The S100 protein family, consisting of over 20 members, constitutes the largest subgroup of calcium binding proteins. These proteins share amino acid sequence similarity as well as the functional EF-hand structure motif, which plays a key role in calcium binding through a helix-loop-helix topology. Proteins containing this motif are involved in virtually all normal and pathological cell functions including gene transcription, inflammatory and immune responses, regulation of protein phosphorylation, transcription factors, anti-microbial responses, Ca2+ homeostasis, the dynamics of cytoskeleton constituents, as well as cell proliferation, differentiation, and death [1–3]. Given the global importance of these proteins, inhibitors of specific S100 proteins are currently being developed as therapeutics for diseases including diabetes mellitus, heart failure, neurological diseases, and several types of cancer [2, 4].
The role of S100 protein in breast cancer is only beginning to emerge. A recent observational study demonstrated upregulation of S100A1, S100A2, S100A4, S100A6, S100A8, S100A9, S100A10, S100A11, and S100A14 in basal-type breast cancers compared to non-basal types. In the same study, it was determined that expression of S100A8 and S100A9 were elevated in high grade compared to low grade tumors and estrogen receptor (ER) negative tumors compared to ER positive tumors . Mechanistic studies demonstrated that overexpression of S100A4 induced metastatic capability in non-metastatic breast cancer cells and stimulated metastasis of benign tumors in transgenic mouse model systems . S100A7 was shown to be upregulated in high-grade ductal carcinoma in situ, and is correlated with poor prognosis in estrogen negative breast cancer. In vitro, S100A7 overexpression increased breast cancer cell growth, invasiveness, and increased tumorigenicity in a xenograft mouse model [7, 8]. On the other hand, S100A2 expression was found to be reduced as breast cancer progressed from carcinoma in situ to carcinoma . Corresponding to this observation, S100A2 has also been proposed as a tumor suppressor in early stage lung carcinogenesis .
We recently performed mass spectrometry analysis of the extracellular matrix of whole breast tissue with the goal of determining underlying differences in the normal breast microenvironment between premenopausal African- and Caucasian-American women . Premenopausal African-American women suffer disproportionately from breast cancer mortality compared to Caucasian women. Both social and biological mechanisms are contributory, including a higher prevalence of aggressive basal-like breast cancers in African-American women. Hornerin, an S100 protein family member, was detected in significantly higher abundance in the Caucasian-American samples. Therefore we further investigated the biological functions of this protein, only to find that little is known.
Hornerin was first characterized in the mouse embryo epidermis and was also detected in the skin, tongue, and forestomach of the adult tissues examined . Hornerin contains a Ca2+ binding EF-hand domain at the N terminus followed by a spacer sequence and an extensive repetitive domain rich in glycine and serine . Its similarity in structural features, expression profile, extensive posttranslational proteolytic processing, and tissue localization to profilaggrin indicated a role in keratinocyte cornification. Additional studies demonstrated the presence of hornerin in regenerating, psoriatic and healthy human skin, and that hornerin is a component of cornified cell envelope [13–15]. While it might initially seem peculiar that a protein involved in cornification of the skin is found in breast tissue, it is important to recall the evolutionary development of the mammary gland. In all mammals, the mammary gland organogenesis arises from a localized thickening of the epidermis. An elevation of the epidermal mammary crest and the development of a milk-line on both sides of the mid-ventral line of the embryo form the mammary buds, which eventually progress to form the functional mammary gland . Indeed, other proteins involved in epidermal/skin function have been shown to perform roles in mammary gland physiology. Neuregulin3 regulates the cell fate of pluripotent epidermal cells, including those that ultimately differentiate into progenitor cells of the mammary gland . Additionally, LMO-4 a member of the LIM-only family of transcriptional co-regulatory proteins functions in both epidermal cell migration and mammary gland differentiation [18, 19].
Herein, we demonstrate hornerin expression in human breast tissue and mammary epithelial and stromal cells, its regulation throughout postnatal mammary developmental stages in murine tissue, as well as its expression in correlation with breast cancer subtypes. Furthermore, we show that proteolytic fragments of hornerin have distinctive intracellular localization and that induction of apoptosis/necrosis upregulates hornerin expression in breast cells. Collectively, these data support a novel role for hornerin in the regulation of mammary cell function.