EGF modulation of the ras‐related rhoB gene expression in human breast‐cancer cell lines

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Abstract
The mRNA levels of the ras-related human rhoA, rhoB and rhoC genes were studied in human breast-cancer cell lines (HBCal), and in normal and immortalized mammary epithelial cells (HMEC) by Northern blot analysis and in situ hybridization. In contrast to the ubiquitous rhoA and rhoC gene expression, dramatic variations in the mRNA level of the rhoB gene were evidenced. The rhoB mRNA level appeared to be inversely correlated to the amounts of the epidermal-growth-factor(EGF) receptors in these cells. The rhoB transcripts were detected at high levels in ZR75-1, MCF7, HSL 53, HSL 59, HSL 90, T47D and SKBR3 HBCal, at hardly detectable levels in BT 20, MDA-MB 231 and H466B HBCal and at intermediate levels in normal and immortalized breast epithelial cells. Rapid and transient induction of the rhoB transcription was observed after EGF treatment in serum-deprived MDA-MB231, T47D and immortalized epithelial cells. In contrast, no modulation of rhoB expression by EGF could be objectified in the MCF7 and ZR75-1 cell lines. Yet a normal function of EGF receptors was evidenced, since the immediate early gene c-fos was rapidly induced, suggesting a constitutive expression of rhoB in these cell lines bypassing the regulation by EGF. In human mammary epithelial cells, rhoB mRNA is rapidly and transiently induced with EGF concentrations known to stimulate cell proliferation. This suggests that the rhoB product might be involved in a cascade that initiates or promotes cell proliferation, and plays an important role in EGF-stimulated growth of breast normal and cancer cells.