Indirubin, an acting component of indigo naturalis, inhibits EGFR activation and EGF-induced CDC25B gene expression in epidermal keratinocytes

Background: Topical indigo naturalis ointment is clinically proved to be an effective therapy for plaque-type psoriasis. Indirubin, as the active component of indigo naturalis, inhibits cell proliferation of epidermal keratinocytes. However, the detailed underlying mechanism is not fully understood.

Objective: To further investigate the anti-proliferating effects of indigo naturalis and indirubin on epidermal keratinocytes.

Methods: The decreased expression of CDC25B in indigo naturalis- or indirubin-treated epidermal keratinocytes, as revealed by cDNA microarray analysis, was studied. The CDC25B expression was examined under different serum concentrations and compared between primary and immortalized keratinocytes. The activation of EGFR and the effect of EGF on the cell proliferation and CDC25B expression were also investigated in epidermal keratinocytes. RT/Real-Time PCR and western blot method were used to analyze the CDC25B expression at the mRNA and protein levels, respectively.

Results: Indigo naturalis and indirubin were confirmed to down-regulate CDC25B expression significantly at both the mRNA and protein levels. The growth-dependent expression of CDC25B was demonstrated by the increased expression in serum-stimulated and immortalized keratinocytes. The activation of EGF receptor, known to be highly expressed in psoriatic lesions, was inhibited by indigo naturalis or indirubin. The cell proliferation and CDC25B expression of epidermal keratinocytes were induced by EGF alone and confirmed to be inhibited by indigo naturalis or indirubin.

Conclusion: Except being a common therapeutic target in various cancers, CDC25B also plays an important role in the hyper-proliferation of epidermal keratinocytes which can be suppressed by anti-psoriatic drug indigo naturalis and its component, indirubin.