Palbociclib, a Selective Inhibitor of Cyclin-dependent Kinase4/6, Blocks HIV-1 Reverse Transcription Through the Control of Sterile α Motif and HD Domain-containing Protein-1 (SAMHD1) Activity

Background: Sterile α motif and HD domain-containing protein-1 (SAMHD1) inhibits HIV-1 reverse transcription by decreasing the pool of intracellular deoxynucleotides. SAMHD1 is controlled by cyclin-dependent kinase (CDK)-mediated phosphorylation. However, the exact mechanism of SAMHD1 regulation in primary cells is unclear. We explore the effect of palbociclib, a CDK6 inhibitor, in HIV-1 replication.

Methods: Human primary monocytes were differentiated into macrophages with monocyte-colony stimulating factor and CD4 T lymphocytes stimulated with phytohaemagglutinin (PHA)/interleukin-2. Cells were treated with palbociclib and then infected with a Green fluorescent protein-expressing HIV-1 or R5 HIV-1 BaL. Viral DNA was measured by quantitative PCR and infection assessed by flow cytometry. Deoxynucleotide triphosphate (dNTP) content was determined using a polymerase-based method.

Results: Pan-CDK inhibitors AT7519, roscovitine and purvalanol A reduced SAMHD1 phosphorylation. HIV-1 replication was blocked by AT7519 (66.4 ± 3.8%; n = 4), roscovitine (47.3 ± 3.9%; n = 4) and purvalanol A (55.7 ± 15.7%; n = 4) at subtoxic concentrations. Palbociclib, a potent and selective CDK6 inhibitor, blocked SAMHD1 phosphorylation, intracellular dNTP levels, HIV-1 reverse transcription and HIV-1 replication in primary macrophages and CD4 T lymphocytes. Notably, treatment of macrophages with palbociclib led to reduced CDK2 activation, measured as the phosphorylation of the T-loop at the Thr160. The antiviral effect was lost when SAMHD1 was degraded by Vpx, providing further evidence for a role of SAMHD1 in mediating the antiretroviral effect.

Conclusions: Our results indicate that SAMHD1-mediated HIV-1 restriction is controlled by CDK as previously suggested but point to a preferential role for CDK2 and CDK6 as mediators of SAMHD1 activation. Our study provides a new signaling pathway susceptible for the development of new therapeutic approaches against HIV-1 infection.