Systemic Lupus Erythematosus: the involvement of PI3K/Akt/mTOR pathway in cellular cycle and in early apoptosis

Abstract

Objective PI3K/Akt/mTOR signaling pathway plays a critical role in many cellular functions. Until now there are no data regarding this signaling pathway and p27kip1 status in peripheral blood mononuclear cells (PBMCs) of patients with Systemic Lupus Erythematosus (SLE). Therefore, we proposed to analyze in SLE PBMCs the activity of Akt, the expression level of their substrate p27kip1 and the relationship with cell cycle progression as well as the contribution of mTOR to the increased apoptosis of SLE PBMCs. Methods Akt and mTOR activities were evaluated by measuring their phosphorylation level, using immunoblotting. The expression level of p27kip1 in PBMCs and lymphocytes was analyzed by immunoblotting and FACS. Quantification of apoptosis and the cell cycle distribution were performed using FACS. Results The phosphorylation level of Akt at Thr308 was statistically significant higher while the expression level of p27kip1 was two times lower in SLE than in HC PBMCs. The percentage of lymphocytes accumulated in S and G2/M cell cycle phases are significantly increased in SLE than in HC. Statistical analysis demonstrated that p27kip1 expression level correlated with the percentages of lymphocytes in cell cycle phases. Moreover, in SLE lymphocytes, p27kip1 expression level was indirectly correlated with apoptosis. The phosphorylation level of mTOR was similar in SLE and HC PBMCs, probably due to the therapy. In SLE lymphocytes, where an increased apoptosis was found, mTOR activity was inverse correlated with apoptosis. Conclusions Increased activity of Akt, observed in SLE lymphocytes, explains the reduced expression of its substrate p27kip1. This defect seems to be involved in SLE lymphocytes passage by G1/S cell cycle checkpoint. Therefore, SLE lymphocytes accumulate in S and G2/M cell cycle phases towards apoptosis or proliferation. The inverse correlation of mTOR activity and apoptosis rather suggested an anti-apoptotic role of this kinase.