Cultured cancer cells frequently rely on the consumption of glutamine and its subsequent hydrolysis by glutaminase (GLS). However, this metabolic addiction can be lost in the tumour microenvironment, rendering GLS inhibitors ineffective in the clinic. Here we show that glutamine-addicted breast cancer cells adapt to chronic glutamine starvation, or GLS inhibition, via AMPK-mediated upregulation of the serine synthesis pathway (SSP). In this context, the key product of the SSP is not serine, but α-ketoglutarate (α-KG). Mechanistically, we find that phosphoserine aminotransferase 1 (PSAT1) has a unique capacity for sustained α-KG production when glutamate is depleted. Breast cancer cells with resistance to glutamine starvation or GLS inhibition are highly dependent on SSP-supplied α-KG. Accordingly, inhibition of the SSP prevents adaptation to glutamine blockade, resulting in a potent drug synergism that suppresses breast tumour growth. These findings highlight how metabolic redundancy can be context dependent, with the catalytic properties of different metabolic enzymes that act on the same substrate determining which pathways can support tumour growth in a particular nutrient environment. This, in turn, has practical consequences for therapies targeting cancer metabolism.

培养的癌细胞通常依赖于谷氨酰胺的消耗及其随后的谷氨酰胺酶 （GLS） 水解。然而，这种代谢成瘾会在肿瘤微环境中丢失，使 GLS 抑制剂在临床上无效。在这里，我们表明，谷氨酰胺成瘾的乳腺癌细胞通过 AMPK 介导的丝氨酸合成途径 （SSP） 上调来适应慢性谷氨酰胺饥饿或 GLS 抑制。在这种情况下，SSP 的关键产物不是丝氨酸，而是 α-酮戊二酸 （α-KG）。从机制上讲，我们发现磷酸丝氨酸氨基转移酶 1 （PSAT1） 在谷氨酸耗尽时具有持续产生 α-KG 的独特能力。对谷氨酰胺饥饿或 GLS 抑制具有抵抗力的乳腺癌细胞高度依赖于 SSP 提供的 α-KG。因此，抑制 SSP 会阻止对谷氨酰胺阻断的适应，从而产生抑制乳腺肿瘤生长的强效药物协同作用。这些发现强调了代谢冗余如何依赖于环境，作用在同一底物上的不同代谢酶的催化特性决定了哪些途径可以支持特定营养环境中的肿瘤生长。反过来，这对针对癌症代谢的疗法具有实际影响。