HSP47 levels determine the degree of body adiposity

Adipose tissue is a specialized energy-storing organ whose mass dynamically adjusts to nutritional and hormonal states. Insulin promotes lipid accumulation during feeding and overeating, whereas glucocorticoids induce lipolysis during fasting. PPARγ is a master regulator of adipocyte differentiation and lipid storage; its dysfunction leads to loss of adipose tissue, while activation (e.g., pioglitazone) expands adiposity. Body adiposity varies widely among individuals due to physiological, environmental (e.g., exercise), pathological (e.g., obesity, cachexia, effects of bariatric surgery), and genetic factors (e.g., SNPs associated with BMI, waist/hip traits). Despite these known contributors, a unified molecular determinant linking these contexts has been lacking. This study hypothesizes that HSP47, a collagen-specific chaperone, is a key determinant of body adiposity. The purpose is to define HSP47’s expression patterns relative to adiposity, hormonal regulation, genetic associations, causal effects on fat mass, and the molecular mechanism connecting extracellular matrix, focal adhesion signaling, and PPARγ stability to adipose expansion.

Prior work establishes: (1) adipose tissue remodeling with nutritional/hormonal cues; (2) PPARγ as essential for adipocyte differentiation and adipose maintenance, with mutations causing lipodystrophy and agonists promoting adipose expansion; (3) exercise and calorie restriction reduce adiposity; (4) bariatric surgery lowers adiposity; (5) GWAS identify numerous loci influencing adiposity distribution (BMI-adjusted waist/hip); (6) focal adhesion components (integrins/FAK) mediate extracellular matrix (ECM)-intracellular signaling; and (7) collagens and ECM composition affect tissue structure and function beyond fibrosis. These backgrounds motivate testing whether a collagen chaperone (HSP47/SerpinH1) may mechanistically link ECM dynamics to PPARγ regulation and adiposity.

Study design integrated in silico transcriptomics/genetics with in vivo mouse models and in vitro/ex vivo cellular assays. Transcriptomic analyses: adipose-enrichment scoring using GTEx tissue expression profiles; GO/KEGG enrichment for top 500 adipose-enriched genes; mining GEO datasets for conditions affecting adiposity (feeding/overeating/obesity/exercise/calorie restriction/bariatric surgery/cachexia) and assessing HSP47 expression; correlation of adipose HSP47 expression with body adiposity traits (fat mass, BMI, waist, hip, lean mass) in the METSIM cohort. Genetic analyses: interrogation of GWAS Catalog for HSP47 (SERPINH1) variants associated with BMI-adjusted waist/hip; eQTL analyses (GTEx, eQTL Dashboard) in cultured fibroblasts and omental adipose tissue for selected variants (rs606452, rs668347, rs645935, rs584961, rs605040, rs646474); allele frequencies from 1000 Genomes via Ensembl. Hormonal regulation: in vitro 3T3-L1 adipocytes and ex vivo mouse adipose explants treated with insulin (± insulin receptor inhibitor OSI-906) or dexamethasone (± GR antagonist RU486); in vivo insulin receptor inhibition (OSI-906, 100 mg/kg) and adipose-specific GR knockout (AGRKO) with corticosterone administration; analysis of HSP47 protein by western blot; validation with independent datasets (chicken anti-insulin serum neutralization; human adipose explants treated with dexamethasone/cortisol). Mouse models: adipose-specific Hsp47 knockout (AdHSP47KO) generated by crossing Hsp47 floxed mice with Adipoq-Cre; pharmacological HSP47 inhibition using Col003 (HSP47i; 100 mg/kg i.p.) under fasting/refeeding or short-term HFD refeeding paradigms; outcomes included body weight, food intake, fat depot masses (VAT/SAT/BAT), lean mass, liver and muscle masses; adipocyte size by H&E and whole-mount lipid staining/confocal microscopy. Systemic metabolism: hepatic glycogen and triglycerides assays; plasma insulin ELISA; glucose and insulin tolerance tests. Mechanistic assays: assessment of ECM collagen (e.g., collagen VI) deposition/folding (monomer/dimer/trimer forms in non-reducing gels), integrin β1–collagen interaction (DTSSP crosslink, co-IP), FAK activation (pFAK), and PPARγ protein levels by western blot in adipose tissues, 3T3-L1 adipocytes with Hsp47 siRNA or HSP47i, RGD peptide competition, and Itgb1 siRNA. PPARγ stability assays: FAK inhibitor (PF-573228) treatment with cycloheximide chase; ubiquitination assays in HEK293T co-expressing FLAG-PPARγ and HA-ubiquitin ± MG132; candidate E3 ligases from BioGRID overlap; MDM2 involvement tested via siRNA and the MDM2 inhibitor MI-773. Gene expression: RT-qPCR for Pparg and target genes (Adipoq, Fabp4, Cd36, Lpl, Glut4, Scd1). Rescue experiments: PPARγ agonist pioglitazone (AdHSP47KO: 15 mg/kg i.p. every other day for 1 week; HSP47i co-treatment: 30 mg/kg) with assessment of WAT mass and adiponectin expression. Standard protocols: RNA isolation, RT-qPCR, western blotting, immunoprecipitation, whole-mount immunofluorescence, histology, and statistical analyses (t-tests, Tukey-Kramer, Wilcoxon; p<0.05 significant). All animal work under Osaka University IACUC approval; C57BL/6J background; detailed conditions for housing, diets (HFD 60% kcal), and interventions provided.

