk6 is the in vivo catalytic subunit of K cyclin in PEL cells using GST-pRb and histone H1 as substrates (16,29). To extend our analysis, we depleted cdk2, cdk4, and cdk6 from BC-3 cell lysates to see which kinase partner was important for the K cyclin-induced phosphorylation of p21 Cip1 .…”

“…We have previously shown that K cyclin associates with p27 Kip1 in vivo in transfected cells and PEL-derived cells (16). To assess the potential physical interactions between K cyclin and p21…”

Kaposi’s Sarcoma-Associated Virus Governs Gene Expression Profiles Toward B Cell Transformation

“…v-CYC interacts with cyclin dependent kinase 6 (CDK6) and promotes G1-S progression (Godden-Kent et al, 1997;Swanton et al, 1997). The Cyclin D-CDK6 complex is to function to exit from G0 to G1 phase (Laman et al, 2001) and the v-cyc/CDK6 complex is resistant to inhibition by CDK inhibitors by p16, p21 and p27 (Jarviluoma et al, 2004). Thus, its real function has not been elucidated and it was reported that ectopic or overexpression of c-CYC evokes rather cell/DNA damage (Koopal et al, 2007).…”