Different Forms of Prolactin Have Opposing Effects on the Expression of Cell Cycle Regulatory Proteins in Differentiated Mammary Epithelial Cells

Abstract

Prolactin (PRL) is a hormone that contributes to both the growth and differentiation of mammary epithelial cells, activities likely to impact breast cancer in opposite ways. Whether PRL causes growth or differentiation has been solely attributed to the coexisting steroidal environment, with PRL stimulating mammary gland growth during pregnancy, and then milk production after the postpartum drop in estrogen and progesterone. However, previous work from our laboratory has shown that the form of PRL may also be an important factor. During pregnancy, unmodified PRL (U-PRL) promotes mammary growth, while an increase in phosphorylated PRL, or administration of a molecular mimic of phosphorylated PRL (S179D PRL), inhibits growth. Unknown, however, is whether these forms of PRL have opposite effects on growth in the absence of steroids and whether effects are directly on mammary epithelial cells. To mimic the glandular epithelium in vitro, we used contact-inhibited, differentiated cells and showed that even with these minimally growing cells that treatment with U-PRL caused increased expression of cyclin D1 and cyclin-dependent kinase 4, increased activity of both cdk4 and cdk2, while having no effect on the inhibitory protein, p21. S179D PRL, by contrast, had no effect on cyclin D1 and cdk4 expression, but increased p21 expression and expression of the vitamin D receptor (VDR). We conclude that increased U-PRL or decreased phosphorylated PRL can directly affect cell cycle control proteins in relatively differentiated mammary epithelial cells, thereby implicating the balance between these two forms of PRL in the early promotion of breast cancer.

S179D prolactin: Antagonistic agony!

“…Thus, one could envisage phosphorylated/S179D PRL simply blocking the effect of unmodified PRL by occupying a receptor, but not inducing a signal. However, as mentioned above, small proportions of phosphorylated PRL/S179D PRL as low as 6-10% have significant effects on cell proliferation Walker,1993,Schroeder et al,2003), Stat 5 activation (Coss et al,1999;Wu et al, 2003;Schroeder et al,2003), and signaling leading to activation of cyclin D1 (Schroeder et al, 2003;Wu et al, 2006). This, as several leading investigators said at the time of our original presentation of this finding, is impossible!…”

“…In vivo, S179D PRL inhibits the growth of the mammary gland during pregnancy in both rats and mice (Kuo et al,2002;Naylor et al,2005). In the normal mouse mammary cell line, HC11, S179D PRL inhibits the proliferation of actively growing cells (Wu et al, 2006) and unmodified PRL and S179D PRL have opposing effects on cell cycle regulatory proteins in contact-inhibited, differentiated cells (Wu et al, 2006). S179D PRL also inhibits the proliferation of human endothelial cells (Ueda et al,2006a).…”

“…S179D PRL also inhibits the proliferation of human endothelial cells (Ueda et al,2006a). In prostate, mammary, and endothelial cells, S179D PRL upregulates expression of the cell cycle regulatory protein, p21 (Wu et al, 2005;Wu et al, 2006;Ueda et al, 2006a). This is associated with upregulation of the vitamin D receptor, but the details of this association vary a little from one cell line to another.…”

“…This is associated with upregulation of the vitamin D receptor, but the details of this association vary a little from one cell line to another. In prostate cancer cell lines, the vitamin D receptor appears to mediate all of the effect on p21 (Wu et al, 2007), whereas in HC11 cells, there also seem to be some vitamin D receptorindependent effects on p21 (Wu et al, 2006). In each case, blockade of ERK signaling inhibits the effect of S179D PRL on p21 (Wu et al, 2005;Wu et al, 2006;Ueda et al, 2006b).…”

S179D prolactin diminishes the effects of UV light on epidermal gamma delta T cells

“…However, studies in which the two major forms of PRL have been individually analyzed have shown them to have very different biological functions. Some of these studies used the naturally phosphorylated material (Krown et al, 1992, Wang and, while others utilized a stable molecular mimic of phosphorylated PRL, S179D PRL (Chen et al, 1998;Xu et al, 2001;Yang et al, 2001;Kuo et al,2002;Yang et al, 2002;Schroeder et al, 2003;Wu et al, 2003;Xu et al, 2003;Naylor et al, 2005;Guzman et al, 2005;Ueda et al, 2006a,b;Wu et al 2006). In the current study, we assessed the effects of either U-PRL or S179D PRL on LC and γδT in mature skin, in the absence or presence of a suberythemal dose of UVB.…”