Breast cancer remains a predominant malignancy worldwide. In the United States, there are more than 3.8 million women living with a history of breast cancer, including over 150,000 women living with metastatic disease [1]. With profound advancements in breast cancer therapeutic management, mortality rates have decreased overall. However, invasive breast cancer remains difficult to manage with current regimens; approximately 13% of women will be diagnosed with invasive breast cancer in their lifetime [1]. Classically, breast cancers (BC) were categorized based on the receptors they expressed, estrogen receptor positive (ER+), progesterone receptor positive (PR+), HER2/Neu amplified, or triple negative breast cancer (TNBC) [2, 3]. More recently, Lehmann et al. introduced a new classification system for breast cancers based on gene expression profiling of the tumors: Basal-like 1, basal-like 2, luminal androgen receptor and mesenchymal [4, 5]. One of the underlying features that determines the clinical behavior of a tumor is the propensity of cancer cells to migrate and invade in distal tissue sites [6–8]. In addition to the complex and heterogenous nature of breast cancers, metastasis and tumor recurrence are obstacles in effectively treating breast cancer [9–11]. There is an urgent need to continue to characterize potential drivers of these processes to formulate effective targeted therapeutic regimens for breast cancer.
Kinase-targeting agents are widely used in anti-cancer therapeutic regimens. Kinases are enzymes that modulate cell signaling pathways responsible for a range of processes including cell proliferation, survival, motility, and apoptosis. Out of the 538 known human kinases, many have key roles in carcinogenesis and metastasis of various cancer types [12, 13]. Kinase inhibitors account for approximately 25% of all currently investigated therapeutics [14]. Currently, several kinase targets are being pursued in difficult-to-treat breast cancer types, such as triple negative breast cancers lacking commonly targeted receptors HER2/Neu and estrogen receptor, including cyclin-dependent kinases (CDKs), BRAF, lipid kinases like phosphatidylinositol 3-kinase (PI3K) alpha, AKT, CHK1 and epidermal growth factor receptor (EGFR) [14, 15]. However, because the human kinome is vast there exist many unexplored targets in cancer [16].
Kinases that regulate key processes in cell survival and division, such as mitosis, are important to study as novel targets in cancer due to frequent upregulation in human cancer [17–19]. The three major families of mitotic kinases are: Polo, Aurora, and the Never-in-mitosis (NEK) kinase families [20, 21]. Of these kinases, the NEK members remain the least studied. NEK family members regulate specific mitotic events, including centrosome separation, spindle assembly and cytokinesis [21, 22]. While many of the eleven members of the NEK family have been extensively characterized, NEK5 function remains widely understudied. Various groups have shown that NEK5 expression is upregulated in specific tumor types (colon, lung, thyroid, breast) compared to normal cells/tissue [23]. NEK5 has the highest endogenous protein expression in testes, and it has been proposed to regulate cilia motility [24]. To support this, NEK5 expression is higher in ciliated tissues, including colon and lung [23].
Recent studies demonstrate a role for NEK5 in regulation of microtubule assembly and activity during mitosis [25]. Because microtubule activation is associated with enhanced cell migration and motility in cancer systems, these findings led us to interrogate a possible role for NEK5 in breast cancer cell migration. Furthermore, NEK5 has been shown to have an integral role in the DNA damage response, through interactions with topoisomerase IIβ [26]. Recently, Pei et al. evaluated NEK5 mRNA and protein expression in breast cancer. Upregulation of NEK5 expression in breast cancer cells promoted tumor progression and silencing of NEK5 suppressed proliferation and inhibited migration and invasion. This important study demonstrated an integral role for NEK5 in cell proliferation and migration pathways in breast cancer through regulation of Cyclin A2 function [27].
In this study, we provide further evidence that supports a potential role for NEK5 in driving breast cancer cell motility and we parse out specific processes in breast cancer biology that are regulated, or not regulated, by NEK5 activity. We employed iterations of patient-derived xenograft (PDX) models as physiologically relevant tools in our project to provide more translational evidence of our work. Given the understudied status of NEK5, and the recent interest in further characterization of the NEK family in cancer biology, our work is crucial to elucidate specific roles for NEK5 in breast cancer to evaluate its potential as a therapeutic target.