Dysregulation of INF2-mediated mitochondrial fission in SPOP-mutated prostate cancer

Abstract

Next-generation sequencing of the exome and genome of prostate cancers has identified numerous genetic alternations. SPOP (Speckle-type POZ Protein) was one of the most frequently mutated genes in primary prostate cancer, suggesting SPOP is a potential driver of prostate cancer development and progression. However, how SPOP mutations contribute to prostate cancer pathogenesis remains poorly understood. SPOP acts as an adaptor protein of the CUL3-RBX1 E3 ubiquitin ligase complex that generally recruits substrates for ubiquitination and subsequent degradation. ER-localized isoform of the formin protein inverted formin 2 (INF2) mediates actin polymerization at ER-mitochondria intersections and facilitates DRP1 recruitment to mitochondria, which is a critical step in mitochondrial fission. Here, we revealed that SPOP recognizes a Ser/Thr (S/T)-rich motif in the C-terminal region of INF2 and triggers atypical polyubiquitination of INF2. These ubiquitination modifications do not lead to INF2 instability, but rather reduces INF2 localization in ER and mitochondrially associated DRP1 puncta formation, therefore abrogates its ability to facilitate mitochondrial fission. INF2 mutant escaping from SPOP-mediated ubiquitination is more potent in prompting mitochondrial fission. Moreover, prostate cancer-associated SPOP mutants increase INF2 localization in ER and promote mitochondrial fission, probably through a dominant-negative effect to inhibit endogenous SPOP. Moreover, INF2 is important for SPOP inactivation-induced prostate cancer cell migration and invasion. These findings reveal novel molecular events underlying the regulation of INF2 function and localization, and provided insights in understanding the relationship between SPOP mutations and dysregulation of mitochondrial dynamics in prostate cancer.

SPOP negatively regulates Toll-like receptor-induced inflammation by disrupting MyD88 self-association

“…TLR4 activation causes the translocation of SPOP to the cytoplasm Recently, it was demonstrated that SPOP is located in both the cytoplasm and nucleus, although it was previously considered a nuclear protein. 34 Because SPOP is associated with MyD88 following TLR4 activation, we sought to determine whether SPOP is translocated to the cytoplasm after TLR4 activation. We isolated nuclear and cytoplasmic fractions and found that SPOP was mainly located in the nucleus, and only a small amount of SPOP was detected in the cytoplasm (Fig.…”

Mitochondrial Dynamics in Type 2 Diabetes and Cancer

“…Similarly, the energy-sensing AMPK increased recruitment of DRP1 to mitochondria via phosphorylation of the MFF and ( 61 ). Speckle-type POZ protein loss-of-function mutations commonly found in primary prostate cancer were associated with increased DRP1 activation, mitochondrial fission, and prostate cancer cell invasion ( 62 ). Recently, loss of expression of the sirtuin SIRT4 was shown to lead to increased mitochondrial fragmentation ( 63 ).…”

Inverted formins: A subfamily of atypical formins

“…Specifically, SPOP poly-ubiquitinates INF2, which leads not to degradation, but to translocation of INF2 from the ER to the cytosol [Jin et al, 2017]. Consequently, SPOP overexpression or targeting to the cytosol (by elimination of its nuclear localization signal) leads to mitochondrial lengthening.…”