Influence of Cholesterol Import on Aspergillus fumigatus Growth and Antifungal Suscepibility

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Description:

Invasive pulmonary aspergillosis is a life-threatening fungal infection commonly observed in immunocompromised patients and has a mortality rate approaching 100% once the disease is disseminated. Aspergillus fumigatus is the most common pathogen. Early diagnosis improves the prognosis but is very difficult since most signs and symptoms are nonspecific. Antifungal therapy, usually based on sterol biosynthesis inhibitors, is also of limited efficacy. In my attempts to discover a diagnostic sterol marker for aspergillosis, I observed that A. fumigatus incorporates large amounts of cholesterol from serum-containing medium. This observation suggested the hypothesis that exogenous cholesterol from the host can be imported by A. fumigatus and used as a substitute for ergosterol in the cell membrane. This proposed mechanism would reduce the efficacy of antifungal drugs that act as sterol biosynthesis inhibitors. Experiments to test this hypothesis were designed to determine the effects of serum-free and serum-containing medium on growth of A. fumigatus in the presence and absence of azole antifungal agents. The results showed a marked increase in growth in the presence of human serum. Cultures in media containing cholesterol but no serum also showed enhanced growth, a result indicating that a non-cholesterol component of serum is not primarily responsible for the increased growth. However, sterol analysis of A. fumigatus cultured in the absence of inhibitors showed little or no change in ergosterol levels. This result suggested that the imported cholesterol was not being used as membrane sterol. However, in parallel experiments using Itraconazole™, an antifungal agent that attenuates sterol biosynthesis by inhibiting the sterol 14a-demethylase (ERG11), ergosterol levels decreased with increasing doses of inhibitor. Moreover, serum-containing medium partially rescued A. fumigatus from the effects of Itraconazole™, and a similar rescue effect was observed with serum-free media containing cholesterol. From the preceding results, it can be concluded that human serum enhances A. fumigatus growth, that cholesterol import rescues Aspergillus from the effects of antifungal agents, that the potency of some azole antifungals is decreased by cholesterol, and that imported cholesterol may substitute for membrane ergosterol in the presence of sterol biosynthesis inhibitors.

Creator(s): Hassan, Saad A.
Creation Date: December 2003
Partner(s):
UNT Libraries
Collection(s):
UNT Theses and Dissertations
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Publisher Info:
Publisher Name: University of North Texas
Place of Publication: Denton, Texas
Date(s):
  • Creation: December 2003
  • Digitized: November 27, 2003
Description:

Invasive pulmonary aspergillosis is a life-threatening fungal infection commonly observed in immunocompromised patients and has a mortality rate approaching 100% once the disease is disseminated. Aspergillus fumigatus is the most common pathogen. Early diagnosis improves the prognosis but is very difficult since most signs and symptoms are nonspecific. Antifungal therapy, usually based on sterol biosynthesis inhibitors, is also of limited efficacy. In my attempts to discover a diagnostic sterol marker for aspergillosis, I observed that A. fumigatus incorporates large amounts of cholesterol from serum-containing medium. This observation suggested the hypothesis that exogenous cholesterol from the host can be imported by A. fumigatus and used as a substitute for ergosterol in the cell membrane. This proposed mechanism would reduce the efficacy of antifungal drugs that act as sterol biosynthesis inhibitors. Experiments to test this hypothesis were designed to determine the effects of serum-free and serum-containing medium on growth of A. fumigatus in the presence and absence of azole antifungal agents. The results showed a marked increase in growth in the presence of human serum. Cultures in media containing cholesterol but no serum also showed enhanced growth, a result indicating that a non-cholesterol component of serum is not primarily responsible for the increased growth. However, sterol analysis of A. fumigatus cultured in the absence of inhibitors showed little or no change in ergosterol levels. This result suggested that the imported cholesterol was not being used as membrane sterol. However, in parallel experiments using Itraconazole™, an antifungal agent that attenuates sterol biosynthesis by inhibiting the sterol 14a-demethylase (ERG11), ergosterol levels decreased with increasing doses of inhibitor. Moreover, serum-containing medium partially rescued A. fumigatus from the effects of Itraconazole™, and a similar rescue effect was observed with serum-free media containing cholesterol. From the preceding results, it can be concluded that human serum enhances A. fumigatus growth, that cholesterol import rescues Aspergillus from the effects of antifungal agents, that the potency of some azole antifungals is decreased by cholesterol, and that imported cholesterol may substitute for membrane ergosterol in the presence of sterol biosynthesis inhibitors.

Degree:
Level: Doctoral
Discipline: Molecular Biology
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Keyword(s): Cholesterol import | Aspergillus fumigatus | antifungal suscepibility tests
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Partner:
UNT Libraries
Collection:
UNT Theses and Dissertations
Identifier:
  • OCLC: 54350861 |
  • ARK: ark:/67531/metadc5539
Resource Type: Thesis or Dissertation
Format: Text
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Access: Use restricted to UNT Community (strictly enforced)
License: Copyright
Holder: Hassan, Saad A.
Statement: Copyright is held by the author, unless otherwise noted. All rights reserved.