Inhibition of Prolactin (PRL)-Induced Proliferative Signals in Breast Cancer Cells by a Molecular Mimic of Phosphorylated PRL, S179D-PRL

Abstract

Posttranslational modifications of prolactin (PRL), including phosphorylation, vary with physiologic state and alter biologic activity. In light of the growing evidence for a role for PRL in proliferation in mammary cancer, we examined the ability of a mimic of phosphorylated human PRL, S179D-PRL, to initiate signals to several pathways in mammary tumor cells alone and in combination with unmodified PRL. Unmodified PRL employed multiple pathways to increase cellular proliferation and cyclin D1 levels in PRL-deficient MCF-7 cells. S179D-PRL was a weak agonist compared with unmodified PRL with regard to cellular proliferation, cyclin D1 levels, and phosphorylation of signal transducer and activator of transcription 5 and ERKs. However, S179D-PRL was a potent antagonist of unmodified PRL to these endpoints. In contrast to the reduced levels of the long isoform of the PRL receptor observed in response to a 3-d incubation with unmodified PRL, S179D-PRL up-regulated expression of this isoform, 4-fold. These studies support the utility of this mutant as a PRL antagonist to proliferative signals in mammary epithelial cells, including a potential role in breast cancer therapeutics.

S179D prolactin: Antagonistic agony!

“…In addition, S179D PRL inhibited Stat5 activation in response to unmodified PRL when both were used together . This latter is also well illustrated in Nb2 cells (Coss et al,1999) and MCF7 cells (Schroeder et al,2003). In addition, partially phosphorylated PRL (i.e.…”

“…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!…”

“…S179D PRL also inhibits the growth of human prostate cancer xenografts in nude mice . The proliferation of breast cancer cells in response to unmodified PRL can also be inhibited by S179D PRL, and in cells from which the autocrine PRL has been knocked out, S179D PRL can be accurately titrated against unmodified PRL (Schroeder et al,2003). Under these circumstances, S179D PRL has a marked effect at about one tenth the concentration of unmodified PRL (Schroeder et al, 2003).…”

“…The proliferation of breast cancer cells in response to unmodified PRL can also be inhibited by S179D PRL, and in cells from which the autocrine PRL has been knocked out, S179D PRL can be accurately titrated against unmodified PRL (Schroeder et al,2003). Under these circumstances, S179D PRL has a marked effect at about one tenth the concentration of unmodified PRL (Schroeder et al, 2003). 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).…”

EGF stimulates Pit-1 independent transcription of the human prolactin pituitary promoter in human breast cancer SK-BR-3 cells through its proximal AP-1 response element

“…In mammals, PRL mainly promotes growth and development of the mammary gland, as well as milk production and secretion (Shiu et al, 1987). PRL is also involved in human mammary tumor development (reviewed in Llovera et al, 2000a;Clevenger et al, 2003) and acts as a mitogen on several human breast cancer lines (Biswas et al, 1987;Fuh and Vonderhaar, 1995;Ginsburg et al, 1995;Llovera et al, 2000b;Acosta et al, 2003;Schroeder et al, 2003). Epidemiological studies suggest a role for PRL in human breast cancer promotion and growth (Holtkamp et al, 1984;Bhatavdekar et al, 1990;Hankinson et al, 1999).…”