Heat-Induced HSAT3 Transcription and Nuclear Stress Body Assembly in Human Myeloid Cells

Heat shock activates heat shock factor 1 (HSF1), which induces classical heat shock genes and promotes transcription of pericentromeric HSAT3 repeats that contribute to nuclear stress body (nSB) formation. Although well characterized in epithelial cells, this response remains poorly understood in myeloid cells. We compared heat-induced HSAT3 transcription and nSB assembly in epithelial and monocytic cell lines, using A549 and THP-1 as models. pHSF1 (Ser326) levels, HSPA1A and HSAT3 expression, and nSB formation were assessed by Western blotting, qPCR, immunofluorescence, RNA-FISH, and 5-ethynyluridine labeling. Heat shock increased pHSF1 (Ser326) levels and HSPA1A expression in both models. A549 cells showed increased HSAT3 transcript levels and prominent nSB formation, whereas undifferentiated THP-1 cells showed little immediate change in HSAT3 transcript levels and did not form prominent nSBs. 5-EU labeling revealed discrete nascent RNA foci in heat-shocked A549, but not THP-1, cells. These foci overlapped with HSF1 and were associated with HSAT3-related hybridization signals. However, click chemistry increased accessibility of genomic satellite sequences, limiting RNA-specific interpretation of the HSAT3 probe signal. PMA-induced differentiation enabled an increase in HSAT3 transcript levels during heat shock and nSB assembly. These findings identify cellular state as a determinant of heat shock-induced satellite transcription and nSB assembly.