Tissue-specific changes in mRNA expression of Abc and Slc transporters in murine pulmonary tuberculosis

Intracellular Staphylococcus aureus Modulates Host Central Carbon Metabolism To Activate Autophagy

Staphylococcus aureus is a facultative intracellular pathogen that invades and replicates within many types of phagocytic and nonphagocytic cells. During intracellular infection, S. aureus is capable of subverting xenophagy and escaping to the cytosol of the host cell. Furthermore, drug-induced autophagy facilitates the intracellular replication of S. aureus, but the reasons behind this are unclear. Here, we have studied the host central carbon metabolism during S. aureus intracellular infection. We found extensive metabolic rerouting and detected several distinct metabolic changes that suggested starvation-induced autophagic flux in infected cells. These changes included increased uptake but lower intracellular levels of glucose and low abundance of several essential amino acids, as well as markedly upregulated glutaminolysis. Furthermore, we show that AMP-activated protein kinase (AMPK) and extracellular signal-regulated kinase (ERK) phosphorylation levels are significantly increased in infected cells. Interestingly, while autophagy was activated in response to S. aureus invasion, most of the autophagosomes detected in infected cells did not contain bacteria, suggesting that S. aureus induces the autophagic flux during cell invasion for energy generation and nutrient scavenging. Accordingly, AMPK inhibition halted S. aureus intracellular proliferation.IMPORTANCEStaphylococcus aureus escapes from immune recognition by invading a wide range of human cells. Once the pathogen becomes intracellular, the most important last resort antibiotics are not effective. Therefore, novel anti-infective therapies against intracellular S. aureus are urgently needed. Here, we have studied the physiological changes induced in the host cells by S. aureus during its intracellular proliferation. This is important, because the pathogen exploits the host cell's metabolism for its own proliferation. We find that S. aureus severely depletes glucose and amino acid pools, which leads to increased breakdown of glutamine by the host cell in an attempt to meet its own metabolic needs. All of these metabolic changes activate autophagy in the host cell for nutrient scavenging and energy generation. The metabolic activation of autophagy could be used by the pathogen to sustain its own intracellular survival, making it an attractive target for novel anti-infectives.

Alterations in gene expression in vitamin D-deficiency: Down-regulation of liver Cyp7a1 and renal Oat3 in mice

The vitamin D-deficient model, established in the C57BL/6 mouse after 8 weeks of feeding vitamin D-deficient diets in the absence or presence of added calcium, was found associated with elevated levels of plasma parathyroid hormone (PTH) and plasma and liver cholesterol, and a reduction in cholesterol 7α-hydroxylase (Cyp7a1, rate-limiting enzyme for cholesterol metabolism) and renal Oat3 mRNA/protein expression levels. However, there was no change in plasma calcium and phosphate levels. Appraisal of the liver revealed an up-regulation of mRNA expressions of the small heterodimer partner (Shp) and attenuation of Cyp7a1, which contributed to hypercholesterolemia in vitamin D-deficiency. When vitamin D-sufficient or D-deficient mice were further rendered hypercholesterolemic with 3 weeks of feeding the respective, high fat/high cholesterol (HF/HC) diets, treatment with 1α,25-dihydroxyvitamin D₃ [1,25(OH)₂ D₃ ], active vitamin D receptor (VDR) ligand, or vitamin D (cholecalciferol) to HF/HC vitamin D-deficient mice lowered the cholesterol back to baseline levels. Cholecalciferol treatment partially restored renal Oat3 mRNA/protein expression back to that of vitamin D-sufficient mice. When the protein expression of protein kinase C (PKC), a known, negative regulator of Oat3, was examined in murine kidney, no difference in PKC expression was observed for any of the diets with/without 1,25(OH)₂ D₃ /cholecalciferol treatment, inferring that VDR regulation of renal Oat3 did not involve PKC in mice. As expected, plasma calcium levels were not elevated by cholecalciferol treatment of vitamin D-deficient mice, while 1,25(OH)₂ D₃ treatment led to hypercalcemia. In conclusion, vitamin D-deficiency resulted in down-regulation of liver Cyp7a1 and renal Oat3, conditions that are alleviated upon replenishment of cholecalciferol.

Metabolic host responses to infection by intracellular bacterial pathogens

The interaction of bacterial pathogens with mammalian hosts leads to a variety of physiological responses of the interacting partners aimed at an adaptation to the new situation. These responses include multiple metabolic changes in the affected host cells which are most obvious when the pathogen replicates within host cells as in case of intracellular bacterial pathogens. While the pathogen tries to deprive nutrients from the host cell, the host cell in return takes various metabolic countermeasures against the nutrient theft. During this conflicting interaction, the pathogen triggers metabolic host cell responses by means of common cell envelope components and specific virulence-associated factors. These host reactions generally promote replication of the pathogen. There is growing evidence that pathogen-specific factors may interfere in different ways with the complex regulatory network that controls the carbon and nitrogen metabolism of mammalian cells. The host cell defense answers include general metabolic reactions, like the generation of oxygen- and/or nitrogen-reactive species, and more specific measures aimed to prevent access to essential nutrients for the respective pathogen. Accurate results on metabolic host cell responses are often hampered by the use of cancer cell lines that already exhibit various de-regulated reactions in the primary carbon metabolism. Hence, there is an urgent need for cellular models that more closely reflect the in vivo infection conditions. The exact knowledge of the metabolic host cell responses may provide new interesting concepts for antibacterial therapies.

Mycobacterium tuberculosis Mce1 protein complex initiates rapid induction of transcription of genes involved in substrate trafficking

The mammalian cell entry (Mce)1 protein complex has an important role during the initial phase of a Mycobacterium tuberculosis (M. tuberculosis) infection. Murine macrophages were infected with M. tuberculosis H37Rv or Δ-mce1 H37Rv, and total RNA was isolated from the host cells at 15, 30 and 60 min, and 4 and 10 h post-infection. With the aim of studying the role for the Mce1 protein complex on host gene expression, the RNA was hybridized onto 44 K whole-genome microarrays. Selected genes were verified by reverse-transcriptase quantitative PCR (RT-QPCR). 'Transport' was the most overrepresented biological process during the first hour post H37Rv infection. Five genes (Abca1 (21.0-fold), Slc16a10 (3.1-fold), Slc6a12 (17.9-fold), Slc6a8 (2.3-fold) and Nr1h3, (5.5-fold)) involved in substrate trafficking were verified by RT-QPCR to be upregulated by >2-fold 1 h post H37Rv infection. By 1 h post Δ-mce1 H37Rv infection, only Abca1 and Slc6a12 were upregulated by >2-fold. A number of other genes, which may be directly involved in substrate trafficking or share the same transcription, were found to have expression profiles similar to the genes involved in substrate trafficking. The Mce1 protein complex has a significant role in the transcriptional activation of genes involved in substrate trafficking during the initial phase of an M. tuberculosis infection.