Cyclin-dependent kinases (CDKs) are involved in regulation of cell cycle and passage through the G1 restriction point relies on CDK4/6 and their association with cyclin D1 (CycD1) whereas entry into S phase requires CDK2 and its association with cyclin E (CycE). The levels of cyclin partners, their phosphorylation status, and the abundance of inhibitors of CDK (CDKI) regulate CDK activity. CKIs may be implicated in signaling pathways that are involved in psoriasis pathogenesis and regulation of G1 and S phases by CDKs is relevant for inflammatory skin diseases that are associated with proliferative cell disorders. To monitor CDK2 and CDK4 activity in human epidermis we used CDKACT fluorescent biosensors that undergo fluorescence enhancement upon phosphorylation by CDKs. By using confocal and Western blot analysis, we evaluated the expression of CDK4/2, Cyc D1/E and CDKI such as p16 (p16), p21 (p21) and p27 (p27). A cohort of 24 patients was asked to participate in the study. Punch biopsies were taken from a chronic plaque and from non-lesional skin of the same patient. CDK4 and its CycD1 partner were overexpressed in psoriatic epidermis. Epidermal QPCR analysis showed that the increased CDK4 expression was not due to enhanced transcription. Compared to normal skin there was no increase in CDK4/CyclinD1 activity. CDK2 and CycE were overexpressed in psoriatic epidermis and this overexpression was correlated with an increase in CDK2-CycE activity. Increased CDK2 expression was not related to enhanced transcription. There was an increased expression of p16 and p21 whereas p27 was decreased in psoriatic epidermis. We have have shown that epidermal CDK2 activity was greatly increased in psoriatic epidermis while CDK4 activity was completely inhibited. These alterations are not associated with changes in CDK transcription and instead involve post-translational control that may be mediated by a dysregulation of CDKI expresion in
psoriatic epidermis.

Psoriatic epidermis is associated with upregulation of CDK2 and inhibition of CDK4 activity