The regulation of PTHrP expression and production by transforming growth factor-β (TGFβ) has been investigated in an epidermal squamous cancer cell line COLO 16. TGFβ₁ treatment increased steady-state levels of PTHrP mRNA and concentrations of PTHrP immunoreactivity in conditioned medium in a time- and dose-dependent manner with a half-maximal effect at 40 pM. An effect of TGFβ₁ on PTHrP mRNA was observed first after 4 h treatment and continued to increase up to 48 h with a concomitant increase in PTHrP immunoreactivity in the culture medium. TGFβ₁ was found to stabilize PTHrP mRNA as assessed by actinomycin C₁ experiments. In addition, a direct effect of TGFβ to increase PTHrP transcription was indicated by nuclear run-on and transient transfection experiments using a CAT promoter/expression construct encompassing the region −1100 bp to −20 bp from the initiating AUG of the human PTHrP gene. The conditioned medium from COLO 16 cells was also shown to contain both latent and active TGFβ at concentrations of 160 pM and 16 pM, respectively, in 72 h conditioned medium. A neutralizing antibody to TGFβ₁ (and TGFβ₂) decreased the level of immunoassayable PTHrP in the medium.