Microtubule acetylation has been shown to regulate actin filament dynamics by modulating signaling pathways that control actin organization, although the precise mechanisms remain unknown. In this study, we found that the downregulation of microtubule acetylation via the disruption ATAT1 (which encodes メ-tubulin N-acetyltransferase 1) inhibited the expression of RhoA, a small GTPase involved in regulating the organization of actin filaments and the formation of stress fibers. Analysis of RHOA promoter and chromatin immunoprecipitation assays revealed that C/EBPモ is a major regulator of RHOA expression. Interestingly, the majority of C/EBPモ in ATAT1 knockout (KO) cells was found in the nucleus as a 27-kDa fragment (referred to as C/EBPモp27) lacking the N-terminus of C/EBPモ. Overexpression of a gene encoding a C/EBPモp27-mimicking protein via an N-terminal deletion in C/EBPモ led to competitive binding with wild-type C/EBPモ at the C/EBPモ binding site in the RHOA promoter, resulting in a significant decrease of RHOA expression. We also found that cathepsin L (CTSL), which is overexpressed in ATAT1 KO cells, is responsible for C/EBPモp27 formation in the nucleus. Treatment with a CTSL inhibitor led to the restoration of RHOA expression by downregulation of C/EBPモp27 and the invasive ability of ATAT1 KO MDA-MB-231 breast cancer cells. Collectively, our findings suggest that the downregulation of microtubule acetylation associated with ATAT1 deficiency suppresses RHOA expression by forming C/EBPモp27 in the nucleus through CTSL. We propose that CTSL and C/EBPモp27 may represent a novel therapeutic target for breast cancer treatment.