Cancer is generally a disease associated with ageing. For many cancer types, incidence increases dramatically in the sixth and seventh decade of life. However, in advanced ageing these increased rates mysteriously decline.

This observation is not new – but the underlying biological mechanisms remain largely unknown.

Enormous progress has been made in our understanding around ageing and cancer. It’s now known, for example, that somatic mutations are accumulated in adult stem cells at a stable speed over the course of life – and that this varies in different tissues. This rate is also affected by exogenous exposures.

It’s become clear that the tumour microenvironment in aged individuals also significantly contributes to tumour development. Important aspects here include ageing-associated chronic inflammation (known as ‘inflammageing’), excessive deposition of extracellular matrix (fibrosis), and hindered surveillance and elimination by the immune system. However, exactly how ageing impacts on the tumour cells of origin is much less studied and understood.

Model problem

This could, at least in part, be due to the models used for studying ageing and cancer. The prevailing models used involve the transplantation of cancer cell lines to young and aged host mice. While there is no doubt these models have significantly advanced our understanding of changes in the aged tumour microenvironment, that is not the case for the cancer cells themselves. Because these models represent advanced stages of tumour growth, they also offer limited insight into the early malignant transformation of the cell of origin.

To assess how ageing impacts tumour initiation, we used genetically engineered mouse models to induce lung cancer in young (12–16 weeks old; equivalent to a human age of 20-30 years) and aged (104–130 weeks old; equivalent to approximately 70 human years) mice. In these models, the oncogenic KRAS G12D mutation is activated and the tumour suppressor p53 is deleted specifically in alveolar type II (AT2) stem cells. These cells – which are the predominant source of alveolar regeneration in tissue homeostasis and injury repair – also give rise to the most common form of human lung cancer.

Our models show the same counterintuitive finding seen previously around advanced ageing and cancer. We found that ageing suppresses tumour formation in aged lung alveolar stem cells. Compared to younger mice, the aged mice showed a 37% increase in median survival and a 40–60% reduction in tumour burden.

As the ability to form tumours is closely tied to the regenerative potential of stem cells, and since both the number and function of stem cells are known to decline with age, the associated loss of “stemness” likely also reduces their tumorigenic potential.

So now the question was, why? To understand the underlying mechanisms, we looked to single-cell RNA sequencing and DNA methylation analysis on tumour cells and the alveolar stem cells. The pathways identified were later confirmed through genetic and pharmacological techniques.

Age and biology

With these approaches, we discovered the loss of stemness and tumorigenic capacity of aged alveolar stem cells was caused by ageing-associated epigenetic reprogramming.

This reprogramming – mostly DNA methylation changes – results in transcriptomic alterations initiating in the aged cell of origin and persisting in tumour cells. Interestingly, we found that one gene of this ageing-associated gene expression program, NUPR1, is key to the loss of stemness and transformation capacity in aged alveolar stem cells. Upregulation of NUPR1 – a stress response mediator – in the aged stem cells, led to a functional iron deficiency and resistance to ferroptosis via lipocalin-2.

Genetic inactivation of the NUPR1– lipocalin-2 axis or iron supplementation restored the stemness and tumorigenic potential in the aged alveolar stem cells. In addition, we saw the opposite effect in the young counterparts. This age-specific function of NUPR1-lipocalin-2 axis demonstrates that aging, beyond a risk factor, can significantly change the biology of cancer initiation and tissue regeneration.

With this work we have seen a direct connection between the age-related decline in stem cell fitness and a reduced potential for cancer initiation. Importantly, it also allows us to consider potential therapeutic strategies that focus on rejuvenating stem cells or targeting altered iron metabolism for cancer prevention and treatment in elderly adults.

The plan for us now is to explore iron’s role in controlling stemness and plasticity further, with future work aimed at clarifying the molecular mechanisms behind these age-related changes, hopefully shedding light on novel approaches for regenerative medicine and cancer therapies.