Targeting PI3K in Cancer: Impact on Tumor Cells, Their Protective Stroma, Angiogenesis, and Immunotherapy

Abstract

The PI3K pathway is hyperactivated in most cancers, yet the capacity of PI3K inhibitors to induce tumor cell death is limited. The efficacy of PI3K inhibition can also derive from interference with the cancer cells' ability to respond to stromal signals, as illustrated by the approved PI3Kδ inhibitor idelalisib in B-cell malignancies. Inhibition of the leukocyte-enriched PI3Kδ or PI3Kγ may unleash antitumor T-cell responses by inhibiting regulatory T cells and immune-suppressive myeloid cells. Moreover, tumor angiogenesis may be targeted by PI3K inhibitors to enhance cancer therapy. Future work should therefore also explore the effects of PI3K inhibitors on the tumor stroma, in addition to their cancer cell-intrinsic impact.

Significance: The PI3K pathway extends beyond the direct regulation of cancer cell proliferation and survival. In B-cell malignancies, targeting PI3K purges the tumor cells from their protective microenvironment. Moreover, we propose that PI3K isoform-selective inhibitors may be exploited in the context of cancer immunotherapy and by targeting angiogenesis to improve drug and immune cell delivery. Cancer Discov; 6(10); 1090-105. ©2016 AACR.

Figure 1. Direct and indirect anti-cancer effects of interference with PI3K isoform activity

Different PI3K isoforms have the potential to interfere with cancer growth and survival, either by acting on the transformed cells directly, by interfering with the supportive stroma and nutrient supply, or by stimulating more potent immune responses against the transformed cells. PI3Kα inhibitors have shown promising results in cancers driven by activating PIK3CA mutations, such as p110α^H1047R. PI3Kα inhibition may also affect nutrient supply by inhibiting angiogenesis, or enhance drug delivery by normalizing vessels, depending on the degree of inhibition and the particular tumor type. Evidence suggests that PTEN-deficient tumors are often (but not always) more sensitive to PI3Kβ inhibitors. Both PI3Kα and PI3Kβ inhibitors may be useful in targeting tumor-associated fibroblasts, although this is speculative at this stage. PI3Kγ inhibition has been shown to reduce the infiltration of tumor-suppressive macrophages, diverting them from an immune suppressive (wound healing) M2 to an immunostimulatory M1 phenotype and reducing the production of fibroblast-stimulating growth factors. PI3Kδ inhibitors can stimulate a more potent CD8⁺ T-cell-mediated cytotoxic anti-tumor response by activating DCs to produce more IL-12 and by inhibiting Treg and MDSCs, which antagonize cell-mediated immunity in tumors. Arrows indicate a stimulatory impact.

Figure 2. Vascular targeting strategies in cancer and possible role of PI3K isoforms therein

PI3K inhibitors, at high doses, have been documented to induce a mild vessel eradication response, whereas at low doses, can lead to vessel normalization, associated with either reduced or no changes in vessel density. Key concepts to therapeutically exploit the dependence of tumors on the vasculature: (1) vessel eradication, aimed at destroying the tumor vasculature and ‘starving the tumor to death’ (146, 147). Problems with this strategy are both intrinsic and acquired resistance to vascular trimming (148, 149), reduced chemotherapy delivery to the tumor and induction of hypoxia, which can accelerate tumor progression (51, 150); (2) vessel normalization, aimed at improving vascular perfusion and oxygenation, allowing enhanced drug delivery and immunotherapy (151, 152). This normalization effect can be the consequence of vessel pruning, with only normal vessels left behind after therapy (153, 154) or of vessel conversion, a biological change in endothelial cells whereby the tubular structures are converted into more physiological, normal vessels (155, 156). The applicability of the normalization approach in the clinic has been limited by the transient window during which vessels are susceptible to normalization, difficulties to predict when and which agent will induce vessel normalization, and a highly context-dependent response, with every tumor relying to different extents on angiogenic cues to stimulate vessel growth (157, 158); (3) the vessel promotion strategy is based on stimulating vessel growth, response, with every tumor relying to different extents on angiogenic cues to stimulate vessel together with promotion of vasodilatation (159) and is aimed at enhancing delivery of chemotherapy and other anti-cancer agents to the tumor (160).

Figure 3. Roles of PI3Kδ in CLL cells and their interaction with their surrounding stroma

CLL cells depend on signaling via the BCR to increase proliferation, adhesion to mesenchymal stromal cells (via VLA-4/V-CAM1), and secretion of the chemokines CCL3 and CCL4. The ligands (antigens) for the BCR may be expressed by the CLL cells themselves or by other cells in the stroma. CCL3 and CCL4 facilitate the recruitment of T-cells which provide CD40 ligand (CD40L), IFN-γ and other agonists that stimulate the CLL cells, as evidenced by increased expression of CD38 on the CLL cells. Other key cell types in the lymph node niche include: (1) mesenchymal stromal cells that secrete chemokines, such as CXCL12 and CXCL13, which bind to CXCR4 and CXCR5 on CLL cells and facilitate their recruitment to and retention in the lymph nodes; and (2) myeloid-derived nurse-like cells that also secrete CXCL13 and promote the survival of CLL cells. PI3Kδ inhibition interferes with many aspects of this intercellular communication, ultimately resulting in the ‘purging’ of CLL cells from their protective lymph node (or bone marrow) environment into circulation where they are more susceptible to undergoing apoptosis.

Figure 4. PI3K pathway components in Treg involved in the regulation of anti-cancer immunity

The activation of Akt by PI3Kδ leads to the phosphorylation of the BACH2 and FOXO1 transcription factors and their retention in the cytoplasm. BACH2 and FOXO1 regulate expression of key genes in Treg differentiation and function, including genes encoding FOXP3, L-selectin, CCR7 and IFNγ. Failure to activate this pathway upon PI3Kδ inhibition may prevent Treg from undergoing a differentiation program and migrating effectively to tumors in order to suppress anti-tumor responses. By contrast, constitutive PI3Kδ activation upon loss of PTEN may destabilize the Treg lineage, loss of FOXP3 and CD25 expression, resulting in loss of suppressive function of the Treg and production of pro-inflammatory cytokines instead of the normal immune-suppressive factors produced by Treg.