Thioredoxin-interacting protein regulates keratinocyte differentiation: Implication of its role in psoriasis

Thioredoxin-interacting protein (TXNIP), also known as Vitamin-D upregulated protein-1 (VDUP-1), interacts with thioredoxin to regulate redox responses and participates in diverse disorders including metabolic, cardiovascular, inflammatory and malignant diseases. Psoriasis is characterized by chronic skin inflammation and an aberrant pattern of keratinocyte differentiation. Clinically, psoriasis is associated with various cardiometabolic comorbidities but studies on TXNIP's biological role in skin disorders are limited. In this study, we investigated TXNIP expression in psoriasis and its regulation in normal human epidermal keratinocytes (NHEKs), and then explored how TXNIP regulated skin keratinocyte differentiation to determine its role in psoriasis pathogenesis. Our immunohistochemical study demonstrated extensive TXNIP expression in the upper and lower epidermis of psoriasis compared to predominant TXNIP expression in the basal layer of normal skin. 1, 25-dihydroxyvitamin D₃  suppressed but TGF-α and EGF enhanced TXNIP expression in NHEKs. An inducer of keratinocyte differentiation, phorbol 12-myristate 13-acetate (PMA), also diminished TXNIP expression, which was reversed by PKC-δ knockdown. TXNIP knockdown reduced PMA-induced involucrin and transglutaminse-1 expression, and increased p63 expression in NHEKs but did not significantly affect cell proliferation. H₂ O₂ -induced ROS production and EGFR phosphorylation decreased in NHEKs with TXNIP knockdown. Furthermore, PMA-induced PKC-δ phosphorylation, TGF-α, and EGF-triggered EGFR phosphorylation were attenuated by TXNIP knockdown. Our results unraveled the regulation and function of TXNIP expression in skin keratinocytes and the cross-regulation between TXNIP and EGFR signaling. These findings imply a role of TXNIP in psoriasis and provide insight into the possible impact of TXNIP regulators on the skin or psoriasis.