Ikaros and Aiolos Inhibit Pre-B-Cell Proliferation by Directly Suppressing c-Myc Expression

Pre-B- cell expansion is driven by signals from the interleukin-7 receptor and the pre-B-cell receptor and is dependent on
cyclin D3 and c-Myc. We have shown previously that interferon regulatory factors 4 and 8 induce the expression of Ikaros
and Aiolos to suppress pre-B- cell proliferation . However, the molecular mechanisms through which Ikaros and Aiolos
exert their growth inhibitory effect remain to be determined. Here, we provide evidence that Aiolos and Ikaros bind to the
c-Myc promoter in vivo and directly suppress c-Myc expression in pre-B cells. We further show that downregulation of c-
Myc is critical for the growth- inhibitory effect of Ikaros and Aiolos. Ikaros and Aiolos also induce expression of p27 and
downregulate cyclin D3 in pre-B cells , and the growth- inhibitory effect of Ikaros and Aiolos is compromised in the absence
of p27. A time course analysis further reveals that downregulation of c-Myc by Ikaros and Aiolos precedes p27 induction
and cyclin D3 downregulation. Moreover, downregulation of c-Myc by Ikaros and Aiolos is necessary for the induction of
p27 and downregulation of cyclin D3. Collectively, our studies identify a pre-B- cell receptor signaling induced inhibitory
network, orchestrated by Ikaros and Aiolos, which functions to terminate pre-B-cell expansion.

Molecular and Cellular Biology, September 2010, p. 4149-4158, Vol. 30, No. 17
0270-7306/10/$12.00+0 doi:10.1128/MCB.00224-10