Inhibition of human immunodeficiency virus type 1 transcription by chemical cyclin-dependent kinase inhibitors

Complexities in the development of cyclin-dependent kinase inhibitor drugs

Abnormalities in the normal regulation of the cell cycle are a hallmark of neoplasia. Drugs directed against the cyclin-dependent kinases (CDKs), which govern the normal orderly progression through the cell cycle, have been proposed to address the pathogenic defect in tumors. Recently, CDK family members that do not regulate the cell cycle directly but instead influence transcription (CDK7, CDK8, and CDK9) and neuronal and secretory cell function (CDK5) have been described. Continued synthetic chemistry efforts have defined important new selective inhibitors of CDKs, and strategies directed at newly described CDK-related targets, such as transcription control, can now be envisaged. CDKs remain important and novel targets whose potential needs to be more fully explored, albeit in light of the newly emerging complexities of their cellular physiology.

Sequential involvement of Cdk1, mTOR and p53 in apoptosis induced by the HIV-1 envelope

Syncytia arising from the fusion of cells expressing the HIV-1-encoded Env gene with cells expressing the CD4/CXCR4 complex undergo apoptosis following the nuclear translocation of mammalian target of rapamycin (mTOR), mTOR-mediated phosphorylation of p53 on Ser15 (p53(S15)), p53-dependent upregulation of Bax and activation of the mitochondrial death pathway. p53(S15) phosphorylation is only detected in syncytia in which nuclear fusion (karyogamy) has occurred. Karyogamy is secondary to a transient upregulation of cyclin B and a mitotic prophase-like dismantling of the nuclear envelope. Inhibition of cyclin-dependent kinase-1 (Cdk1) prevents karyogamy, mTOR activation, p53(S15) phosphorylation and apoptosis. Neutralization of p53 fails to prevent karyogamy, yet suppresses apoptosis. Peripheral blood mononuclear cells from HIV-1-infected patients exhibit an increase in cyclin B and mTOR expression, correlating with p53(S15) phosphorylation and viral load. Cdk1 inhibition prevents the death of syncytia elicited by HIV-1 infection of primary CD4 lymphoblasts. Thus, HIV-1 elicits a pro-apoptotic signal transduction pathway relying on the sequential action of cyclin B-Cdk1, mTOR and p53.

Inhibition of HTLV-1 transcription by cyclin dependent kinase inhibitors

HTLV-1 is the etiologic agent for adult T-cell leukemia/lymphoma (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), where viral replication and transformation are largely dependent upon modification of regulatory and host cell cycle proteins. The mechanism of HTLV-1 transformation appears to be distinct from that of many known chronic or acute leukemia viruses and is related to the viral activator Tax. Here we show that cyclin E, can associate tightly with the coactivator p300 and Pol II complex in HTLV-1 infected cells. The cyclin E associated complex is kinase active and phosphorylates the carboxy terminal domain of RNA Pol II. More importantly, p21/Waf1, a well-known cdk inhibitor at the G1/S border, inhibits transcription of HTLV-1 in both transfections and in in vitro transcription assays. Finally, specific cdk chemical inhibitors, functionally similar to cellular cdkIs, such as p21/Waf1 which inhibits cyclin E/cdk2 activity, also inhibit transcription of the HTLV-1 promoter. In particular, Purvalanol A, with an IC50 of 0.035 microm inhibits activated, but not basal transcription, as well as HTLV-1 infected cells. Collectively, the role of cyclin E/cdk2 in HTLV-1 infected cells and its involvement in RNA Pol II phosphorylation is discussed.

Pharmacological cyclin-dependent kinase inhibitors inhibit replication of wild-type and drug-resistant strains of herpes simplex virus and human immunodeficiency virus type 1 by targeting cellular, not viral, proteins

Pharmacological cyclin-dependent kinase (cdk) inhibitors (PCIs) block replication of several viruses, including herpes simplex virus type 1 (HSV-1) and human immunodeficiency virus type 1 (HIV-1). Yet, these antiviral effects could result from inhibition of either cellular cdks or viral enzymes. For example, in addition to cellular cdks, PCIs could inhibit any of the herpesvirus-encoded kinases, DNA replication proteins, or proteins involved in nucleotide metabolism. To address this issue, we asked whether purine-derived PCIs (P-PCIs) inhibit HSV and HIV-1 replication by targeting cellular or viral proteins. P-PCIs inhibited replication of HSV-1 and -2 and HIV-1, which require cellular cdks to replicate, but not vaccinia virus or lymphocytic choriomeningitis virus, which are not known to require cdks to replicate. P-PCIs also inhibited strains of HSV-1 and HIV-1 that are resistant to conventional antiviral drugs, which target viral proteins. In addition, the anti-HSV effects of P-PCIs and a conventional antiherpesvirus drug, acyclovir, were additive, demonstrating that the two drugs act by distinct mechanisms. Lastly, the spectrum of proteins that bound to P-PCIs in extracts of mock- and HSV-infected cells was the same. Based on these observations, we conclude that P-PCIs inhibit virus replication by targeting cellular, not viral, proteins.

