TWEAK and RIPK1 mediate a second wave of cell death during AKI

Abstract

Acute kidney injury (AKI) is characterized by necrotic tubular cell death and inflammation. The TWEAK/Fn14 axis is a mediator of renal injury. Diverse pathways of regulated necrosis have recently been reported to contribute to AKI, but there are ongoing discussions on the timing or molecular regulators involved. We have now explored the cell death pathways induced by TWEAK/Fn14 activation and their relevance during AKI. In cultured tubular cells, the inflammatory cytokine TWEAK induces apoptosis in a proinflammatory environment. The default inhibitor of necroptosis [necrostatin-1 (Nec-1)] was protective, while caspase inhibition switched cell death to necroptosis. Additionally, folic acid-induced AKI in mice resulted in increased expression of Fn14 and necroptosis mediators, such as receptor-interacting protein kinase 1 (RIPK1), RIPK3, and mixed lineage domain-like protein (MLKL). Targeting necroptosis with Nec-1 or by genetic RIPK3 deficiency and genetic Fn14 ablation failed to be protective at early time points (48 h). However, a persistently high cell death rate and kidney dysfunction (72-96 h) were dependent on an intact TWEAK/Fn14 axis driving necroptosis. This was prevented by Nec-1, or MLKL, or RIPK3 deficiency and by Nec-1 stable (Nec-1s) administered before or after induction of AKI. These data suggest that initial kidney damage and cell death are amplified through recruitment of inflammation-dependent necroptosis, opening a therapeutic window to treat AKI once it is established. This may be relevant for clinical AKI, since using current diagnostic criteria, severe injury had already led to loss of renal function at diagnosis.

Necroptosis in Acute Kidney Injury

“…Models of toxic AKI such as oxalate nephropathy or cisplatin-induced AKI showed consistent results, that is, Ripk3- deficient mice were protected when compared to wild type controls [8, 10]. In folic acid-induced AKI, Ripk3 -deficient mice did not display a phenotype in the early phase but was protective in the later phase of injury, which implies divergent functions in the early and late phase of injury; however, this remains to be confirmed by other groups [11, 12]. …”

“…Folic acid exposure is another crystal-related toxic AKI model. Martin-Sanchez et al [11, 12] demonstrated in 2 publications that Nec-1 does not affect the early phase of folic acid-induced tubular cell injury, a phase that is driven predominately by ferroptosis but rather the subsequent inflammatory AKI phase. Obviously, ferroptosis and necroptosis can act in sequence during certain forms of AKI and ferroptosis inhibition increases the protective effect of Nec-1 [13].…”

“…Indeed, Mlkl –/– mice rather displayed an aggravated phenotype in early folic acid-induced AKI and although a possible contribution in kidney regeneration was discussed, this remains entirely speculative to date [12]. In another publication of the same group, Mlkl –/– mice were protected from folic acid-induced AKI when analyzed at a later time point, which suggests a more complex disease pathophysiology that remains to be further clarified [11]. …”

Cell Death Pathways Drive Necroinflammation during Acute Kidney Injury

“…Whereas apoptosis does not cause and immune response, pathways of regulated necrosis may initiate a detrimental immune cell infiltrate that further deteriorates the primary damage [8]. The second wave of death may also be dependent on necrosis and thereby fuel a vicious circle of necroinflammation [13].…”

The clinical relevance of necroinflammation—highlighting the importance of acute kidney injury and the adrenal glands

“…However, in the case of folic acid induced nephropathy, the data are less clear. While it was first believed that necroptosis was of minor importance in this setting [92], both ferroptosis and necroptosis, depending on the timing of the interference, may be important [93]. This may reflect the independent features of regulated necrosis as a primary damage and a second wave of renal failure associated with immune cell infiltration (necroinflammation) [93].…”