Abstract
Candida albicans is a pathogenic yeast responsible for causing infection in patients under immunosuppressive therapy, and is responsible for recurrent vulvovaginal candidiasis. Although, number of C. albicans derived intrinsic factors and the host factors have been found responsible for virulence and favouring the disease process, there is no antimycotic drug which proves out to be completely fungicidal, suppressing recurrent infections. This work deals with the study of Candida albicans-host cell interaction, to give an insight of the disease process, which could lead to new therapeutic interventions. Earlier studies on D. uninucleata revealed presence of a biologically active 3-HETE derived from exogenously fed Arachidonic acid. It was found, that it is not only important for its own life cycle, but also shows biological activity in human cells. We tried to find if a similar mechanism occurs in pathogenic fungi. It was found that although C. albicans does not contain arachidonic acid, it is able to grow on arachidonic acid as a sole carbon source as efficiently as it can utilise linoleic acid. Metabolism of arachidonic acid by C. albicans occurs both by mitochondrial dependent and independent pathways as shown by inhibitor studies. Arachidonic acid was shown to block the alternative oxidase pathway of energy generation in Candida albicans, and was efficiently metabolised to other fatty acids, carbohydrates, and proteins. Arachidonic acid metabolism by Candida albicans produced a novel compound 3,18-diHETE, which was closely related to previously discovered 3-HETE by Dr. S. Nigam's group. It was also observed that 3,18-diHETE was mainly associated with the cells bearing germ-tube and hyphael forms. This production was found to be aspirin sensitive. We found that C. albicans grown in aspirin showed diminished immunofluorescence with anti-3-OH-oxylipins antibody, indicating decreased amounts of 3,18 diHETE. Apart from this, there was diminished germ-tube formation, which is essential for infection, in C. albicans in presence of aspirin. Aspirin suppressed not only cell growth, but it also reduced the adhesion of C. albicans to the host cells. These observations prompted us to explore a therapeutic role for aspirin. When given along with clotrimazole, aspirin was found to reduce minimum inhibitory concentration (MIC) of clotrimazole against C. albicans. The action of aspirin was directed towards inhibiting 3-hydroxylation mechanism in C. albicans and of clotrimazole towards 18-hydroxylation mechanism and ergosterol synthesis, which occurs via cytochrome p450. This demonstrates a novel approach to potentiate the action of antimycotic drug clotrimazole. Moreover, aspirin is a cyclooxygenase-2 inhibitor, which inhibits formation of PGE2, and thus the infectivity of C. albicans. Our results thus showed that aspirin may be a suitable drug for the therapy of recurrent Candidiasis.
It is known that host factors are also responsible for the establishment of infection. HDL, which is lowered in sepsis patients, and therefore substituted, was found to further increase virulence by inducing germtube formation in C. albicans.
The first step of infection includes adhesion of pathogen to host cells. C. albicans which adheres to host cells rapidly is found to be more virulent, and is able to proceeds further to establish the infection process. Our results demonstrate that aspirin is able to decrease the adhesion of C. albicans to HeLa cell, thus decreasing its virulence.
Upon infection with C. albicans signal transduction pathways are triggered in host cells (HeLa cells). We found upregulation of COX-2 in HeLa cells upon infection with C. albicans as well as increase in PGE2 production. Not only C. albicans but also 3-HETE, upregulated COX-2, showing that 3-HETE was important compound which mediated cell signaling during infection. Using specific chemical inhibitors to various signal transduction pathways, it was established that Protein Kinase C (PKC) and p38 MAP kinase pathway were primarily involved in the COX-2 upregulation. In other systems, involving cytokines and LPS, control of COX-2 transcription involved ERK1/2 and JNK MAP kinase pathways. Further studies were performed to evaluate the exact sequence of activation of kinases involved. GF 203190X (PKC inhibitor) prevented the phosphorylation of p38 MAP kinase, while other inhibitors failed to have any effect. Thus, PKC acts as an upstream factor phosphorylating p38 MAP kinase. C. albicans induced p38 MAP kinase was also observed to be involved in modulating cytoskeletol changes host cell. This effect involved the activation, of phosphorylated HSP27, a factor involved in the modulation of actin, which was abrogated by p38 MAP kinase inhibitor SB 202190.We find that inhibition of free radicals, NAC inhibits COX-2 and PGE2 upregulation suggesting involvement of reactive oxygen species and NFkB during the infection of HeLa cells with C. albicans.
The control of transcription occur by the various transcription factors binding to the concerned gene promoter. NFkB is one of the important transcription factors implicated in the COX-2 transcription. The role of NFkB was studied using a NFkB-dependent reporter plasmid and a dominant negative plasmid for IkB (prevents the release of active NFkB). NFkB-dependent transcription was triggered upon infection with C albicans. However, this could be only partially abrogated by the IkB dominant negative plasmid, moreover PI3-kinase pathway is involved in COX-2 upregulation via NFkB pathway. Thus C. albicans brings about redundant signalling for the upregulation and control of COX-2 transcription.
Extensive programmed cell death (apoptosis) was observed after 24 h in Hela cells infected with C. albicans as shown by genomic DNA laddering and TUNEL assay. After 6 h postinfection, upregulation of caspase-3 activity was observed. Caspase-3 is an effector caspase which is involved in the cleavage of cellular proteins during apoptosis. Further more, using DNA microarray technology, we showed that C. albicans upregulated PI-3-kinase in HeLa cells. PI-3-kinase is a prominent mediator of anti-apoptotic functions via AKT and p65 NFkB. Its inhibition by wortmannin increased the caspase-3 activity in infected cells. IkB dominant negative transfected cells showed no caspase-3 activity upon infection. This result was surprising as p65 NFkB, whose release is prevented by the dominant negative plasmid, is an anti-apoptotic molecule. This paradoxical reaction was clarified by the observation of truncated forms of p65 NFkB in C. albicans infected HeLa cells. Carboxy terminal truncation of p65 NFkB by caspase-3 renders the molecule inactive thereby preventing its anti-apoptotic function. The pro-apoptotic subunits of NFkB, p50 and c-REL, however, were upregulated in C. albicans infected cells. Thus, the anti-apoptotic pathway, from PI-3-kinase via AKT to p65 NFkB is rendered ineffective by the cleavage of p65 NFkB by caspase-3. |
