Intracellular Calcium links Milk Stasis to Lysosome Dependent Cell Death during Early Mammary Gland Involution

Involution of the mammary gland after lactation is a dramatic example of coordinated cell death. Weaning results in the distension of the alveolar structures by the accumulation of milk, which, in turn, activates STAT3 and initiates a caspase-independent but lysosome-dependent cell death (LDCD) pathway. Although the importance of STAT3 and LDCD in early mammary involution is well established, it has not been entirely clear how milk stasis activates STAT3. In this report, we demonstrate that protein levels of the PMCA2 calcium pump are significantly downregulated within 2-4 hours of experimental milk stasis. Reductions in PMCA2 expression correlate with an increase in cytoplasmic calcium in vivo as measured by multiphoton intravital imaging of GcAMP6f fluorescence. These events occur concomitant with the appearance of nuclear pSTAT3 expression and prior to significant activation of LIF, IL6 and TGFb3 production or LDCD. Interestingly, we observed the activation of TFEB led by activation of TGFb signaling and deactivation of mTOR signaling in the involution of mammary gland. Finally, we demonstrate that increased intracellular calcium activates STAT3 by degrading SOCS3 mediated by TGFb signaling. In summary, these data suggest that intracellular calcium serves as an important biochemical signal linking milk stasis to STAT3 activation, increased lysosome biogenesis, and lysosome-mediated cell death. mRNA profile in control, high calcium (10mM calcium with ionomycin) and TGFb3 treated MCF10A cells