Breast cancer: the menacing face of Janus kinase

The Multifaceted Role of STAT3 in Mammary Gland Involution and Breast Cancer

Since seminal descriptions of signal transducer and activator of transcription 3 (STAT3) as a signal transducer and transcriptional regulator, which is most usually activated by phosphorylation of a specific tyrosine residue, a staggering wealth of research has delineated the key role of this transcription factor as a mediator of mammary gland postlactational regression (involution), and paradoxically, a pro-survival factor in breast cancer and some breast cancer cell lines. STAT3 is a critical regulator of lysosomal-mediated programmed cell death (LM-PCD) during mammary gland involution, where uptake of milk fat globules, and consequent high levels of free fatty acids, cause permeabilisation of lysosomal vesicle membranes, in turn leading to cathepsin protease leakage and cell death. A recent proteomic screen of STAT3-induced changes in lysosomal membrane protein components has highlighted wide-ranging effects of STAT3, which may coordinate LM-PCD via the stimulation of endocytosis, intracellular trafficking, and lysosome biogenesis. In parallel, STAT3 regulates the acute phase response during the first phase of involution, and it contributes to shaping the pro-tumourigenic 'wound healing' signature of the gland during the second phase of this process. STAT3 activation during involution is important across species, although some differences exist in the progression of involution in dairy cows. In breast cancer, a number of upstream regulators can lead to STAT3 activation and the effects of phosphorylation of STAT3 are equally wide-ranging. Recent studies have implicated microRNAs in some regulatory pathways. In this review, we will examine the multifaceted role of STAT3 in mammary gland involution and tumourigenesis, incorporating a review of these fundamental processes in tandem with a discussion of recent developments in this field.

The interleukin-6/Janus kinase/STAT3 pathway in pleomorphic adenoma and carcinoma ex pleomorphic adenoma of the lacrimal gland

PURPOSE

Pleomorphic adenoma (PA) is the most common tumour of the lacrimal gland, but very little is known about its biology. It has a tendency to recur and an ability to transform into the high-grade malignancy carcinoma ex pleomorphic adenoma (ca-ex-PA), which is also largely unexplored. In this study, we examine the expression of the interleukin-6/Janus kinase/STAT3 (IL-6/JAK/STAT3) pathway components in PA and ca-ex-PA.

METHODS

Sixteen PAs and two ca-ex-PAs were examined with immunohistochemistry. Seven PAs were subjected to microdissection and subsequent qPCR.

RESULTS

The IL-6/JAK/STAT3 pathway was overexpressed in PA compared to normal lacrimal gland. Overexpression of phosphorylated JAK1 (p-JAK1) and cyclin D1 was significantly overexpressed in ductal cells compared with myoepithelial cells in PA. A shift from p-JAK1 to p-JAK2 and p-Tyk2 overexpression was seen between PA and ca-ex-PA, combined with a high p-STAT3 expression in the latter.

CONCLUSION

The IL-6/JAK/STAT3 pathway is overexpressed in PA, and this overexpression was even more pronounced in ca-ex-PA, with a shift in the JAKs mediating STAT3 phosphorylation. Future studies are needed to clarify whether PA and ca-ex-PA could be treated with targeted therapy directed against components of the IL-6/JAK/STAT3 pathway.

Stat3 modulates chloride channel accessory protein expression in normal and neoplastic mammary tissue

Mammary gland regression at the cessation of lactation (involution) is an exquisitely orchestrated process of cell death and tissue remodelling in which Stat3 signalling has an essential role. The involution microenvironment of the mammary gland is considered to be pro-tumourigenic and a proportion of cases of pregnancy-associated breast cancer are suggested to originate in tandem with involution. However, the apparent paradox that STAT3 is required for cell death in normal mammary gland, but is associated with breast cancer cell survival, has not been resolved. Herein, we investigate Stat3-mediated regulation of expression of members of the calcium-activated chloride channel regulator (CLCA) family of proteins during involution and mammary carcinogenesis. Using the conditionally immortal mammary epithelial cell line KIM-2, together with mice exhibiting mammary epithelial cell-specific deletion of Stat3 during lactation, we demonstrate that expression of mCLCA1 and mCLCA2 is elevated in concert with activation of Stat3. By contrast, murine CLCA5 (mCLCA5), the murine orthologue of human CLCA2, is significantly upregulated at 24, 72 and 96 h of involution in Stat3 knockout mice, suggesting a reciprocal regulation of these proteins by Stat3 in vivo. Interestingly, orthotopic tumours arising from transplantation of 4T1 murine mammary tumour cells exhibit both phosphorylated Stat3 and mCLCA5 expression. However, we demonstrate that expression is highly compartmentalized to distinct subpopulations of cells, and that Stat3 retains a suppressive effect on mCLCA5 expression in 4T1 tumour cells. These findings enhance our understanding of the regulation of CLCA channel expression both in vitro and in vivo, and in particular, demonstrate that expression of mCLCA1 and mCLCA2 during involution is profoundly dependent upon Stat3, whereas the relationship between mCLCA5 and Stat3 activity is reciprocal and restricted to different subpopulations of cells.