Roscovitine inhibits ongoing DNA synthesis in human cervical cancer

The effect of roscovitine, a purine analogue and cyclin dependent kinase inhibitor, on DNA synthesis rate in tissue mini-units obtained from human cervical cancers was investigated. Roscovitine (100 microM) gave a DNA synthesis rate inhibition by 61% (P<0.0001; range 23-93%) within 30 min of incubation. This inhibitory effect was concentration-dependent. The results suggest that the inhibition of tumor DNA synthesis rate is due to a direct effect on the DNA synthesis machinery via presently unknown mechanisms. In addition, the potential application of CDKs inhibitors as preventive agents is discussed.

The cyclin-dependent kinase inhibitor roscovitine inhibits the transactivating activity and alters the posttranslational modification of herpes simplex virus type 1 ICP0

The cyclin-dependent kinase (cdk) inhibitor Roscovitine (Rosco) reduces transcription of herpes simplex virus early genes significantly, even in the presence of wild-type levels of immediate-early (IE) viral proteins, suggesting that the transactivating functions of IE proteins may require the activities of one or more Rosco-sensitive cdk (L. M. Schang, A. Rosenberg, and P. A. Schaffer, J. Virol. 73:2161-2172, 1999). Based on this observation, we sought to determine whether Rosco alters the transactivating activity and posttranslational modification state of the IE protein, infected cell protein 0 (ICP0), in KOS6beta-infected Vero cells. KOS6beta is a KOS-derived recombinant virus containing an ICP0-inducible ICP6 promoter::lacZ cassette. To monitor ICP0's transactivating activity, KOS6beta-infected cells were released from a cycloheximide (CHX)-mediated protein synthesis block into medium with or without Rosco, and beta-galactosidase activity was measured. Rosco inhibited the ability of ICP0 to transactivate the ICP6 promoter by 50-fold. This inhibition was shown not to be a consequence of inhibition of ICP6 basal promoter activity or aberrant nuclear localization of ICP0. Rosco also altered the electrophoretic mobility of a portion of ICP0 molecules derived from KOS-infected cells following reversal of a CHX block. Notably, however, Rosco had only a minimal effect on the phosphorylation state of ICP0. We conclude that ICP0's transactivating activity requires Rosco-sensitive cdks and hypothesize that these cdks regulate the functions of cellular enzymes which modify ICP0, and are, consequently, required for its transactivating activity. Thus, we propose that Rosco regulates ICP0's posttranslational state by mechanisms other than, or in addition to, phosphorylation.

Suppression of HIV-1 expression by inhibitors of cyclin-dependent kinases promotes differentiation of infected podocytes

The glomerular lesions of HIV-associated nephropathy (HIVAN) are associated with the expression of HIV-1 in podocytes. Infected podocytes proliferate and lose several differentiation markers in vivo and in vitro, which suggests that HIV-1 gene expression induces these changes. Flavopiridol and roscovitine, newly identified inhibitors of cyclin-dependent kinase-9, markedly decrease HIV-1 promoter activity in cell lines of various lineages. In this study, the inhibitors were used to determine whether suppression of HIV-1 transcription in infected podocytes correlated with an inhibition of proliferation and a return to the differentiated phenotype. Dose-response analysis showed that both flavopiridol and roscovitine reversibly suppressed HIV-1 transcription in podocytes in vitro at an IC(50) of 25 nM and 3 microM, respectively. Despite equivalent suppression of HIV-1 transcription, roscovitine was a more effective inhibitor of podocyte proliferation than flavopiridol. Suppression of HIV-1 transcription by flavopiridol or roscovitine was marked by re-expression of the podocyte differentiation markers, synaptopodin and podocalyxin. These results suggest that inhibition of HIV-1 transcription decreases podocyte proliferation and permits the reexpression of differentiation markers. Thus, suppression of HIV-1 transcription by selective cyclin-dependent kinase-9 inhibitors may be a useful therapeutic strategy for the treatment of HIVAN.