Absence of DICER in Monocytes and Its Regulation by HIV-1

Abstract

MicroRNAs (miRNAs) are a class of small RNA molecules that function to control gene expression and restrict viral replication in host cells. The production of miRNAs is believed to be dependent upon the DICER enzyme. Available evidence suggests that in T lymphocytes, HIV-1 can both suppress and co-opt the host's miRNA pathway for its own benefit. In this study, we examined the state of miRNA production in monocytes and macrophages as well as the consequences of viral infection upon the production of miRNA. Monocytes in general express low amounts of miRNA-related proteins, and DICER in particular could not be detected until after monocytes were differentiated into macrophages. In the case where HIV-1 was present prior to differentiation, the expression of DICER was suppressed. MicroRNA chip results for RNA isolated from transfected and treated cells indicated that a drop in miRNA production coincided with DICER protein suppression in macrophages. We found that the expression of DICER in monocytes is restricted by miR-106a, but HIV-1 suppressed DICER expression via the viral gene Vpr. Additionally, analysis of miRNA expression in monocytes and macrophages revealed evidence that some miRNAs can be processed by both DICER and PIWIL4. Results presented here have implications for both the pathology of viral infections in macrophages and the biogenesis of miRNAs. First, HIV-1 suppresses the expression and function of DICER in macrophages via a previously unknown mechanism. Second, the presence of miRNAs in monocytes lacking DICER indicates that some miRNAs can be generated by proteins other than DICER. Human immunodeficiency virus, type 1 (HIV-1) infects multiple types of cells, including CD4ϩ T-cells, macrophages, monocytes, dendritic cells, and microglial cells (1-5). It has also been reported that latently infected hematopoietic stem cells can be detected in HIV-1-positive patients (6, 7). However, despite the range of susceptible cell types, HIV-1 is typically discussed in the context of T-cell infection and the resulting depletion of CD4 ϩ T-cells. Conversely, research into HIV-1 infection in the context of monocytes and macrophages remains underdeveloped. Monocytes and macrophages have been previously shown to harbor both productive and latent infections, serving as a part of the viral reservoir that could potentially be essential to chronic infection (8, 9). Unlike T-cells, macrophages can resist the cytotoxic effects of HIV-1 infection and persist for several months while producing new infectious viral particles (10 -12). In fact, it has been observed that latently infected macrophages are responsible for a large amount of viral resurgence after cessation of anti-retroviral therapies (13). Interestingly, we have previously observed that monocytes differ greatly from T-cells in their expression of miRNA-related enzymes. Specifically, monocytes appeared to be deficient for the DICER enzyme (14). This observation was intriguing because previous reports have indicated that HIV-1 can manipulate the host's RNA inter...

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