Regulation of HC11 Mouse Mammary Epithelial Cell Differentiation by the Rac/Stat3 axis
Studying cellular differentiation can reveal valuable information underlying the mechanisms that mediate cell fate. The HC11 mouse mammary epithelial cell line has long been used as a model to study lactogenic differentiation in vitro. Differentiation of HC11 cells requires the addition of hydrocortisone, insulin, and prolactin, added at confluence. Recently, our lab demonstrated that engagement of E-cadherin in HC11 cells, which is favoured under conditions of confluence, triggers a dramatic increase in the activity of the cRac1, small GTPase, which, in turn, activates Stat3 (Signal Transducer and Activator of Transcription-3). Therefore, we hypothesized that the E-cadherin/Rac/Stat3 axis may play a role in HC11 cell differentiation. My results demonstrate that this pathway positively regulates HC11 mammary epithelial cell differentiation and that it is a key mediator of cell fate. Particularly, the strength of the Rac signal is key for the outcome of the differentiation process; cRac is critically required for differentiation, and at low levels, mutationally activated RacV12 is able to increase differentiation, presumably by reinforcing the E-cadherin/Rac differentiative signal. However, high RacV12 expression blocked differentiation while inducing neoplastic transformation. On the other hand, despite the fact that Stat3 was also required for differentiation, constitutively active Stat3 expression was found to block differentiation at all levels. In conclusion, hyperactive RacV12 may have opposing functions, depending upon its level of expression. As a result, a complete Rac inhibition may not be necessary in order to reverse a Rac-driven, neoplastic phenotype in a clinical setting, since the residual, low levels of Rac may be actually promoting cellular differentiation, a finding which could have important therapeutic implications.