Investigations in hormonal regulation of the mammary gland

Abstract

During puberty and pregnancy, oestrogen, progesterone (Pg) and prolactin (PRL) drive mammary gland development via a network of transcription factors and paracrine signals. These factors not only coordinate normal mammary development but can also contribute to breast cancer. This thesis investigates interactions between oestrogen, Pg and PRL and draws implications for breast cancer. Oestrogen inhibits growth hormone (GH) signalling by inducing suppressors of cytokine signalling (SOCS). Given the similarities between GH and PRL, experiments were performed to determine whether oestrogen induces SOCS to inhibit PRL signalling in mammary cells. Although the SOCS protein, CIS, was induced by oestrogen in breast cancer cells, PRL signalling was unaffected. Similarly, oestrogen did not suppress PRL-induced milk expression in a mammary cell line. These results argue against oestrogen modulation of PRL signalling via regulation of SOCS proteins. PRL and Pg are critical for mammary alveologenesis during pregnancy. The transcription factor, Elf5, is also essential for this process and is an important mediator of PRL action. Experiments were undertaken to determine the relationship between Pg and Elf5 in vivo. Elf5 was induced by Pg and cooperated with Pg to promote alveolar bud formation. Further, RANKL was identified as a likely mediator of Pg’s effects on Elf5 expression. These results establish Elf5 as a point of convergence between PRL and Pg signalling in the mammary gland. Elf5 promoter methylation is proposed to enforce lineage commitment in the blastocyst, so experiments were undertaken to characterise Elf5 promoter methylation in mammary cells. Elf5 is expressed in luminal cells, and accordingly, promoter methylation was reduced in luminal cells relative to the stem cell containing basal population. During pregnancy, the abundance of luminal cells increases, so there was an overall decrease in Elf5 promoter methylation. Thus, the Elf5 promoter displays lineage specific methylation in the mammary gland. Loss of promoter methylation in luminal cells may be a prerequisite for hormonal induction of Elf5 expression during pregnancy. This thesis provides novel information regarding interactions between the hormonal drivers of mammary development. The implications of this work extend beyond normal development to pharmacological control of lactation and hormonal control of breast carcinogenesis.