Mitochondrial quality control mediated by PINK1 and PRKN: links to iron metabolism and tumor immunity

ABSTRACT

Mitochondrial quality control is an essential process required to maintain cellular homeostasis and functions. Mutations of PINK1 and PRKN/PARK2 contribute to the risk of Parkinson disease. Our recent findings indicate that depletion of Pink1 and Prkn promotes pancreatic tumorigenesis in KRAS-driven engineered mouse models. Mechanistically, PINK1- and PRKN-mediated autophagic degradation of mitochondrial iron importers (e.g., SLC25A37 and SLC25A28) suppresses pancreatic tumor growth by attenuating mitochondrial iron accumulation, inflammasome activation, HMGB1 release, and subsequent immune checkpoint expression. Consequently, pharmacological or genetic inhibition of mitochondrial iron-dependent signals prolongs animal survival and reverses pancreatic tumor phenotype in vivo. Thus, PINK1- and PRKN-mediated immunometabolism provides new insights into the tumor microenvironment and could be a suitable target for new pancreatic cancer treatments.

The clearance of unused proteins or damaged organelles by macroautophagy/autophagy is critical for the regulation of cellular homeostasis in various species, including humans. Selective mitochondrial clearance by autophagy, namely mitophagy, is an important mitochondrial quality control pathway. The best-studied mitophagy pathway in mammalian cells involves PINK1 and PRKN/Park2, genes both known to be linked to human Parkinson disease. PINK1 and PRKN may also play a role in mitochondrial quality control through regulation of mitochondrial dynamics and balance. Recently, we demonstrated that PINK1 and PRKN are novel regulators of mitochondrial iron trafficking, which is important for the suppression of pancreatic tumorigenesis [1].

Figure 1.

Schematic depicting the role of PINK1- and PRKN-mediated mitochondrial quality control in pancreatic tumorigenesis. PINK1-PRKN pathway-mediated degradation of SLC25A37 and SLC25A28 increases mitochondrial iron accumulation, which results in the HIF1A-dependent Warburg effect, and AIM2-dependent inflammasome activation in tumor cells. AIM2-mediated HMGB1 release further induces expression of CD274/PD-L1 in pancreatic tumorigenesis.

Tumor suppressor role of PINK1 and PRKN in the pancreas

The role of autophagy in cancer initiation and therapy is complex and can exert multifaceted effects, depending on the context. Different autophagy regulators have been reported to promote or suppress pancreatic cancer growth by interfering with different metabolism pathways such as carbohydrate metabolism, fatty acid beta-oxidation, and amino acid transport. We demonstrate that PINK1 and PRKN significantly suppress oncogenic KRAS-driven pancreatic tumorigenesis by inhibiting mitochondrial iron-mediated chronic inflammation and immune dysfunction. Global genetic ablation of Pink1 or Prkn renders mice (termed KCP1 or KCP2 mice, respectively) extremely sensitive to oncogenic KRAS-driven initiation of pancreatic intraepithelial neoplasms and stromal response. Pancreatitis carries a high lifetime risk of pancreatic cancer. Furthermore, loss of Pink1 or Prkn in mice also accelerates pancreatitis-associated pancreatic tumorigenesis. These findings indicate that Pink1 and Prkn are potential tumor suppressors. It remains unknown whether conditional knockout of Pink1 or Prkn in the pancreas have similar pancreatic tumor phenotypes compared to global knockout mice.

