1,25-Dihydroxyvitamin D3-3-bromoacetate, a Novel Vitamin D Analog Induces Immunosuppression Through PI3K/Akt/mTOR Signaling Cascade

Purpose: The molecular mechanism responsible for the immunomodulatory effect of 1,25-dihydroxyvitamin D3 (Vit-D) is still not well elucidated. Unavoidable systemic toxicity of Vit-D has encouraged to develop more potent and less toxic Vit-D analogs, such as 1,25-dihydroxyvitamin D3-3-bromoacetate (BE). Our aim was to explore the immunosuppressive effect of BE and its molecular mechanism in autoimmune diseases.

Method: Magnetically sorted CD3(+) T cells (T cells) from PBMCs of psoriasis and autoimmune arthritis patients were cultured with/without BE and Vit-D followed by proliferation (MTT, CFSE dilution assays) and apoptosis assays (annexin V). Immunoblot was performed to determine the signaling cascade responsible for the antiproliferative effect.

Results: In MTT assay, BE (OD: 0.64±0.08) markedly inhibited the anti-CD3/CD28 stimulated proliferation of T cells (OD: 1.8±0.30, p<0.001) and at equivalent doses, the inhibitory effect was more than that of Vit-D (OD: 0.91±0.11, p<0.05). The antiproliferative effect of BE was extended to activated CD4(+) and CD8(+) memory T cells (CD45RA(-)CD11a(+)) without much effect on the naïve T cells. BE induced more apoptosis of T cells (45.01±4.27%, p<0.01) compared to untreated cells (3.45±1.8%), and the proapoptotic effect was markedly more than that of Vit-D (26.1±2.05%, p<0.05). BE effectively inhibited the anti-CD3/CD28-induced phosphorylation of Akt and mTOR and in both, BE showed more potency than Vit-D (p<0.05).

Conclusion: Topical Vit-D is being used successfully in psoriasis for years. However, its potency is less compared to topical corticosteroids. The de novo BE showed significantly more immunosuppression than conventional Vit-D and the immunosuppressive effect is PI3K/Akt/mTOR dependent. Our results indicate that BE could be an effective therapeutic agent for psoriasis and other T-cell-mediated autoimmune diseases.