Hypothesis

In aged tissues, persistent type I/II interferon signaling drives transcriptional upregulation of SOCS1 and SOCS3, yet the accumulated SOCS proteins are largely ineffective at inhibiting JAK‑STAT signaling because chronic ligand exposure overwhelms the ubiquitin‑proteasome system (UPS). The resulting proteasome saturation leads to accumulation of polyubiquitinated SOCS species that cannot bind JAKs or receptors, producing a decoupling of SOCS mRNA induction from functional feedback. This mechanism explains the observed JAK‑STAT hyperactivity despite high SOCS transcript levels in aging.

Mechanistic Basis

SOCS proteins function as E3 ubiquitin ligase adaptors that recruit JAKs and cytokine receptors to the proteasome for degradation. Each round of SOCS‑mediated ubiquitination consumes one SOCS molecule, which is itself degraded after delivering its cargo. In young cells, de novo SOCS synthesis keeps pace with this consumptive cycle. In aging, sustained IFN elevates SOCS transcription but also increases the load of ubiquitinated substrates (including damaged proteins, organelles, and immune complexes). Proteasome capacity becomes limiting, causing a backlog of polyubiquitinated SOCS that accumulates in insoluble aggregates or aggresomes. These sequestered SOCS retain ubiquitination motifs but sterically lack access to JAK catalytic sites or receptor tyrosine residues, rendering them unable to execute feedback. Consequently, JAK‑STAT signaling remains active, driving ISG expression and senescence-associated phenotypes.

Testable Predictions

1. Aged cells exposed to continuous IFN will show higher SOCS1/3 mRNA and total protein levels compared with young cells, but a reduced proportion of SOCS that is soluble and capable of co‑immunoprecipitating with JAK1/JAK2 or IFNAR1.
2. Proteasome activity (measured by fluorogenic peptide cleavage) will be significantly lower in aged cells after prolonged IFN treatment, correlating with increased accumulation of polyubiquitinated SOCS detected by K48‑linked ubiquitin immunoblots.
3. Pharmacological proteasome activation (e.g., with IU1) or enhancement of autophagy (e.g., with spermidine) will restore SOCS‑JAK binding and reduce ISG expression in aged cells without altering SOCS transcription.
4. Genetic knockdown of the aggresome‑forming HDAC6 will decrease insoluble SOCS aggregates and rescue JAK‑STAT inhibition in aged tissues.

Experimental Design

• Cell models: Primary lung MO‑ifn monocytes, tendon stem/progenitor cells, and muscle satellite cells isolated from young (3‑month) and aged (24‑month) mice.
• IFN stimulation: Treat cells with uniform IFN‑β (100 ng/mL) for 0, 6, 12, 24, and 48 hours to model chronic exposure.
• Readouts:
  • qRT‑PCR for SOCS1/3 and ISGs (Ifit1, Mx1).
  • Western blot for total SOCS1/3, K48‑ubiquitinated SOCS (immunoprecipitate SOCS then blot for ubiquitin), and soluble vs insoluble fractions (RIPA supernatant vs pellet).
  • Co‑immunoprecipitation of SOCS1/3 with JAK1/JAK2 or IFNAR1.
  • Proteasome activity assay using Suc‑LLVY‑AMC substrate.
  • Aggressome detection by immunofluorescence for HDAC6 and ubiquitin co‑localization.
• Interventions: Parallel treatment with MG132 (proteasome inhibitor) to exacerbate accumulation, IU1 (USP14 inhibitor) to boost proteasome function, and spermidine to induce autophagy.
• Expected outcome: Aged cells will exhibit a divergence between SOCS mRNA/protein increase and functional JAK‑STAT inhibition, reversible by proteasome or autophagy enhancement but not by transcriptional modulation alone.

This hypothesis directly links the consumptive nature of SOCS‑mediated feedback to age‑related proteostatic decline, offering a clear, falsifiable framework to distinguish true negative feedback regulation from pathway exhaustion in chronic interferon signaling.