P-REX1 creates a positive feedback loop to activate growth factor receptor, PI3K/AKT and MEK/ERK signaling in breast cancer

Abstract

Phosphatidylinositol 3-kinase (PI3K) promotes cancer cell survival, migration, growth, and proliferation by generating phosphatidylinositol 3,4,5-trisphosphate (PIP3) in the inner leaflet of the plasma membrane. PIP3 recruits pleckstrin homology (PH) domain-containing proteins to the membrane to activate oncogenic signaling cascades. Anti-cancer therapeutics targeting the PI3K/AKT/mTOR pathway are in clinical development. In a mass spectrometric screen to identify PIP3-regulated proteins in breast cancer cells, levels of the Rac activator PIP3-dependent Rac exchange factor 1 (P-REX1) increased in response to PI3K inhibition, and decreased upon loss of the PI3K antagonist PTEN. P-REX1 mRNA and protein levels were positively correlated with ER expression, and inversely correlated with PI3K pathway activation in breast tumors as assessed by gene expression and phosphoproteomic analyses. P-REX1 increased activation of Rac1, PI3K/AKT, and MEK/ERK signaling in a PTEN-independent manner, and promoted cell and tumor viability. Loss of P-REX1 or inhibition of Rac suppressed PI3K/AKT and MEK/ERK, and decreased viability. P-REX1 also promoted insulin-like growth factor-1 receptor (IGF-1R) activation, suggesting that P-REX1 provides positive feedback to activators upstream of PI3K. In support of a model where PIP3-driven P-REX1 promotes both PI3K/AKT and MEK/ERK signaling, high levels of P-REX1 mRNA (but not phospho-AKT or a transcriptomic signature of PI3K activation) were predictive of sensitivity to PI3K inhibitors among breast cancer cell lines. P-REX1 expression was highest in ER+ breast tumors compared to many other cancer subtypes, suggesting that neutralizing the P-REX1/Rac axis may provide a novel therapeutic approach to selectively abrogate oncogenic signaling in breast cancer cells.

Interplay between PREX2 mutations and the PI3K pathway and its effect on epigenetic regulation of gene expression in NRAS-mutant melanoma

“…This is in line with earlier studies that demonstrated RAC1 can directly bind and activate PI3K. [11][12][13] This mechanism adds another layer of complexity in Rac1-PI3K interaction which is known to involve complex feedback loops including actin cytoskeletal and local phosphorylated lipid changes to sustain activation of the pathway. 14 Next we wondered whether the PI3K/AKT pathway in turn exerts any regulatory effects on PREX2.…”

Heregulin/ErbB3 Signaling Enhances CXCR4-Driven Rac1 Activation and Breast Cancer Cell Motility via Hypoxia-Inducible Factor 1α

“…HRG and other ErbB ligands translocate P-Rex1 to the plasma membrane in a PI3K-dependent manner, leading to its activation. The requirement for P-Rex1 in HRG-induced Rac1 activation, ruffle formation, and motility, as well as its role in mammary tumorigenesis, has been unambiguously established by means of RNA interference (RNAi)-mediated loss of function approaches (36,40,43,44). Consistent with the established requirement for G␤␥ subunits in P-Rex1 activation, the GEF is also an effector of GPCRs (45).…”

Truncating PREX2 mutations activate its GEF activity and alter gene expression regulation in NRAS-mutant melanoma

“…As pharmacologic agents can have nonspecific effects, future studies using genetic approaches such as dominant negative Rac1 mutants will be helpful to strengthen our conclusion. This observation expands on the role of Rac1 in activating the PI3K pathway as documented recently (29)(30)(31). Additionally, as a well-known modulator of growth signaling, the increase in IGF2 expression in PREX2 mutants is also expected to contribute to the activation of the PI3K/Akt pathway.…”

“…We hypothesized that the increased GEF activity of PREX2 mutants and resulting activation of Rac1 contributes to this activation of Akt based on several reports that have shown Rac1 can activate PI3K/Akt signaling by directly binding to PI3K (29)(30)(31). To test this hypothesis in our model system, we expressed a constitutively active Rac1 construct, Q61L, in primary melanocytes.…”