• HSP47 expression is adipose-enriched and associates with body adiposity in humans: increases with feeding, overeating, and obesity; decreases with exercise, calorie restriction, bariatric surgery, and cachexia. Quantitatively: feeding increased HSP47 in human SAT (GSE154612, n=11, P=0.0013); obese vs lean adipocytes (GDS3602) showed higher HSP47 (n=10 each, P=0.0071); monozygotic twins discordant for adiposity had higher HSP47 in high-adiposity co-twins (GSE92405, n=25 pairs, P=0.024); exercise training decreased HSP47 (GSE116801, n=10, P=0.0114); low-calorie diet reduced HSP47 (GSE77962, n=51 pre/post, P=0.0039); bariatric surgery reduced HSP47 (GSE72158, n=42, P=0.000521); cachexia decreased HSP47 (GSE20571, P=0.000276).
• HSP47 expression in human adipose correlates with adiposity traits (METSIM, GSE70353): fat mass R²=0.0294 (P<0.0001); BMI R²=0.0168 (P<0.0001); waist circumference R²=0.0307 (P<0.0001); hip circumference R²=0.0121 (P=0.0022); inversely with lean mass R²=0.0204 (P<0.0001).
• Hormonal regulation: insulin upregulates and glucocorticoids downregulate HSP47. In 3T3-L1 adipocytes: insulin (100 nM) increased HSP47 protein (P<0.0001) and dexamethasone (100 nM) decreased it (P=0.0052). Ex vivo adipose explants: insulin and dexamethasone effects both P<0.0001. IR inhibitor OSI-906 blocked insulin-induced HSP47 (ctrl vs insulin P=0.0013; insulin vs insulin+IRi P=0.0019). GR antagonist RU486 blocked dexamethasone’s effect (dex P=0.0003; dex vs dex+GRi P=0.0005). In vivo IR inhibition reduced adipose HSP47; AGRKO increased HSP47, especially with corticosterone (P<0.001). Anti-insulin serum in chicken lowered adipose Hsp47 to fasting levels (Feed vs anti-Insulin P=0.0215). Human adipose explants: dexamethasone 10 nM (P=0.0014) and 1000 nM (P=0.0002) decreased HSP47.
• Genetic association: HSP47 (SERPINH1) variants linked to BMI-adjusted waist/hip (rs606452-A, rs668347-T, rs645935-T, rs584961-A) increase HSP47 expression (significant or trend) in fibroblasts/adipose tissue; adipose eQTL significance for rs606452 (P=2.21e-2) and rs584961 (P=2.06e-2). Allele frequencies across populations: rs606452-A ~30.4%, rs668347-T ~29.6%, rs645935-T ~25.6%, rs584961-A ~11%.
• Causality in mice: AdHSP47KO mice showed reduced fat mass with unchanged body weight/food intake. VAT (epididymal) mass decreased (P=0.0396), SAT (inguinal) mass decreased (P=0.0015), total fat mass decreased (P=0.004); lean mass unchanged; similar in females and during short-term HFD. Pharmacologic HSP47 inhibition (Col003, HSP47i) reduced VAT (P=0.0109), SAT (P=0.00039), total fat mass (P=0.000842) without affecting body weight/food intake; adipocyte size was significantly smaller in both genetic and pharmacologic models.
• Systemic effects: partial fat loss redirected energy storage to liver with increased hepatic glycogen/triglycerides and impaired glucose/insulin tolerance with elevated plasma insulin.
• Mechanism: HSP47 promotes collagen folding and secretion; HSP47 ablation reduces extracellular collagen around adipocytes and decreases focal adhesion signaling (pFAK) and PPARγ protein in adipose tissue and 3T3-L1 cells. Representative statistics: in AdHSP47KO adipose tissue, pFAK decreased (P=0.000466) and PPARγ decreased (P=2.19e−6). HSP47i in vivo reduced pFAK (P=0.011) and PPARγ (P=0.00074). Hsp47 siRNA in 3T3-L1 reduced pFAK (P=0.0239) and PPARγ (P=0.0121) and diminished collagen dimer (P=0.00043) and trimer (P=0.003) forms. HSP47i reduced collagen–integrin β1 interaction (P=0.00783) alongside pFAK (P=0.0058) and PPARγ (P=1.59e−5). Blocking collagen–integrin binding with RGD peptides or Itgb1 siRNA decreased pFAK and PPARγ.
• PPARγ stability control: FAK inhibition reduced PPARγ protein (0.5 μM P=0.0063; 1 μM P<0.0001) without lowering Pparg mRNA, accelerated PPARγ decay under cycloheximide, and increased PPARγ ubiquitination; MG132 rescued PPARγ. MDM2 interacts with PPARγ and mediates degradation; MDM2 inhibition (MI-773) partially rescued FAKi- or HSP47i-induced PPARγ reductions (e.g., HSP47i vs HSP47i+MDM2i 4 μM, P=0.0073). PPARγ target genes (Adipoq, Fabp4, Cd36, Lpl, Scd1 ± Glut4) were reduced by HSP47 ablations in vitro and in vivo.
• Functional rescue: Pioglitazone restored WAT mass in AdHSP47KO (Ctrl vs KO total WAT P=0.0096; KO vs KO+Pio P=0.0068) and in HSP47i-treated mice (Ctrl vs HSP47i P=0.0122; HSP47i vs HSP47i+Pio P=0.0122), indicating that reduced PPARγ activity underlies the low adiposity phenotype.

