Abstract
T-cell lymphomas (TCL) are aggressive and clinically heterogeneous malignancies originating from mature or immature T cells. Standard combination chemotherapy protocols, yield limited long-term benefit, and most patients experience relapse or refractory disease. In recent years, histone deacetylase inhibitors (HDACi), have emerged as promising alternatives. Although HDACi are FDA-approved for select TCL subtypes, their clinical benefit remains limited, with overall response rates—comprising complete and partial responses—typically ranging from 20% to 40%, depending on the disease subtype and treatment context. Chromatin regulation, including its accessibility state, is increasingly recognized as a critical factor influencing the therapeutic response to HDACi. However, the upstream regulators of chromatin architecture in TCL are not fully understood. In this sense, thyroid hormones (THs) have been implicated in chromatin remodeling and oncogenic signaling in other cancers. Our most recent work demonstrated that THs promote TCL cell proliferation, survival, and angiogenesis via activation of the integrin αvβ3–JAK/STAT signaling pathway. The role of THs in regulating the epigenetic landscape and affecting HDACi therapy response in these hematologic malignancies has not been previously explored.
We investigated the impact of THs on romidepsin-induced anti-tumoral effects and epigenetic modulation in TCL models. In vitro assays were performed using TCL cell lines representing immature (EL-4 and CUTLL-1) and mature (HuT78, OCI-Ly13.2, OCI-Ly12 and Karpas299) subtypes. Cells were treated with vehicle, physiological concentrations of THs (T3: 1 nM; T4: 100 nM), romidepsin, or their combination. After 48 hours, cell viability and apoptosis (via caspase 3/7 activity) were assessed. Histone acetylation (H3K27 and H4K16) was analyzed by Western blot after 24 hours, and global HDAC activity was measured at 6 and 24 hours using a luminescence-based assay. In vivo, we employed a syngeneic EL4 TCL model in C57BL/6 mice with euthyroid or hypothyroid status. Hypothyroidism was induced by administering propylthiouracil (PTU, 0.5 mg/mL in drinking water) for 14 days.
We first evaluated the in vitro activity of romidepsin across our panel of TCL cell lines under standard culture conditions. Romidepsin induced a dose-dependent reduction in cell viability, with IC50 values ranging from 4.5 to 17 nM, confirming its potent anti-tumoral effect across all TCL subtypes tested. To specifically assess the role of THs, we performed subsequent experiments in TH-depleted culture conditions to minimize background hormonal effects. We found that physiological concentrations of THs significantly increased the anti-tumoral activity of romidepsin in all TCL cell lines, with Karpas299 cells representing the only exception. The observed increase in romidepsin efficacy caused a 30–60% greater decrease in cell viability and a 25–70% increase in apoptosis, depending on the TCL subtype (p < 0.05). Mechanistically, this enhanced effect was accompanied by epigenetic alterations, with acetylation levels of histone marks H3K27ac and H4K16ac increasing significantly by approximately 30% and 50%, respectively, both indicative of open chromatin and active transcriptional states (p < 0.05). Additionally, global HDAC activity decreased by 20 to 50% depending on the TCL subtype (p < 0.05), supporting the notion that THs potentiate romidepsin's inhibitory action on HDACs.
To assess the in vivo relevance of these findings, we employed a syngeneic TCL murine model with euthyroid or hypothyroid status. Mice were subcutaneously injected with 3×10⁵ EL-4 cells, and once tumors reached a palpable size, animals were randomized to receive daily intraperitoneal injections of either (i) vehicle or (ii) romidepsin (0.5 mg/kg). Romidepsin treatment significantly reduced tumor size in both groups (p < 0.05 vs. vehicle). Notably, the magnitude of tumor growth inhibition was significantly greater in euthyroid mice compared to hypothyroid counterparts, suggesting that systemic thyroid status may play a key role in influencing treatment HDACi response in TCL patients.
All together, our findings highlight the importance of thyroid status as a modulator of romidepsin efficacy in patients with TCL and provide a strong basis for further study of TH-regulated epigenetic mechanisms during HDAC inhibitor treatment, which may contribute to improved therapies and identification of novel targets.
