Latently infected CD4+ T cells are the main barrier to complete clearance of HIV infection, but it is unclear what mechanisms govern latent HIV infection in vivo. To address this question, we developed a new panel of reverse transcription droplet digital polymerase chain reaction (RT-ddPCR) assays specific for different HIV transcripts that define distinct blocks to transcription. We applied this panel of assays to CD4+ T cells freshly isolated from HIV-infected patients on suppressive antiretroviral therapy (ART) to quantify the degree to which different mechanisms inhibit HIV transcription. In addition, we measured the degree to which these transcriptional blocks could be reversed ex vivo by T cell activation (using anti-CD3/CD28 antibodies) or latency-reversing agents. We found that the main reversible block to HIV RNA transcription was not inhibition of transcriptional initiation but rather a series of blocks to proximal elongation, distal transcription/polyadenylation (completion), and multiple splicing. Cell dilution experiments suggested that these mechanisms operated in most of the HIV-infected CD4+ T cells examined. Latency-reversing agents exerted differential effects on the three blocks to HIV transcription, suggesting that these blocks may be governed by different mechanisms.

The persistence of latent HIV proviruses in long-lived CD4+ T cells despite antiretroviral therapy (ART)1–3 is a major obstacle to viral eradication4–6. Because current candidate latency-reversing agents (LRAs) induce HIV transcription but fail to clear these cellular reservoirs,7–8 new approaches for killing these reactivated latent HIV reservoir cells are urgently needed. HIV latency depends upon transcriptional quiescence of the integrated provirus and circumvention of immune defense mechanisms4–6,9. These defenses include cell-intrinsic innate responses that use pattern-recognition receptors (PRR) to detect viral pathogens and subsequently induce apoptosis of the infected cell10. Retinoic acid-inducible gene I (RIG-I) forms one class of pattern-recognition receptors that mediates apoptosis and elimination of infected cells after recognition of viral RNA11–14. Here we show that acitretin, an FDA-approved retinoic-acid derivative, enhances RIG-I signaling ex vivo, increases HIV transcription, and induces preferential apoptosis of HIV-infected cells. These effects are abrogated by RIG-I knockdown. Acitretin also decreases proviral DNA levels in CD4+ T cells from HIV-infected subjects on suppressive ART, an effect amplified by combination with suberoylanilide hydroxamic acid (SAHA), a histone deacetylase inhibitor. Pharmacologic enhancement of an innate cellular defense network could provide a means to eliminate reactivated cells in the latent HIV reservoir.

Latently-infected CD4+ T cells are widely considered to be the major barrier to a cure for HIV. Much of our understanding of HIV latency comes from latency models and blood cells, but most HIV-infected cells reside in lymphoid tissues such as the gut. We hypothesized that tissue-specific environments may impact the mechanisms that govern HIV expression. To assess the degree to which different mechanisms inhibit HIV transcription in the gut and blood, we quantified HIV transcripts suggestive of transcriptional interference (U3-U5; "Read-through"), initiation (TAR), 5' elongation (R-U5-pre-Gag; "Long LTR"), distal transcription (Nef), completion (U3-polyA; "PolyA"), and multiple splicing (Tat-Rev) in matched peripheral blood mononuclear cells (PBMCs) and rectal biopsies, and matched FACS-sorted CD4+ T cells from blood and rectum, from two cohorts of ART-suppressed individuals. Like the PBMCs, rectal biopsies showed low levels of read-through transcripts (median = 23 copies/106 cells) and a gradient of total (679)>elongated(75)>Nef(16)>polyadenylated (11)>multiply-spliced HIV RNAs(<1) [p<0.05 for all], demonstrating blocks to HIV transcriptional elongation, completion, and splicing. Rectal CD4+ T cells showed a similar gradient of total>polyadenylated>multiply-spliced transcripts, but the ratio of total to elongated transcripts was 6-fold lower than in blood CD4+ T cells (P = 0.016), suggesting less of a block to HIV transcriptional elongation in rectal CD4+ T cells. Levels of total transcripts per provirus were significantly lower in rectal biopsies compared to PBMCs (median 3.5 vs. 15.4; P = 0.008) and in sorted CD4+ T cells from rectum compared to blood (median 2.7 vs. 31.8; P = 0.016). The lower levels of HIV transcriptional initiation and of most HIV transcripts per provirus in the rectum suggest that this site may be enriched for latently-infected cells, cells in which latency is maintained by different mechanisms, or cells in a "deeper" state of latency. These are important considerations for designing therapies that aim to disrupt HIV latency in all tissue compartments.

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Aquaporin 3 (AQP3), a member of the aquaglyceroporin family, which transports water and glycerol, is robustly expressed in epidermis and plays an important role in stratum corneum hydration, permeability barrier function, and wound healing. PPAR and LXR activation regulates the expression of many proteins in the epidermis and thereby can affect epidermal function. Here we report that PPARgamma activators markedly stimulate AQP3 mRNA expression in both undifferentiated and differentiated cultured human keratinocytes (CHKs). The increase of AQP3 mRNA by PPARgamma activator occurs in a dose- and time-dependent fashion. Increased AQP3 mRNA levels are accompanied by an increase in AQP3 protein in undifferentiated keratinocytes and a significant increase in glycerol uptake. Activation of LXR, RAR, and RXR also increase AQP3 mRNA levels in undifferentiated and differentiated CHKs, but to a lesser extent. PPARdelta activation stimulates AQP3 expression in undifferentiated CHKs but decreases expression in differentiated CHKs. In contrast, PPARalpha activators do not alter AQP3 expression. AQP9 and AQP10, other members of aquaglyceroporin family, are less abundantly expressed in CHKs and their expression levels are not significantly altered by treatment with LXR, PPAR, RAR, or RXR activators. Finally, when topically applied, the PPARgamma activator, ciglitazone, induces AQP3 but not AQP9 gene expression in mouse epidermis. Our data demonstrate that PPAR and LXR activators stimulate AQP3 expression, providing an additional mechanism by which PPAR and LXR activators regulate epidermal function.