The study demonstrates that HSP47, a collagen-specific chaperone, is a central determinant of body adiposity by linking extracellular collagen matrix dynamics to intracellular PPARγ stability via integrin/FAK signaling. Expression analyses across multiple human cohorts and conditions show HSP47 tightly tracks with adiposity phenotypes and responds to hormonal cues reflecting nutritional status: insulin elevates and glucocorticoids suppress HSP47. Genetic variants that raise HSP47 expression in adipose tissue are associated with increased adiposity traits, suggesting population-level impact. Mouse models establish causality: genetic or pharmacological HSP47 ablation reduces fat mass and adipocyte size without altering intake or weight acutely, but shifts energy storage to the liver and impairs glucose/insulin tolerance, consistent with partial lipodystrophy-like redistribution. Mechanistically, reduced HSP47 compromises collagen folding/secretion and collagen–integrin engagement, lowering FAK activation and destabilizing PPARγ via ubiquitin–proteasome pathways involving MDM2. Restoration of PPARγ activity with pioglitazone rescues adipose mass, functionally linking the HSP47–collagen–integrin/FAK axis to PPARγ-mediated adipose expansion. These findings integrate physiological, hormonal, and genetic determinants of adiposity into a cohesive mechanism, highlighting the ECM as an active regulator of adipocyte lipid storage rather than a passive scaffold.

HSP47 is abundant in adipose tissue and functions as a key determinant of body adiposity. Its expression mirrors adiposity across physiological and pathological contexts, is upregulated by insulin and downregulated by glucocorticoids, and genetic variants elevating HSP47 expression associate with higher adiposity traits. Adipose-specific or pharmacologic HSP47 inhibition in mice lowers fat mass and adipocyte size, implicating HSP47 causally. Mechanistically, HSP47 sustains collagen folding/secretion and collagen–integrin interactions to activate FAK signaling, which stabilizes PPARγ and maintains its transcriptional activity necessary for adipose expansion. Therapeutically, modulation of this axis affects adiposity; PPARγ agonism can rescue the fat-loss phenotype from HSP47 ablation. Future research should: (1) delineate tissue-specific and temporal dynamics of HSP47 in long-term metabolic settings; (2) explore selective modulation of HSP47–collagen interactions to influence adiposity without adverse metabolic trade-offs; (3) assess clinical relevance of HSP47 variants in diverse populations and interactions with lifestyle factors; and (4) evaluate safety/efficacy of HSP47-targeted interventions with careful titration to preserve systemic metabolic homeostasis.

• HSP47 is not the sole determinant of adiposity; other genetic, inflammatory, oxidative, endocrine, and metabolic factors contribute.
• Mouse studies primarily used normal diet or short-term HFD paradigms; long-term outcomes and chronic metabolic adaptations were not assessed.
• Pharmacologic inhibition (Col003) and genetic knockout may have off-target or compensatory effects; increased liver mass and impaired glucose/insulin tolerance indicate potential metabolic liabilities when reducing adipose tissue via HSP47.
• Human data are associative for expression and genetics; although eQTLs support increased HSP47 expression in adipose tissue, causal effects in humans remain to be established.
• Tissue specificity: effects were characterized mainly in white adipose depots; broader impacts on other tissues and ECM remodeling require further evaluation.