STATMonomer Depletion Drives IFN Signaling Exhaustion in Aged Tissues via Age‑Dependent Ubiquitin‑Proteasome Activation

Aging is associated with elevated baseline interferon-stimulated gene (ISG) signatures but a blunted ISG response to acute interferon rechallenge, a phenotype that resembles pathway exhaustion rather than classic SOCS-mediated negative feedback. While SOCS proteins can inhibit JAKs and target STATs for degradation, no study has directly quantified whether the observed deficit in STAT signaling stems from SOCS upregulation or from a depletion of total STAT monomer pools. We hypothesize that in aged tissues, chronic low‑grade interferon signaling triggers an age‑specific increase in the activity of a ubiquitin‑ligase complex (e.g., the TRIM family member TRIM21 or the E3 ligase FBXW7) that preferentially ubiquitinates unphosphorylated STAT1 and STAT3, targeting them for proteasomal degradation. This leads to a reduction in the cytoplasmic pool of STAT monomers available for phosphorylation upon acute stimulation, producing the observed exhaustion signature without a compensatory rise in SOCS expression.

Testable predictions:

1. Total STAT1 and STAT3 protein levels (not just phosphorylated forms) will be significantly lower in basal conditions in tissues from old mice (or humans) compared with young controls, while SOCS1 and SOCS3 mRNA and protein levels remain unchanged or only modestly increased.
2. Proteasome inhibition (e.g., with MG132 or bortezomib) ex vivo will restore total STAT monomer levels and rescue ISG transcription upon acute interferon stimulation in aged cells, whereas SOCS knock‑down will have little effect.
3. Ubiquitination of STAT1 and STAT3 will be higher in aged tissues, and this increase will correlate with heightened expression or activity of the candidate E3 ligase (TRIM21/FBXW7).
4. Genetic knock‑down of the putative E3 ligase in aged mice will prevent STAT monomer loss, normalize baseline ISG signatures, and restore acute interferon responsiveness.

Experimental approach:

• isolate primary cells (e.g., muscle satellite cells, microglia, keratinocytes) from young and aged mice; measure total STAT1/3 by Western blot or quantitative mass spectrometry, phospho‑STAT by flow cytometry, and SOCS levels by qPCR and immunoblot.
• assess ubiquitination of STAT proteins using immunoprecipitation followed by anti‑Ub blot.
• treat aged cells with proteasome inhibitor or siRNA against the candidate E3 ligase, then rechallenge with interferon‑β or IL‑27 and quantify ISG induction (e.g., MX1, ISG15) by RT‑qPCR.
• in vivo, deliver isoform‑specific shRNA to the E3 ligase via AAV in aged mice, then challenge with systemic interferon and evaluate ISG induction in target tissues.

If STAT monomer levels are unchanged with age, or if proteasome inhibition fails to rescue acute ISG responses, the hypothesis would be falsified, supporting the alternative view that SOCS‑mediated regulation dominates. Conversely, confirmation would reveal a novel mechanism of interferon signaling exhaustion driven by age‑dependent ubiquitin‑proteasome activity, distinct from SOCS feedback, and point to therapeutic strategies that replenish STAT pools rather than suppress SOCS.