Stem cell control in the lung by an autocrine injury-activated Igf complex.

Stem cells proliferate after injury to repair damaged tissue, and chronic injury can promote cancer. However, the injury-activated signals and regulatory mechanisms, and their relationship to cancer, are poorly understood. Here, we identified insulin-like growth factor 2 (Igf2) as an injury-activated mitogen for lung neuroendocrine stem cells, which are facultative airway progenitors and a cell of origin of small-cell lung cancer in mice. Igf2 was constitutively produced by the stem cells but sequestered in the niche by coexpressed Igf binding proteins (Igfbps). Airway injury released Igf2 and induced proliferation by transiently activating Igf2 receptors and repressing retinoblastoma (Rb) tumor suppressor. Permanent pathway activation by Rb deletion initiated continuous stem cell division. Thus, beyond their classical hormonal roles in physiology, growth, and aging, Igf proteins operate locally and rapidly with Igfbp and Rb to control injury-induced stem cell proliferation and tumor initiation. Editor's summary: Stem cells are rapidly activated for tissue repair after injury, but the regulatory signals and mechanisms remain obscure. Zhang et al. identified the protein Igf2 as the key proliferative signal. Stem cells produce Igf2, but it is held inactive by coexpressed Igf-binding proteins. Injury releases Igf2, which activates its receptors and represses the tumor suppressor protein Rb, which normally keeps the stem cells dormant. Rb loss causes uncontrolled growth. Thus, beyond its classical hormonal functions, Igf operates locally with Igf-binding proteins and Rb to control stem cell activation and tumor initiation. —Stella M. Hurtley INTRODUCTION: Adult stem cells are rapidly, but transiently, activated by injury to repair damaged tissue. Repeated or chronic injury predisposes to cancer. Although there has been progress in identifying and characterizing mammalian stem cells and homeostatic mitogens that regulate their proliferation, injury-activated mitogens, the mechanisms that keep them inactive until injury, and whether and how these contribute to cancer is unclear. Here, we investigated these questions for a population of mouse lung epithelial stem cells called NEstem. NEstem are rare stem cells intermingled with other pulmonary neuroendocrine (NE) cells in innervated neurosensory clusters that monitor airway status and serve as a stem cell niche. NEstem are facultative stem cells that proliferate extensively after airway injury, forming large clonal repair patches that restore the surrounding epithelium. NEstem can also be activated permanently by compound deletion of tumor suppressors retinoblastoma (Rb) and p53. This initiates small-cell lung cancer (SCLC), one of the deadliest cancers. RATIONALE: We sought to identify the injury-induced mitogen for NEstem, the mechanisms by which injury activates the mitogen and stimulates stem cell proliferation, and the relationship of this tissue repair pathway to the SCLC tumor suppressors. We reasoned that this could reveal the molecular logic of how injury triggers the critical first step in stem cell activation (division) and answer questions in the fields of tissue repair, stem cells, and cancer. RESULTS: To identify the mitogenic signal, we developed a mouse lung-slice culture assay and screened candidate ligands for an effect on NEstem proliferation. Only insulin-like growth factors 1 and 2 (Igf1 and Igf2), which are well-known hormones that regulate growth, physiology, and aging, induced their proliferation. Igf2 and its receptors were both expressed by and required for injury-induced activation of NEstem, thus functioning in an autocrine fashion. Although Igf2 was produced constitutively by NEstem, it was sequestered and stabilized in an inactive form in the niche by coexpressed Igf binding proteins (Igfbps). Pharmacologic release of Igf from latent Igf-Igfbp complexes, or destruction of Igfbp with PAPP-A (pregnancy-associated plasma protein A, also called pappalysin-1) and PAPP-A2 (pappalysin-2) proteases expressed by sensory neurons in the niche, immediately triggered NEstem proliferation, as did injury. Igf signaling repressed Rb function, which normally enforces stem cell quiescence. Persistent pathway activation by Rb deletion resulted in continuous stem cell proliferation. CONCLUSION: We have shown that in addition to its classical hormonal roles, Igf operates locally in a lung stem cell niche as a latent autocrine complex with Igfbp, poised for activation by injury. Injury releases Igf, presumably by proteolytic destruction of Igfbp by PAPP-A and/or PAPP-A2. Once freed, Igf activates its signaling pathway, which inhibits Rb and releases this cell-cycle checkpoint to initiate stem cell proliferation. This poised control pathway in tissue repair may be co-opted in cancer because Rb loss immediately initiates uncontrolled NEstem proliferation and tumorigenesis. This local, rapidly activatable Igf-Rb stem cell control pathway may also operate in other neuroendocrine and neural stem cells and tumors of the body and brain. Injury activation of autocrine Igf complexes in the stem cell niche.: (Top) NEstem intermingled with other NE cells along airways. They constitutively produce Igf2-Igfbp, which are latent complexes in the niche. Innervating sensory neurons express Igfbp proteases PAPP-A and PAPP-A2. Dying cells send a signal (possibly a protease activator), freeing Igf2. (Bottom) During homeostasis, Igf is sequestered, Rb is active, and NEstem are quiescent. Upon injury, Igf is freed, Igf signaling inhibits Rb, and NEstem proliferate (self-renewal). In tumors, Rb is lost and continuous (Igf-independent) proliferation occurs. [ABSTRACT FROM AUTHOR]