TI - Discussion . AB - The mechanisms whereby HIV-1 subverts cell -cycle controls in infected renal epithelium have been unclear . Here , we demonstrate that podocytes expressing HIV-1 genes display the hallmarks of cyclin D1 -dependent cell -cycle progression , specifically , that cyclin D1 transcript and protein expression are markedly up-regulated , and that levels of PHOSphorylated pRb (Ser780) , a target of cyclin D1K-4/6 , are induced in vitro and in vivo . In addition , preliminary micro-array gene profiling of HIV-1-infected tubular epithelium showed that cyclin D1 expression is up-regulated several fold when compared to uninfected tubular epithelium ( Michael Ross , personal communication ) . This suggests that the HIV-1 -induced proliferation of infected renal epithelium may be caused by a cyclin D1 -dependent mechanism , unlike other viruses that bypass a requirement for G1-phase cyclins to trigger aberrant cell -cycle progression . Moreover , normal cellular controls on cell -cycle progression appears to be dysregulated by HIV-1 expression because cell cell contact and differentiation , events that normally lead to the down-regulation of cyclin D1 expression [25] , did not significantly alter cyclin D1 protein levels in HIV-1 infected podocytes . This suggests a novel stabilization of cyclin D1 protein in podocytes by HIV-1 . We are exploring whether this occurs from enhanced translation of cyclin D1 message and decreased degradation of cyclin D1 protein and whether other D-type cyclins may be involved in PHOSphorylating pRb in HIVAN . Prior studies in HIV-1 transgenic animals support the model that HIV-1 gene products interact with endogenous mitogenic pathways in infected renal epithelium. [11-14] . Kidney transplants between normal and transgenic Tg26 siblings suggested that specific HIV-1 proteins within the kidney , and not circulating factors , transform infected renal epithelium [13] . Indeed , HIV-1 transgenic mice generated with an HIV-1 proviral construct mutated in the Nef "PXXP" SH3 -binding motif for Src-family kinases failed to develop nephropathy [31] , and a recent in vitro analysis of podocytes infected with single gene mutants of HIV-1 further suggest that Nef plays a central role in causing the proliferative phenotype in HIVAN [8] . Utilizing these same single gene mutant viruses , we showed here that Nef appears to be play a role in the up-regulation of cyclin D1 by HIV-1 in vitro . Importantly , these single gene mutants do not exclude the possibility that other HIV-1 genes may cooperate with nef to induce cyclin D1 , particularly tat , which is present in every virus . Cumulatively , however , these observations do suggest a mechanism whereby HIV-1 expression in renal epithelium may induce cyclin D1 : activation of Src-family kinases by Nef would be expected to up-regulate cyclin D1 gene expression and promote the synthesis and stabilization of cyclin D1 protein via Src-Ras-MAPK and Src-PI3K-mTOR signal transduction pathways , respectively [32-34] . Interestingly , our unpublished in vitro observations and published data showing marked expression of basic fibroblast growth factor in the interstitium of HIVAN kidneys [35] suggest that soluble mitogens may be important cofactors with HIV-1 expression to induce cyclin D1 . Thus , interrupting HIV-1 gene expression should down-regulate cyclin D1 and correct the phenotypic abnormalities in infected podocytes . In support of this notion , we show here that cyclin D1 expression parallels HIV-1 expression before , during , and after suppression of HIV-1 transcription with flavopiridol , and recently , we showed that longer periods of treatment with flavopiridol ameliorate the phenotypic abnormalities of infected podocytes [7] .