cancers Review Targeting PI3K/AKT/mTOR Signaling Pathway in Breast Cancer Huayi Li †, Lorenzo Prever † , Emilio Hirsch and Federico Gulluni * Department of Molecular Biotechnology and Health Sciences, University of Turin, 10126 Turin, Italy; [email protected] (H.L.); [email protected] (L.P.); [email protected] (E.H.) * Correspondence: [email protected] † Equal contributors. Simple Summary: PI3K signaling pathway plays an essential role in many cellular processes and is frequently altered in breast cancer, leading to increased tumor growth and reduced survival. Small molecule inhibitors have been developed that target the three key elements of this pathway: PI3K, AKT, and mTOR. Despite demonstrating promising preclinical activity, intrinsic and acquired resistance, as well as high levels of adverse reactions, partially limited the therapeutic efficacy of PI3K/AKT/mTOR inhibitors. To increase therapeutic benefit, drug combinations and schedules need to be explored to identify those with the highest efficacy and lowest toxicity rate. In addition, defining appropriate patient subpopulations, for either monotherapy or drug combinations, and identifying predictive biomarkers remain a challenge. Abstract: Breast cancer is the most frequently diagnosed cancer and the primary cause of cancer death in women worldwide. Although early diagnosis and cancer growth inhibition has significantly improved breast cancer survival rate over the years, there is a current need to develop more effective systemic treatments to prevent metastasis. One of the most commonly altered pathways driving Citation: Li, H.; Prever, L.; Hirsch, E.; breast cancer cell growth, survival, and motility is the PI3K/AKT/mTOR signaling cascade. In Gulluni, F. Targeting the past 30 years, a great surge of inhibitors targeting these key players has been developed at a PI3K/AKT/mTOR Signaling rapid pace, leading to effective preclinical studies for cancer therapeutics. However, the central Pathway in Breast Cancer. Cancers role of PI3K/AKT/mTOR signaling varies among diverse biological processes, suggesting the need 2021, 13, 3517. https://doi.org/ for more specific and sophisticated strategies for their use in cancer therapy. In this review, we 10.3390/cancers13143517 provide a perspective on the role of the PI3K signaling pathway and the most recently developed PI3K-targeting breast cancer therapies. Academic Editor: Alakananda Basu Keywords: PI3K; inhibitor; AKT; mTOR; breast cancer; clinical trial; metastasis; class II PI3K Received: 9 June 2021 Accepted: 10 July 2021 Published: 14 July 2021 1. Introduction Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in Phosphoinositide 3-kinase (PI3K) is a group of lipid kinases that phosphorylate the 0 published maps and institutional affil- 3 -OH group of phosphatidylinositol (PI) at plasma and intracellular membranes. They are iations. split into three different classes according to their structure, binding partners, and substrate specificity [1–3]. Among them, the most well-studied PI3K is class I PI3K, which generate PI(3,4,5)P3 (PIP3) starting from PI(4,5)P2 (PIP2). PIP3 is mainly produced at the plasma membrane in response to different stimuli and allows for the recruitment of a myriad of phospholipid effectors, including serine/threonine kinase AKT and 3-phosphoinositide- Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. dependent protein kinase-1 (PDK-1), which are the central mediators of the PI3K pathway. This article is an open access article PIP3 also facilitates PDK1 and AKT interaction, resulting in phosphorylation of AKT at distributed under the terms and thrPhosphorylated AKT promotes protein synthesis, cell growth, and cell survival and conditions of the Creative Commons motility by activating many downstream kinases, including the mammalian target of the Attribution (CC BY) license (https:// rapamycin (mTOR) complex [1]. On the other hand, the tumor suppressor phosphatase creativecommons.org/licenses/by/ and tensin homolog (PTEN) dephosphorylates PIP3, counteracting PI3K signaling [4] 4.0/). (Figure1). Cancers 2021, 13, 3517. https://doi.org/10.3390/cancers13143517 https://www.mdpi.com/journal/cancers Cancers 2021, 13, x FOR PEER REVIEW 2 of 20 Cancers 2021, 13, 3517 the rapamycin (mTOR) complex [1]. On the other hand, the tumor suppressor phospha-2 of 19 tase and tensin homolog (PTEN) dephosphorylates PIP3, counteracting PI3K signaling [4] (Figure 1). Figure 1. Signaling by the phosphatidylinositol-3-kinase (PI3K)/AKT/mammalian target of the ra- Figure 1. Signaling by the phosphatidylinositol-3-kinase (PI3K)/AKT/mammalian target of the pamycin (mTOR) pathway and the respective inhibitors. rapamycin (mTOR) pathway and the respective inhibitors. ClassClass I PI3KI