Studies on Phase Transitions in Sporothrix Schenckii: Possible Involvement of Protein Kinase C
Protein kinase C (PKC) is a signal transducing enzyme, that has been related to the regulation of proliferative and morphogenetic processes in many eukaryotic systems. The yeast to mycelium transition in Sporothrix schenckii involves both morphogenesis and proliferation and has been reported by us to be stimulated by calcium ions but can occur in the absence of this cation. The work reported here suggests the involvement of PKC in the induction of the yeast to mycelium transition in S. schenkii in the absence of extracellular calcium ions. Phorbol-12-myristate-13-acetate (PMA), a tumor promoting agent and PKC activator was found to stimulate germ tube formation and germ tube growth by yeast cells induced to undergo transition to the mycelium form in a concentration dependent manner with an optimal stimulatory concentration of 2 µM. This same PMA concentration had a stimulatory effect on DNA and RNA synthesis in cells induced to undergo the yeast to mycelium transition and was found to inhibit cell duplication and bud formation in yeast cells induced to re-enter the budding cycle. Polymyxin B, an inhibitor of PKC, inhibited germ tube formation by yeast cells at concentrations of 50 and 100 µM, at all time intervals tested. This inhibition could be overcome if PMA 2 µM or calcium 1 mM were added to the medium, suggesting that the inhibition obtained in the presence of this antibiotic was due to an inhibition of PKC. These results support the involvement of PKC in the control of the dimorphic expression in S. schenckii. Other PKC inhibitors tested, H-7 and staurosporine gave unexpected results, probably because of their lack of specificity and the fact that they inhibit other protein kinases.
KeywordsYeast Cell Germ Tube Phorbol Ester Mycelium Form Yeast Cell Cycle
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