SAT-009 SSRI Use in the Peripartum Period Regulates Mammary Gland Parathyroid Hormone Related Protein (PTHrP) by a Serotonylation-Dependent Mechanism

During lactation, a woman experiences a considerable amount of bone loss and recent studies suggest bone deficits persist years postpartum. Furthermore, selective serotonin uptake inhibitors (SSRIs), which are often prescribed to women experiencing peripartum depression, have been linked to osteopenia. Serotonin signaling can increase parathyroid hormone related protein (PTHrP), a bone remodeling protein which liberates calcium for the milk. Additionally, fluoxetine (a common SSRI) results in increased mammary gland serotonin content and PTHrP, and treatment during the peripartal period reduced maternal bone mineral density. One proposed mechanism of serotonin action is by its covalent addition to proteins by transglutaminase (TG2), termed serotonylation. We therefore investigated whether the combination of fluoxetine and lactation can exacerbate maternal bone loss and the underlying mechanism. We hypothesized that SSRI-induced serotonin signaling in the lactating mammary gland increases PTHrP through a serotonylation-dependent mechanism. Treatment of mouse mammary epithelial cells (HC11) with fluoxetine significantly upregulates PTHrP gene expression and the concentration of its downstream effector, cAMP, over control (P < 0.0004). Furthermore, treatment of the HC11 cells with fluoxetine in addition to a TG2 inhibitor, monodansylcadaverine, restores PTHrP mRNA expression to levels observed in the control. Small g-proteins have emerged as a common target protein for serotonylation. Currently, our data suggest that the g-proteins, RhoA and Rab4, are potential serotonylation targets in the mammary gland. Together these data suggest that the molecular process of serotonyation in HC11 cells links serotonin signaling to increased PTHrP expression. Future work is directed at using the cre-lox system to genetically ablate serotonylation using a WAPCre/TG2Flox transgenic mouse to determine whether decreasing serotonylation in vivo in the mammary gland during lactation improves maternal bone mass.

Prolactin regulates ZNT2 expression through the JAK2/STAT5 signaling pathway in mammary cells

The zinc transporter ZnT2 ( SLC30A2) plays an important role in zinc secretion into milk during lactation. The physiological process of mammary gland secretion is regulated through complex integration of multiple lactogenic hormones. Prolactin plays a primary role in this regulation through the activation of various signaling cascades including Jak2/STAT5, mitogen-activated protein kinase (MAPK), p38, and phosphatidylinositol 3-kinase (PI3K). The precise mechanisms that regulate the transfer of specific nutrients such as zinc into milk are not well understood. Herein we report that prolactin increased ZnT2 abundance transcriptionally in cultured mammary epithelial (HC11) cells. To delineate the responsible mechanisms, we first determined that prolactin-mediated ZnT2 induction was inhibited by pretreatment with the Jak2 inhibitor AG490 but not by the MAPK inhibitor PD-98059. Using a luciferase reporter assay, we demonstrated that ZnT2 promoter activity was increased by prolactin treatment, which was subsequently abolished by expression of a dominant-negative STAT5 construct, implicating the Jak2/STAT5 signaling pathway in the transcriptional regulation of ZnT2. Two putative consensus STAT5 binding sequences in the ZnT2 promoter were identified (GAS1:−674 to −665 and GAS2:−377 to −368). Mutagenesis of the proximal GAS2 element resulted in complete abrogation of PRL-induced ZnT2 promoter activity. The promoter incorporating the distal GAS1 mutation was only able to respond to very high PRL concentrations. Results from both the mutagenesis and gel shift assays indicated that a cooperative relationship exists between GAS1 and GAS2 for PRL-induced activation; however, the proximal GAS2 plays a more critical role in STAT5-mediated signal transduction compared with the GAS1 element. Finally, chromosome immunoprecipition assay further confirmed that prolactin activates STAT5 binding to the ZnT2 promoter in vivo. Taken together, these results illustrate that prolactin regulates the transcription of ZnT2 through activation of the Jak2/STAT5 signaling pathway to assist in providing optimal zinc for secretion into milk during lactation.

Selective Response to Insulin Versus Insulin-Like Growth Factor-I and -II and Up-Regulation of Insulin Receptor Splice Variant B in the Differentiated Mouse Mammary Epithelium

The terminal differentiation of the mouse mammary gland epithelium during lactation has been shown to require IGFs and/or superphysiological levels of insulin. It has been suggested that IGF receptor I (IGF-IR), in addition to its well-established role in the mammary gland during puberty and pregnancy, serves as the principal mediator of IGFs at this stage of development. However, our analysis of the expression levels of IGF-IR and the two insulin receptor (IR) splice variants, IR-A and IR-B, has revealed a 3- to 4-fold up-regulation of IR-B transcripts and a 6-fold down-regulation of IGF-IR transcripts and protein during terminal differentiation in the developing mammary gland. IR-B expression was also more than 10-fold up-regulated in murine mammary epithelial cell line HC11 during differentiation in vitro. As already described for the human form, murine IR-B cloned from HC11 exhibited selectivity for insulin as compared with IGFs. When differentiated HC11 cells were stimulated by 10 nm insulin, a concentration that is unable to activate IGF-IR, induction of milk protein and lipid synthetic enzyme gene expression, lactate production, and phosphorylation of Akt were observed. In contrast, on differentiated HC11 cells 10 nm IGF-I or 10 nm IGF-II were able to exert growth-promoting effects only. The lack of response of differentiated cells to low levels of IGFs could not be explained by inactivation of IGFs by IGF binding proteins. Our results suggest a previously unrecognized predominant role for IR-B in the differentiated mammary epithelium.