ABCA12 (ATP binding cassette transporter, family 12) is a cellular membrane transporter that facilitates the delivery of glucosylceramides to epidermal lamellar bodies in keratinocytes, a process that is critical for permeability barrier formation. Following secretion of lamellar bodies into the stratum corneum, glucosylceramides are metabolized to ceramides, which comprise ϳ50% of the lipid in stratum corneum. Gene mutations of ABCA12 underlie harlequin ichthyosis, a devastating skin disorder characterized by abnormal lamellar bodies and a severe barrier abnormality. Recently we reported that peroxisome proliferator-activated receptor (PPAR) and liver X receptor activators increase ABCA12 expression in human keratinocytes. Here we demonstrate that ceramide (C 2 -Cer and C 6 -Cer), but not C 8 -glucosylceramides, sphingosine, or ceramide 1-phosphate, increases ABCA12 mRNA expression in a doseand time-dependent manner. Inhibitors of glucosylceramide synthase, sphingomyelin synthase, and ceramidase and small interfering RNA knockdown of human alkaline ceramidase, which all increase endogenous ceramide levels, also increased ABCA12 mRNA levels. Moreover, simultaneous treatment with C 6 -Cer and each of these same inhibitors additively increased ABCA12 expression, indicating that ceramide is an important inducer of ABCA12 expression and that the conversion of ceramide to other sphingolipids or metabolites is not required. Finally, both exogenous and endogenous ceramides preferentially stimulate PPAR␦ expression (but not other PPARs or liver X receptors), whereas PPAR␦ knockdown by siRNA transfection specifically diminished the ceramide-induced increase in ABCA12 mRNA levels, indicating that PPAR␦ is a mediator of the ceramide effect. Together, these results show that ceramide, an important lipid component of epidermis, up-regulates ABCA12 expression via the PPAR␦-mediated signaling pathway, providing a substrate-driven, feed-forward mechanism for regulating this key lipid transporter.

Keratinocytes require abundant cholesterol for cutaneous permeability barrier function; hence, the regulation of cholesterol homeostasis is of great importance. ABCA1 is a membrane transporter responsible for cholesterol efflux and plays a pivotal role in regulating cellular cholesterol levels. We demonstrate that ABCA1 is expressed in cultured human keratinocytes (CHKs) and murine epidermis. Liver X receptor (LXR) activation markedly stimulates ABCA1 mRNA and protein levels in CHKs and mouse epidermis. In addition to LXR, activators of peroxisome proliferator-activated receptor (PPAR)a, PPARb/d, and retinoid X receptor (RXR), but neither PPARg nor retinoic acid receptor, also increase ABCA1 expression in CHKs. Increases in cholesterol supply induced by LDL or mevalonate stimulate ABCA1 expression, whereas inhibiting cholesterol synthesis with statins or cholesterol sulfate decreases ABCA1 expression in CHKs. After acute permeability barrier disruption by either tape-stripping or acetone treatment, ABCA1 expression declines, and this attenuates cellular cholesterol efflux, making more cholesterol available for regeneration of the barrier. In addition, during fetal epidermal development, ABCA1 expression decreases at days 18-22 of gestation (term 5 22 days), leaving more cholesterol available during the critical period of barrier formation. Together, our results show that ABCA1 is expressed in keratinocytes, where it is negatively regulated by a decrease in cellular cholesterol levels or altered permeability barrier requirements and positively regulated by activators of LXR, PPARs, and RXR or increases in cellular cholesterol levels.-Jiang, Y. J., B. Lu, P. Kim, P. M. Elias, and K. R. Feingold. Regulation of ABCA1 expression in human keratinocytes and murine epidermis.

ATP-binding cassette (ABC) transporter, family 12 (ABCA12), a member of the ABC superfamily, facilitates the delivery of lipids to lamellar bodies (LB) in keratinocytes, which is critical for permeability barrier function. Recently, gene mutations of ABCA12 were found to underlie Harlequin ichthyosis and lamellar ichthyosis, two devastating skin disorders. Previously we and others have demonstrated that peroxisome proliferators-activated receptors (PPARs) and liver X receptor (LXR) activation improved epidermal permeability barrier homeostasis by stimulating keratinocyte differentiation, lipid synthesis, and increasing LB formation/secretion. Here we report that both PPAR-gamma and -beta/delta activators markedly stimulate ABCA12 mRNA expression in cultured human keratinocyte (CHK) in a dose- and time-dependent manner. Increased ABCA12 mRNA levels are accompanied by an increase in ABCA12 protein, suggesting biological importance of this upregulation. LXR activators also increase ABCA12 mRNA levels in CHK, but to a lesser extent. In contrast, activators of PPAR-alpha, RAR, RXR, or vitamin D receptor did not alter ABCA12 expression. Two major ABCA12 alternative transcripts and their corresponding proteins are also expressed and upregulated by PPAR or LXR activator in both undifferentiated and differentiated CHK. Together, our data demonstrate that PPAR and LXR activators increase ABCA12 expression, providing an additional mechanism by which PPAR and LXR activators promote epidermal permeability barrier homeostasis.