Estrogen is believed to contribute to the development of the autoimmune disorder systemic lupus erythematosus (SLE) (lupus) in women. We hypothesized that estrogen might promote the development of lupus by altering apoptosis of bone marrow cells, perhaps through regulation of the apoptotic proteins Bax and Bcl-2. We compared the effects of estrogen (E2) and thrombopoietin (TPO) on the expression of Bax or Bcl-2 in bone marrow cells isolated from female non-lupus (NZW or NZB parental strains) or lupus-prone (NZB and NZW cross; NZB/W) mice. We report that the basal level of Bax in parental bone marrow cells was significantly greater than that of cells from NZB/W animals. Treatment of NZB or NZW marrow cells with E2 resulted in a significant decrease in Bax expression, which was completely reversed upon co-treatment with TPO. Bax expression was not significantly altered by E2 and/or TPO in NZB/W cells. Bcl-2 levels did not differ between murine strains under basal or hormone-treated conditions. Lower basal expression of Bax protein was associated with significantly less apoptosis for NZB/W marrow cells. In addition, there were significantly greater numbers of cells in bone marrow of lupus-susceptible animals. Our results indicate that bone marrow cells of NZB/W animals differ physiologically from those of NZW or NZB mice, and suggest that decreased expression of Bax in bone marrow precursors may lead to decreased apoptosis of mature blood cells in lupus-susceptible mice.
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