S6 kinase 2 potentiates interleukin‐3‐driven cell proliferation

Interleukin‐3 (IL‐3) mediates hematopoietic cell survival and proliferation via several signaling pathways such as the Janus kinase/signal transducer and activator of transcription pathway, mitogen‐activated protein kinase (MAPK) pathway, and phosphoinositide‐3 kinase (PI‐3K) pathway. Mammalian target of rapamycin (mTOR) is one of the downstream targets of the PI‐3K pathway, and it plays an important role in hematopoiesis and immune cell function. To better elucidate how mTOR mediates proliferation signals from IL‐3, we assessed the role of S6 kinase 2 (S6K2), one of the downstream targets of mTOR, in IL‐3 signaling. We show that S6K2 is activated by IL‐3 in the IL‐3‐dependent Ba/F3 cell line and that this is mediated by mTOR and its upstream activator PI‐3K but not by the MAPK kinase/extracellular signal‐regulated kinase pathway. S6K2 is also activated in primary mouse bone marrow‐derived mast cells upon IL‐3 stimulation. Expression of a rapamycin‐resistant form of S6K2, T388E, in Ba/F3 cells provides a proliferation advantage in the absence or presence of rapamycin, indicating that S6K2 can potentiate IL‐3‐mediated mitogenic signals. In cells expressing T388E, rapamycin still reduces proliferation at all doses of rapamycin, showing that mTOR targets other than S6K2 play an important role in IL‐3‐dependent proliferation. Cell‐cycle analysis shows that T388E‐expressing Ba/F3 cells enter S phase earlier than the control cells, indicating that the proliferation advantage may be mediated by a shortened G1 phase. This is the first indication that S6K2 plays a role in IL‐3‐dependent cell proliferation.