PI3K (phosphoinositide 3-kinase) is a family of enzymes that act as a central switch for cell growth, survival, and metabolism. When a cell receives a signal to grow or respond to insulin, PI3K is one of the first molecules to relay that message inside the cell. It does this by modifying a fat molecule in the cell membrane, creating a chemical tag that activates a cascade of downstream signals. Because PI3K sits at this critical junction, mutations that make it overactive are among the most common drivers of cancer.
How PI3K Works Inside Cells
PI3K’s job is to convert one type of membrane fat (called PIP2) into another (called PIP3). This might sound minor, but PIP3 acts as a docking signal on the inner surface of the cell membrane. When PIP3 appears, it recruits two key proteins: Akt (also called protein kinase B) and PDK1. PDK1 activates Akt by adding a chemical tag at one site, and a second complex called mTORC2 adds a tag at another site. Only when both tags are in place is Akt fully switched on.
Once active, Akt fans out to regulate dozens of processes. It promotes cell growth, blocks programmed cell death, triggers nutrient uptake, and helps cells divide. This chain of events, often called the PI3K/Akt/mTOR pathway, is one of the most important signaling networks in human biology. It responds to growth factors, hormones like insulin, and immune signals, making it relevant to nearly every tissue in the body.
The Three Classes of PI3K
Not all PI3Ks are identical. The family is divided into three classes, each with different structures and roles.
- Class I is the most studied and the most relevant to cancer and metabolism. These enzymes work in pairs: a catalytic subunit (one of four types, labeled p110α, β, δ, or γ) paired with a regulatory subunit that keeps the enzyme in check until the right signal arrives. Class I PI3Ks are the only ones that produce PIP3, the key docking signal described above.
- Class II has three members in humans. These enzymes produce different lipid signals and play roles in membrane trafficking and blood vessel formation. They don’t require a dedicated partner subunit the way Class I does.
- Class III is a single enzyme found in all organisms with complex cells. It produces a simpler lipid signal and is essential for autophagy, the process cells use to recycle damaged components.
When people refer to “PI3K” in the context of cancer or drug development, they almost always mean Class I, and usually the p110α subunit specifically.
PI3K’s Role in Insulin and Blood Sugar
PI3K is essential to how your body responds to insulin. When insulin binds to a cell’s surface receptor, PI3K activates and triggers Akt, which then pushes glucose transporters (GLUT4) to the cell surface so sugar can enter. Akt also stimulates the cell to convert glucose into stored energy (glycogen) and promotes the chemical breakdown of glucose for fuel. At the same time, Akt blocks the liver from manufacturing new glucose.
This explains why drugs that inhibit PI3K frequently cause high blood sugar as a side effect. When PI3K is blocked, cells become less responsive to insulin. Glucose transport capacity drops to roughly 60% of normal, the liver ramps up glucose production, and glycogen stores get broken down rather than built up. In clinical trials of the PI3K-targeting cancer drug alpelisib, 64% of patients developed elevated blood sugar.
PI3K and the Immune System
Two of the Class I catalytic subunits, p110δ and p110γ, are concentrated in immune cells rather than spread throughout the body. p110δ sits downstream of the T cell receptor and co-stimulatory signals, making it critical for T cell activation and the generation of regulatory T cells that prevent the immune system from attacking the body’s own tissues. p110γ responds to chemokine receptors, the signals that guide immune cells to sites of infection or inflammation, and also contributes to T cell activation.
In animal studies, knocking out either isoform suppresses alloreactive T cells, the immune cells responsible for transplant rejection, and delays organ rejection. However, losing p110δ also impairs regulatory T cells, which complicates its usefulness as a drug target in transplant medicine. These immune-specific isoforms have become targets for treating blood cancers and inflammatory diseases precisely because blocking them can dampen harmful immune responses without affecting PI3K signaling in every cell in the body.
PI3K Mutations in Cancer
The gene encoding the p110α subunit, called PIK3CA, is one of the most frequently mutated genes across human cancers. Mutations in PIK3CA lock the enzyme in an always-on state, sending constant growth and survival signals regardless of whether the cell has received any external instruction to grow. Cancers of the endometrium, breast, and colon carry PIK3CA mutations at especially high rates, as do certain benign skin tumors.
These mutations cluster in specific hotspots on the gene. Two FDA-approved diagnostic tests can detect 11 of the most common PIK3CA mutations from either a tissue biopsy or a blood sample (liquid biopsy). In breast cancer specifically, PIK3CA mutation status is now used as a biomarker to guide treatment. Patients with hormone receptor-positive, HER2-negative advanced breast cancer who carry a PIK3CA mutation may be eligible for targeted therapy with a PI3K inhibitor combined with hormonal treatment.
PI3K Inhibitors as Cancer Treatment
Because the PI3K pathway is so central to cancer cell survival, blocking it has been a major focus of drug development. PI3K inhibitors work by preventing the enzyme from producing PIP3, effectively cutting off the growth and survival signals downstream. These drugs can target all Class I isoforms at once (pan-PI3K inhibitors) or focus on a single isoform, which tends to reduce side effects.
The most common side effects reflect how deeply PI3K is woven into normal physiology. High blood sugar is the most frequent concern, occurring in roughly two-thirds of patients in some trials. Digestive issues are also common: in the pivotal trial for alpelisib, 58% of patients experienced diarrhea and 45% experienced nausea. These side effects are considered “on-target,” meaning they happen because the drug is doing exactly what it was designed to do, just in healthy tissues as well as cancerous ones. Managing blood sugar with monitoring and medication is a routine part of treatment with these drugs.
Why PI3K Matters Beyond Cancer
PI3K’s influence extends well beyond tumor biology. Its role in insulin signaling makes it relevant to type 2 diabetes research. Its immune functions connect it to autoimmune diseases, transplant rejection, and allergic inflammation. Its involvement in cell survival links it to neurodegenerative conditions where too many or too few cells die.
The challenge with targeting PI3K therapeutically is the same thing that makes it biologically important: it does too many things in too many places. Blocking it in a tumor also blocks it in the liver, immune cells, and fat tissue. The trend in drug development has moved toward isoform-selective inhibitors that affect only one version of PI3K, and toward combination strategies that allow lower doses. Newer approaches, including degrader molecules that destroy the PI3K protein entirely rather than just blocking its activity, are being explored to improve selectivity and overcome resistance that can develop with traditional inhibitors.