Histone deacetylase classes I and II regulate Kaposi's sarcoma-associated herpesvirus reactivation

Area of Science:

• Epigenetics and Viral Latency
• Molecular Virology
• Cancer Biology

Background:

• Primary effusion lymphoma (PEL) cells harbor latent Kaposi's sarcoma-associated herpesvirus (KSHV).
• The viral Rta gene promoter in KSHV-infected PEL cells exhibits bivalent chromatin organization, akin to cellular developmental genes.
• Histone deacetylase inhibitors (HDACis) are known to reactivate latent KSHV, altering viral genome topology and chromatin structure, but reactivation is often heterogeneous.

Purpose of the Study:

• To identify an HDAC inhibitor cocktail that achieves uniform KSHV reactivation in PEL cells.
• To elucidate the specific roles of different histone deacetylases (HDACs) in regulating KSHV lytic switch and reactivation.
• To understand the impact of HDAC activity on viral reactivation progression after Rta expression.

Main Methods:

• Utilized various HDAC inhibitors with differing specificities to treat KSHV-infected PEL cells.
• Assessed viral reactivation by measuring lytic cycle gene expression and production of infectious virus.
• Investigated the effects of ectopic HDAC expression and specific HDAC inhibitors (e.g., Tubacin) on KSHV reactivation.
• Employed immunofluorescence to examine HDAC6 expression and localization during viral reactivation.

Main Results:

• Class I HDAC inhibitors were sufficient for KSHV reactivation, with Valproic acid (VPA) showing the highest potency (75% cell reactivation).
• Trichostatin A (TSA) induced less widespread reactivation and partially inhibited VPA-stimulated reactivation.
• VPA, but not TSA, significantly increased infectious virus production, indicating a crucial role for HDACs post-Rta expression.
• Ectopic HDACs 1, 3, and 6 inhibited TPA-stimulated reactivation, while ectopic HDACs 1 and 6 independently stimulated reactivation, highlighting complex stoichiometry.
• Tubacin, an HDAC6 inhibitor, also inhibited VPA-stimulated reactivation, and HDAC6 expression increased during reactivation.

Conclusions:

• Optimal KSHV reactivation requires the inhibition of Class I and IIa HDACs, alongside the maintenance of HDAC6 (Class IIb) activity.
• HDAC stoichiometry within cellular complexes plays a critical role in regulating the KSHV lytic switch.
• HDAC6 is a key regulator of KSHV reactivation progression, with its expression and localization changing during the process.