Coronary heart disease (CHD) is a major cause of disability and premature death throughout the world.¹ Epidemiological studies have demonstrated an inverse association between physical fitness and the incidence of CHD or all-cause mortality in healthy or asymptomatic participants. Physical fitness is typically expressed as cardiorespiratory fitness (CRF) and is assessed by exercise tolerance testing²; however, it is rare for clinicians to consider CRF when evaluating future risk of CHD.³

Cardiovascular disease (CVD) is the leading cause of death worldwide.¹ Low blood levels of high-density lipoprotein cholesterol (HDL-C) are an independent risk factor for CVD.^2,3 Cross-sectional data provide strong evidence that people who are more physically active have higher HDL-C levels.^4-6 Thus, the value of regular aerobic exercise in increasing serum HDL-C level and in reducing the risk of CVD has received widespread acceptance.⁷ In contrast, results of aerobic exercise studies vary considerably, depending on the exercise program (eg, duration, intensity, or frequency) and characteristics of subjects at baseline.⁸ However, few studies have examined which characteristics would affect the response of HDL-C level to exercise training.

When transitioning from the environment, pathogenic microorganisms must adapt rapidly to survive in hostile host conditions. This is especially true for environmental fungi that cause opportunistic infections in immunocompromised patients since these microbes are not well adapted human pathogens. Cryptococcus species are yeast-like fungi that cause lethal infections, especially in HIV-infected patients. Using Cryptococcus deneoformans in a murine model of infection, we examined contributors to resistance to antifungal drug exposure and demonstrated that transposon mutagenesis drives the development of 5-fluoroorotic acid (5FOA) resistance. Inactivation of target genes URA3 or URA5 primarily reflected the insertion of two transposable elements (TEs): the T1 DNA transposon and the TCN12 retrotransposon. Consistent with in vivo results, increased rates of mutagenesis and resistance to several antifungal drugs were found when Cryptococcus was incubated at 37° compared to 30° in vitro, a condition that mimics the temperature shift that occurs during the environment-to-host transition. Inactivation of the RNAi pathway, which suppresses TE movement in many organisms, was not sufficient to elevate TE movement at 30° to the level observed at 37°. We propose that temperature-dependent TE mobilization in Cryptococcus is an important mechanism that enhances microbial adaptation and promotes pathogenesis and drug resistance in the human host. SIGNIFICANCE STATEMENT The incidence of infections due to fungal pathogens has dramatically increased in the past few decades with similar increases in human populations with weakened or suppressed immune systems. Understanding the mechanisms by which organisms rapidly adapt during human infection to enhance virulence and evolve drug resistance is important for developing effective treatments. We find that transposon mobilization in the human pathogen Cryptococcus promotes mutations in the genome resulting in enhanced drug resistance in a mouse infection model. Thermotolerance is a key virulence determinant for pathogenic fungi during the environment-to-host transition, and we demonstrate that a temperature increase is sufficient to trigger transposon mobilization in vitro. The link between temperature stress and transposon-associated mutations may significantly impact adaptation to the host during infection, including the acquisition of drug resistance.

Previous studies have demonstrated that polyunsaturated fatty acids (PUFAs) suppress sterol regulatory element-binding protein 1c (SREBP-1c) expression and, thus, lipogenesis. In the current study, the molecular mechanism for this suppressive effect was investigated with luciferase reporter gene assays using the SREBP-1c promoter in HEK293 cells. Consistent with previous data, the addition of PUFAs to the medium in the assays robustly inhibited the SREBP-1c promoter activity. Deletion and mutation of the two liver X receptor (LXR)-responsive elements (LXREs) in the SREBP-1c promoter region eliminated this suppressive effect, indicating that both LXREs are important PUFA-suppressive elements. The luciferase activities of both SREBP-1c promoter and LXRE enhancer constructs induced by co-expression of LXRα or -β were strongly suppressed by the addition of various PUFAs (arachidonic acid > eicosapentaenoic acid > docosahexaenoic acid > linoleic acid), whereas saturated or mono-unsaturated fatty acids had minimal effects. Gel shift mobility and ligand binding domain activation assays demonstrated that PUFA suppression of SREBP-1c expression is mediated through its competition with LXR ligand in the activation of the ligand binding domain of LXR, thereby inhibiting binding of LXR/retinoid X receptor heterodimer to the LXREs in the SREBP-1c promoter. These data suggest that PUFAs could be deeply involved in nutritional regulation of cellular fatty acid levels by inhibiting an LXR-SREBP-1c system crucial for lipogenesis.