Effect of dibutyryl cyclic AMP on lipid synthesis in Microsporum gypseum

Influence of W-7, a calmodulin antagonist on phospholipid biosynthesis in Candida albicans

AIM

This study was undertaken to investigate the role of calmodulin in phospholipid biosynthesis in Candida albicans using W-7, a calmodulin antagonist.

METHODS

Cells were grown as shake cultures in the absence and presence of W-7 at different concentrations. Changes in cell mass, phospholipid content and incorporation of labelled precursor into phospholipid and activities of respective enzymes have been studied.

RESULTS

Decreased incorporation of labelled acetate into total lipids and phospholipids was observed in the presence of 40 microm of W-7 which was not as a consequence of altered growth of Candida in the presence of calmodulin antagonist. Further, a significant decrease in the levels of calmodulin and CaM dependent protein kinase activity was observed in cells grown with different concentrations of W-7. This was accompanied by decreased/increased activity of phosphatidic acid phosphatase and phospholipase A, respectively in W-7 grown cells as compared to controls.

CONCLUSIONS

These findings suggest definite involvement of calmodulin in the regulation of phospholipid metabolism in Candida albicans.

Dermatophyte lipids-Composition and regulation of phospholipids

This review presents the extensive work carried out on lipid components of dermatophytes, their biosynthesis, turnover and regulation. It emerges from the work done so far that the pathways of lipid biosynthesis/ degradation and the lipid composition in dermatophytes are similar to those in yeasts and other fungi. Second messengers (Ca(2+), cAMP) were demonstrated to have a regulatory role in phospholipid metabolism and they mainly act by stimulating Ca(2+)/CaM or cAMP dependent protein kinase(s). Both these kinases were purified and characterized inMicrosporum gypseum. Further work is being carried out to elucidate the molecular mechanism of regulation of phospholipid metabolism by these second messengers.

Correlation between intracellular cAMP levels and phospholipids of Microsporum gypseum

Atropine, a modulator of cAMP has been used to examine the relationship between phospholipids and intracellular levels of cAMP in Microsporum gypseum. A decreased phospholipid content was observed in atropine grown cells as a result of reduced levels of intracellular cAMP. This decline was caused by the inhibitory effect of atropine on adenylate cyclase. Lowered phospholipid content was supported by decreased [14C]acetate incorporation as well as reduced activities of key enzymes of phospholipid biosynthesis. In vitro supplementation of atropine in control cells also caused inhibition in lipid synthesis indicating similar effects of atropine and its metabolites. These results in conjunction with our previous report, in which enhanced levels of cAMP resulted in increased phospholipid synthesis, suggest a direct correlation between phospholipid biosynthesis and intracellular levels of cAMP in M. gypseum.

Influence of aminophylline on the lipids in Microsporum gypseum

The effect of aminophylline on the lipid synthesis of Microsporum gypseum has been examined. A decreased incorporation of [14C]acetate into lipids was observed when the cells were incubated for 1 h with aminophylline which was reflected in all the individual lipid fractions. However, cells grown with aminophylline in the growth medium exhibited increased levels of total phospholipids, which was probably due to a rise in intracellular cAMP as these cells exhibited 4-fold increased levels of cAMP. Decreased activity of phosphodiesterase by aminophylline accounts for the increased cAMP levels. Increased phospholipid content in aminophylline grown cells was further supported by the increased incorporation of [14C]acetate into phospholipids as well as increased activities of phospholipid biosynthetic enzymes in comparison to non-supplemented cells.

Studies on the mode of action of tolnaftate in Microsporum gypseum

Studies were performed on the mode of action of tolnaftate and resistance to this drug in Microsporum gypseum. Cells grown in the presence of tolnaftate (at the IC 50) showed a reduced content of total phospholipids and sterols whereas there was an increase in total RNA content. Incubation of cells with tolnaftate (at 10 x MIC), followed by addition of different macromolecule precursors revealed inhibition of the biosynthesis of all macromolecules except for RNA. The activity of membrane-bound enzymes did not change on treatment with tolnaftate (10 x MIC) whereas an increase in the leakage of intracellular 32P was observed. The content of total phospholipids was higher in tolnaftate-resistant cells, whereas the content of total sterols, DNA, RNA and protein was comparable to that of susceptible cultures. Activity of phosphodiesterase decreased and 5'-nucleotidase increased in tolnaftate-resistant cells. Our results suggest that the antifungal activity of tolnaftate is due to differential action on various targets site(s) which are modified in strains resistant to the drug.