PI3K includes includes four four highly highly homologous homologous catalytic catalytic subunits, subunits, p110α, p110p110βα,, p110 p110γ,β, andp110 p110γ, andδ, which p110δ can, which associate can associate with five with regulatory five regu subunits,latory subunits, collectively collectively referred referred to as p85-typeto as p85-type regulatory regulatory subunits subuni (Figurets (Figure1)[ 5, 1)6]. [5,6]. Whereas Whereas p110 p110α andα and p110 p110β isoformsβ isoforms are are ubiquitouslyubiquitously expressed, expressed, p110 p110δ andδ and p110 p110γ γexpression expression is is largely largely restricted restricted to to hematopoietic hematopoietic cellscells [7 [7,8].,8]. Dysregulation Dysregulation of of phosphoinositide phosphoinositide kinases, kinases, primarily primarily in in class class IA IA PI3K, PI3K, have have beenbeen discovered discovered in ain number a number of human of human diseases, dise withases, mutationswith mutations leading leading to either to increased either in- orcreased decreased or decreased enzymatic enzymatic activity being activity critically being critically involved involved in cancer in [ 9cancer], developmental [9], develop- disordersmental disorders [10], and [10], primary and primary immune immune deficiencies deficiencies [11–13]. [11–13]. For an extensive For an extensive description descrip- of thetion PI3K of the pathway, PI3K pathway, the reader the can reader refer can to the refer following to the following reviews [ 1reviews,14,15]. [1,14,15]. SeveralSeveral studies studies suggest suggest thatthat thethe PI3K/AKT/mTORPI3K/AKT/mTOR pathway pathway is is often often genetically genetically al- alteredtered in in human human cancers cancers [15,16]. [15,16]. Although Although many many small small molecule molecule inhibitors inhibitors targeting targeting the thePI3K/AKT/mTOR PI3K/AKT/mTOR signaling signaling pathway pathway were were pre-clinically pre-clinically studied, studied, only only some some of the of thePI3K PI3Kand andmTOR mTOR inhibitors inhibitors are currently are currently approved approved for the for treatment the treatment of human of human cancers cancers in the inclinic. the clinic. Here Herewe summarize we summarize the themost most recent recent advances advances in in the inhibitioninhibition ofof the the PI3K/AKT/mTORPI3K/AKT/mTOR signalingsignaling pathwaypathway inin breastbreast cancer.cancer. 2.2. Genetic Genetic Alterations Alterations of of the the PI3K PI3K Pathway Pathway in in Breast Breast Cancer Cancer and and Clinical Clinical Implications Implications TheThe PI3K/AKT/mTOR PI3K/AKT/mTOR pathway pathway is is frequently frequently deregulated deregulated in breastin breast cancer cancer by differentby differ- mechanisms,ent mechanisms, leading leading to increased to increased PI3K activity PI3K activity and/or and/or loss of PI3Kloss of inhibitory PI3K inhibitory functions func- as welltions as as mutation well as mutation in tumor suppressorin tumor suppressor genes like genes INPP4B like and INPP4B PTEN and phosphatases. PTEN phosphatases. Among PI3KAmong genes, PI3K PIK3CA genes, isPIK3CA one of is the one most of the frequently most frequently altered, withaltered, mutations with mutations occurring occur- at tworing hotspot at two regions:hotspot regions: an acidic an cluster acidic (E542, cluste E545,r (E542, and E545, Q546) and in Q546) the helical in the domain helical domain and a histidineand a histidine residue (H1047)residue in(H1047) the kinase in the domain. kinase Whereasdomain. mutationsWhereas mutations on the helical on the domain helical mainlydomain rely mainly on the rely loss on of the p85-dependet loss of p85-depend inhibitoryet inhibitory activity, activatingactivity, activating mutations mutations in the α catalyticin the catalytic subunit subunit of p110 ofdirectly p110α directly stimulate stimulate lipid kinase lipid activity kinase byactivity facilitating by facilitating allosteric al- motionslostericrequired motions forrequired catalysis for oncatalysis membranes on membranes [17]. [17]. MutationsMutations in in the the catalytic catalytic domaindomain areare thethe mostmost frequent genetic alterations alterations found found in in more than one third of early breast cancer tumors. In particular, PIK3CA activating more than one third of early breast cancer tumors. In particular, PIK3CA activating mu- mutations comprise up to 47% of HR+/HER2– (luminal A), 33% of HR+/HER2+ (lumi- tations comprise up to 47% of HR+/HER2– (luminal A), 33% of HR+/HER2+ (luminal B), nal B), 39% of HR-/HER2+ (HER2-enriched), and 8–25% of basal-like/triple negative 39% of HR-/HER2+ (HER2-enriched), and 8–25% of basal-like/triple negative breast cancer breast cancer subtypes [18–25]. The