Research Objective

Cancer is the second leading cause of death worldwide, resulting in nearly 8 million deaths each year (WHO, 2013). It is expected that over 12 million people will be diagnosed with the disease this year. In addition to the physical and emotional burden such a diagnosis incurs on the patient, their family, and caregivers, cancer also imparts a significant economical toll on society. According to a report from the National Institute of Health in 2008, the total cost of cancer in the United States totalled $ 201.5 billion, with total health expenditures from direct medical costs reaching $77.4 billion. Despite progress in therapeutic intervention and increasing public awareness regarding

preventative measures, the incidence and prevalence of cancer remain largely unchanged in recent years, and in some cancer types and patient populations, increases were

observed.

For the majority of malignancies, age remains the number one demographic risk factor. The median age at the time of diagnosis is approaching 70 years and is expected to continue to increase. Compounding the cancer burden is the fact that, in the United States alone, the older population (defined as those aged 65 or older) is expected to more than double from approximately 40 million persons in 2010 to over 90 million by 2060 (Administration on Aging, 2013). With this increase in the elderly population, a

subsequent rise in cancer incidence and prevalence is anticipated. Data from the National Cancer Institute’s Surveillance, Epidemiology, and End Results (SEER) Program suggest that the number of cancer patients will more than double from nearly 1.4 million in the

Breast cancer rates steadily climb with age. Over 80% of diagnoses occur in women over the age of 50 and more than one-third in women over the age of 70 (Walker

et al., 2007). Moreover, there is limited data regarding age-related changes in both

diseased and non-diseased breast tissue. Tumor heterogeneity and the hormonal influence on non-diseased breast tissue and malignant transformation necessitate the need for age- related studies in an effort to develop efficacious, yet selective, therapeutics for particular patient populations. For this reason, the present study was designed to assess changes in two kinase signaling pathways that are typically dysregulated in cancer, including breast carcinomas. Namely, the expression and activation profiles of the mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K) pathways were examined in both cellular and animal breast cancer models of aging.

The MAPK and PI3K signaling cascades are critical mediators of numerous cellular functions, including proliferation, metabolism, differentiation, growth, and survival. Dysfunction of these pathways has been noted in various cancer types, and components of these cascades have been the recent target of novel inhibitors aimed at restoring a homeostatic balance. Despite advancements in drug development, low bioavailability, insufficient tumor response, off-target toxicities, and therapeutic resistance have limited the clinical utility of many of these inhibitors that have entered patient trials. Several points of convergence have been established between the two cascades, and crosstalk within the familial MAPK network between ERK1/2 and ERK5 and also between the MAPK cascade and PI3K pathways hold the potential for

The relationship between the MAPK and PI3K pathways in breast cancer has not yet been fully delineated; in particular, to our knowledge, the potential for crosstalk between the terminal MAPK, ERK5, and the PI3K/Akt pathway has not been explored. Therefore, an objective of the current study was to explore potential compensation within the MAPK pathway and between the MAPK and PI3K pathways in breast cancer. The effect of pathway inhibition on cell proliferation, migration, and invasion were assessed in breast cancer cell models with varied hormonal profiles.

Numerous pre-clinical studies have examined the role of the MEK5-ERK5 pathway in breast cancer, implicating its enhancement of tumor survival, increased proliferation, and decreased apoptosis (Drew et al., 2012). Moreover, in analyses of human breast tumors, MEK5 was found to be overexpressed in more than half, while ERK5 was overexpressed in one-fifth of the patient samples examined; these findings were correlated with a decrease in disease-free survival (Montero et al.,2009). In other tumor types, such as oral squamous cell carcinoma and prostate cancer, elevated ERK5 has been correlated with increased metastatic events (Sticht et al., 2008; Mehta et al., 2003). Together, these studies indicate the critical role the MEK5-ERK5 pathway plays in breast carcinomas and indicates it as a viable target for novel anticancer agents.

Inhibitors of the MEK1/2-ERK1/2 pathway previously believed to be specific for the cascade have since been identified as non-selective and also capable of affecting the MEK5-ERK5 pathway. With the recent discovery of the MEK5 inhibitors, BIX02188 and BIX02189, and the ERK5 inhibitor, XMD8-92, all touted as specific to this pathway, came the hope of elucidating the unique MEK5-ERK5 cellular functions. However, in

growth-factor and cell-line specific, as results have varied based on the stimulus, model and selected concentration of inhibitor, suggesting the need for further investigation of these compounds. Pre-clinical studies and assessments of human breast tumors highlight the critical impact the MEK5-ERK5 cascade has in malignant transformation and tumor progression, indicating it as a putative target for potential clinical therapeutics. To that end, the current study also aimed to characterize a series of novel small molecule

compounds targeting selective MEK5 inhibition in cellular breast cancer models. Sparing the relative MEK1/2-ERK1/2 pathway while selectively targeting the MEK5-ERK5 and/or PI3K pathways, where appropriate, may reduce off-target toxicity and optimize treatment regimens.