PI3K regulatory subunit p85alpha plays a tumor suppressive role in the transformation of mammary epithelial cells

Abstract

Hyperactivation of the phosphatidylinositol 3-kinase (PI3K) pathway is one of the most common events in human cancers. Class IA PI3Ks are heterodimers of a p110 catalytic and a p85 regulatory subunit that coordinate the cellular response to extracellular stimuli. Activating mutations in class IA PI3K catalytic isoform p110α are well established as causative in a number of cancer types. More recently, mutation or loss of the class IA regulatory isoform p85α (encoded by PIK3R1) has emerged as contributing to oncogenesis. In this dissertation, we use both in vitro and in vivo approaches to examine the role of p85α as a tumor suppressor in the transformation of mammary epithelial cells.

Using publically available online databases, we find heterozygous deletion of PIK3R1 occurs in 19-26% of breast tumors. Moreover, PIK3R1 expression is significantly decreased in breast tumors compared to normal breast tissue. In human mammary epithelial cells expressing dominant negative p53 (DDp53-HMECs), RNAi-mediated knockdown of PIK3R1 increases PI3K/AKT activation in response to growth factor stimulation and leads to transformation as assessed by anchorage-independent growth. PIK3R1 knockdown also augments transformation of DDp53-HMECs by oncogenes, including activated HER2/neu. In a mouse model of HER2/neu-driven breast cancer, genetic ablation of Pik3r1 accelerates mammary tumor development. Transformation driven by p85α loss is largely mediated by signaling through catalytic isoform p110α, as selective pharmacological inhibition of p110α but not p110β effectively blocks colony formation of PIK3R1 knockdown DDp53-HMECs and growth of Pik3r1 knockout tumors. Mechanistically, we find that partial reduction of p85α increases the amount of p85-p110α bound to activated receptors, augmenting PI3K signaling and oncogenic transformation.

Together the work presented in this dissertation suggests that p85α depletion selectively targets a free negative regulator pool of this regulatory subunit that modulates PI3K activation under normal conditions, and transforms cells when lost. Furthermore, our work indicates that p85α plays a tumor suppressive role in the pathogenesis of breast tumors. Isoform-selective PI3K inhibitors are currently emerging in the clinic, and may offer improved specificity and reduced toxicity over first-generation pan-PI3K inhibitors. Our findings suggest p110α-selective therapies may be an effective treatment for breast cancers with reduced p85α expression.