HRI Kinase Modulation by BTdCPU as a Therapeutic Strategy for Bortezomib Resistance in Prostate Cancer

Heme-Regulated Inhibitor (HRI) kinase is a key serine-threonine kinase that regulates eIF2α phosphorylation and Integrated Stress Signaling (ISR). The present study used parental and bortezomib-resistant PC3 prostate cancer cells to examine whether pharmacologic modulation of this pathway, particularly with BTdCPU, retains activity in a resistant setting and how it relates to stress-signaling markers.

We initially assessed the cytotoxic effects of the HRI activator BTdCPU, along with its inhibitors hemin and ZnPP, in bortezomib-resistant cells using the MTT assay. A single BTdCPU + bortezomib concentration pair was then examined using the real-time iCELLigence system and AO/EB morphology. Western blotting was performed to evaluate total HRI, phosphorylated eIF2, Hsp70, Hsp60, and polyubiquitin (PolyUb) conjugates. Public-dataset analyses were used to provide descriptive clinical context.

The IC50 values for bortezomib in parental and resistant cells were 44.34 nM and 1.151 µM, respectively, indicating that the resistant cells were approximately 26 times more resistant to bortezomib than the parental cells. The IC50 of BTdCPU in parental and resistant cells was found to be 1.268 µM and 1.971 µM, respectively. Notably, BTdCPU-induced HRI activation appears to act through a mechanism distinct from classical proteasome inhibition, as it promotes eIF2α phosphorylation independently of stress markers such as Hsp70 and polyubiquitin (PolyUb) conjugate accumulation, which are observed in parental cells treated with bortezomib, but not in resistant cells treated similarly. Descriptive bioinformatic analyses also indicated differential HRI expression across prostate cancer subgroups and according to TP53 status.

The current findings suggest that cancer cell resistance may, in part, arise from the overactivation of the HRI/eIF2α signaling pathway, which is likely triggered by the increased expression of PolyUb conjugates and Hsp70. These findings support further investigation of HRI/ISR-related signaling in proteasome-inhibitor resistance; however, the present data are pharmacologic and associative rather than causal, and they do not establish direct HRI dependence or downstream ISR effector involvement.

The findings indicate that BTdCPU, either alone or in combination with bortezomib, warrants further investigation as a potential strategy for overcoming bortezomib resistance in both p53-wild type and p53-mutant cancer cells.