PINK1 and PRKN promote mitochondrial iron importer degradation

Consistent with the well-established role of PINK1 and PRKN in mitophagy, we demonstrate that circulating mitochondrial DNA, fragmented mitochondria, and mitophagy deficiency-associated markers such as VDAC (voltage-dependent anion channel), COX4I1/COXIV (cytochrome c oxidase subunit 4I1), and TOMM20 (translocase of outer mitochondrial membrane 20) in pancreases were increased in Pink1- or Prkn-deficient mice during KRAS-driven pancreatic tumorigenesis. Accumulating evidence has established that communication exists between iron metabolism in the mitochondria and the cytosol. These findings have revealed that dysfunction of mitochondrial iron trafficking plays an important role in the pathology of mitochondrial diseases, including cancer. Remarkably, we observed that 2 key mitochondrial iron importers (SLC25A37 and SLC25A28) are significantly upregulated at the protein level (but not at the mRNA level) in pancreases from KCP1 or KCP2 mice. Activation of the ubiquitin-proteasome system is required for the degradation of mitochondrial outer membrane proteins such as TOMM20 during PINK1- or PRKN-mediated mitophagy, whereas activation of the autophagy pathway is required for the PINK1- or PRKN-mediated degradation of mitochondrial inner membrane proteins such as SLC25A37 and SLC25A28. We further demonstrate that SQSTM1, but not other cargo receptors (NCOA4 [nuclear receptor coactivator 4], OTPN [optineurin], and CALCOCO2/NDP52 [calcium binding and coiled-coil domain 2]), is required for PINK1- and PRKN-mediated degradation of mitochondrial iron importers. Characterization of mitophagy-dependent and -independent roles of PINK1 or PRKN in controlling pancreatic tumorigenesis remains challenging in vivo.

Mitochondrial iron mediates abnormal immunometabolism in pancreatic tumorigenesis

Iron is predominantly consumed by the mitochondria for the biosynthesis of heme and iron-sulfur clusters. This process is associated with abnormal mitochondrial energy metabolism, oxidative stress, and immune response. We observe that the Warburg effect (also termed aerobic glycolysis) is increased in Pink1- or Prkn-deficient mice during KRAS-driven pancreatic tumorigenesis. Similar to knockout of Hif1a (hypoxia inducible factor 1, alpha subunit), pancreatic tumorigenesis is delayed in KCP1 and KCP2 mice treated with vitamin E (a lipophilic peroxyl radical scavenger) or deferiprone (a mitochondrial iron chelator). We further demonstrate that lactate, a normal product of anaerobic cells, is a mediator of AIM2 (absent in melanoma 2) inflammasome activation in KCP1 and KCP2 mice. Excessive inflammasome activation can cause pyroptosis, a form of regulated cell death, which promotes inflammasome-associated cytokine (e.g., IL1B [interleukin 1 beta]) and damage-associated molecular pattern (e.g., HMGB1 [high mobility group box 1]) release. Intracellular and extracellular HMGB1 play different roles in pancreatic tumorigenesis. Extracellular HMGB1 can induce immune-checkpoint CD274/PD-L1 expression via AGER/RAGE [advanced glycosylation end product-specific receptor], but not TLR4 (toll-like receptor 4) or TLR9 (toll-like receptor 9). Although CD274 could serve as a negative predictor for the overall survival of pancreatic cancer patients, immunotherapeutic agents, including anti-CD274 antibody, lack efficacy in human pancreatic cancer.

In summary, we demonstrate in mouse models that Pink1 and Prkn deficiency accelerate pancreatic tumorigenesis through mitochondrial iron-dependent immunometabolic dysfunction. Our study further suggests that manipulating the PINK1 and PRKN pathway may improve immunotherapy in pancreatic cancer, as abnormal PRKN, SLC25A37, and AIM2 mRNA expression is associated with poor prognosis in human pancreatic cancer.

Funding Statement

This work was supported by grants from the US National Institutes of Health (R01GM115366, R01CA160417, R01CA211070, R01GM127791, and R01GM053396), the Natural Science Foundation of Guangdong Province (2016A030308011), the American Cancer Society (Research Scholar Grant RSG-16-014-01-CDD), the National Natural Science Foundation of China (31671435, 81400132, and 81772508), and Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2017);National Institutes of Health [R01GM115366, R01CA160417, R01CA211070, R01GM127791, and R01GM053396].

Acknowledgments

We thank Christine Burr (Department of Surgery, University of Pittsburgh) for her critical reading of the manuscript.

Disclosure statement

No potential conflicts of interest were disclosed.