Autocrine human growth hormone inhibits placental transforming growth factor-β gene transcription to prevent apoptosis and allow cell cycle progression of human mammary carcinoma cells

Abstract

Multiple cellular effects of human growth hormone (hGH) are mediated by an indirect mechanism requiring transcriptional activation of genes encoding protein effector molecules such as insulin-like growth factor-1. Such protein effector molecules then act directly to mediate the cellular functions of hGH. We report here that autocrine hGH production by mammary carcinoma cells specifically results in the transcriptional repression of the p53-regulated placental transforming growth factor-β (PTGF-β) gene. Transcriptional repression of the PTGF-β gene does not require the p53-binding sites in the PTGF-β promoter, and autocrine hGH also desensitized the response of the PTGF-β promoter to p53 overexpression. Transcriptional repression of the PTGF-β gene is accompanied by consequent decreases in its protein product, Smad-mediated transcription, and its cellular effects that include cell cycle arrest and apoptosis. PTGF-β specifically inhibited the autocrine hGH-stimulated expression of cyclin D1 required for autocrine hGH-stimulated mammary carcinoma cell cycle progression. Thus, one mechanism by which autocrine hGH promotes an increase in mammary carcinoma cell number is by transcriptional repression of protein effector molecules that promote cell cycle arrest and apoptosis. Such transcriptional repression of negative regulatory factors, such as PTGF-β, may also be requisite for direct stimulation of mammary carcinoma cell mitogenesis by hGH.