Blocking pyruvate carboxylase sensitises cancer cells to metformin in cell lines and mouse xenografts
Chinese researchers mapped a circuit that converts excess NADH to NADPH in cells with a stalled respiratory chain; blocking it with ZY444 sensitised cell lines to metformin.
A team from the Beijing Institute of Hepatology, Beijing Youan Hospital, Capital Medical University, described in The Innovation (7(9), 101365, published 8 September 2026) a metabolic circuit that helps cells survive a stalled mitochondrial respiratory chain. The circuit relies on pyruvate carboxylase (PC), malate dehydrogenases MDH1/2 and malic enzyme 1 (ME1), and converts excess NADH into cytosolic NADPH sciencedirect.com
The Innovationsciencedirect.comOpen source ↗. The mechanism was mapped in four cell lines under pharmacological blockade, and the combination effect was tested in mouse xenografts.
How the NADH drain works
When the respiratory chain is impaired, the NADH/NAD+ ratio rises. The resulting reductive stress impairs biosynthesis and cell viability. In the described circuit, PC produces oxaloacetate in mitochondria, and MDH2 reduces it to malate using mitochondrial NADH. Malate is exported and oxidised by ME1 to pyruvate and NADPH. Alternatively, oxaloacetate becomes aspartate, is exported and is rebuilt in the cytosol by GOT1 and MDH1, which consume cytosolic NADH.
The authors tested whether NADPH itself is the purpose of the circuit. They expressed TPNOX, an engineered oxidase that converts NADPH to NADP+. Under respiratory blockade it restored proliferation, and the rescue disappeared when ME1, MDH1, MDH2 or PC was knocked down. The authors conclude that the main function of the circuit is NADH homeostasis rather than NADPH production.
Glucose-6-phosphate dehydrogenase (G6PD) also took part. Its loss made cells hypersensitive to respiratory inhibitors, yet the catalytically dead mutant R198P rescued growth as well as the wild-type protein. G6PD binds ME1 directly, and purified G6PD raised the activity of purified ME1 in vitro, so it acts as a scaffold sciencedirect.com
The Innovationsciencedirect.comOpen source ↗.
Blocking PC in cells and xenografts
The PC inhibitor ZY444 sensitised HeLa, SK-Hep1 and 143B cells to metformin. Adding CB839, which blocks glutamine-fuelled metabolism, suppressed growth more than either pair of agents. In nude mice carrying HeLa or SK-Hep1 xenografts, single agents had little effect, while the triple combination nearly abolished tumour progression.
PC had to be located inside mitochondria: removing its targeting sequence abolished the rescue until a targeting sequence was restored. This differs from a report that places PC in the cytosol.
The results come from cell culture and xenografts under pharmacological blockade, not from spontaneous tumours. ZY444 is a tool compound, not a drug.
