SELF-RENEWAL OF PLURIPOTENT EMBRYONIC STEM-CELLS IS MEDIATED VIA ACTIVATION OF STAT3

The propagation of embryonic stem (ES) cells in an undifferentiated pl uripotent state is dependent on leukemia inhibitory factor (LIF) or re lated cytokines. These factors act through receptor complexes containi ng the signal transducer gp130, The downstream mechanisms that lead to ES cell self-renewal have not been delineated, however. In this study , chimeric receptors were introduced into ES cells. Biochemical and fu nctional studies of transfected cells demonstrated a requirement for e ngagement and activation of the latent trancription factor STAT3. Deta iled mutational analyses unexpectedly revealed that the four STAT3 doc king sites in gp130 are not functionally equivalent. The role of STAT3 was then investigated using the dominant interfering mutant, STAT3F. ES cells that expressed this molecule constitutively could not be isol ated. An episomal supertransfection strategy was therefore used to ena ble the consequences of STAT3F expression to be examined. In addition, an inducible STAT3F transgene was generated. In both cases, expressio n of STAT3F in ES cells growing in the presence of LIF specifically ab rogated self-renewal and promoted differentiation. These complementary approaches establish that STAT3 plays a central role in the maintenan ce of the pluripotential stem cell phenotype, This contrasts with the involvement of STAT3 in the induction of differentiation in somatic ce ll types. Cell type-specific interpretation of STAT3 activation thus a ppears to be pivotal to the diverse developmental effects of the LIF f amily of cytokines. Identification of STAT3 as a key transcriptional d eterminant of ES cell self-renewal represents a first step in the mole cular characterization of pluripotency.