Therapeutic Effect of Cyclin-Dependent Kinase 4/6 Inhibitor on Dermal Fibrosis in Murine Models of Systemic Sclerosis

Objective One of the histologic characteristics of systemic sclerosis (SSc) is an increased number of dermal myofibroblasts, and transforming growth factor beta (TGF beta) plays a crucial role in the promotion of myofibroblast differentiation from fibroblasts, leading to dermal fibrosis. This study was undertaken to 1) examine whether inhibition of the cell cycle with a cyclin-dependent kinase 4/6 (CDK4/6) inhibitor suppresses the proliferation of fibroblasts and their differentiation into myofibroblasts, and 2) assess the therapeutic effects of a CDK4/6 inhibitor, administered as monotherapy or in combination with a TGF beta receptor (TGF beta R) inhibitor, on dermal fibrosis in murine models of SSc. Methods Fibroblasts obtained from the skin of patients with SSc were cultured in the presence or absence of TGF beta. The effects of palbociclib, a CDK4/6 inhibitor, on fibroblast proliferation and TGF beta-induced differentiation into myofibroblasts were examined using bromodeoxyuridine uptake assays as well as immunofluorescence and immunoblotting analyses. Murine models of HOCl- and bleomycin-induced dermal fibrosis were used to study the effect of a CDK4/6 inhibitor on dermal fibrosis, with the CDK4/6 inhibitor treatment administered as monotherapy or in combination with galunisertib, a TGF beta R inhibitor. Results Addition of a CDK4/6 inhibitor to the cell cultures suppressed the proliferation of human dermal SSc fibroblasts and their TGF beta-induced differentiation into myofibroblasts, without inhibiting canonical and noncanonical TGF beta signals. In murine models of dermal fibrosis, treatment of mice with a CDK4/6 inhibitor decreased dermal thickness and collagen content, as well as dermal fibroblast proliferation and the numbers of myofibroblasts. Combination therapy with the CDK4/6 inhibitor and TGF beta R inhibitor resulted in additive antifibrotic effects. Mechanistically, the CDK4/6 inhibitor suppressed the expression of cellular communication network 2 and cadherin-11, which are proteins that have important roles in the development and progression of fibrosis. Conclusion Results of this study demonstrate the therapeutic effect of a CDK4/6 inhibitor on dermal fibrosis when administered as monotherapy or in combination with a TGF beta R inhibitor. CDK4/6 inhibitors, including palbociclib used in the present study, may represent novel agents for the treatment of SSc, which, if used in combination with a TGF beta R inhibitor, might result in increased efficacy.