Targeted deletion of TGFß1 in basal keratinocytes causes profound defects in stratified squamous epithelia and aberrant melanocyte migration.

Transforming Growth Factor Beta 1 (TGFß1) is a multifunctional cytokine that regulates proliferation, apoptosis, and epithelial-mesenchymal transition of epithelial cells. While its role in cancer is well studied, less is known about TGFß1 and regulation of epithelial development. To address this, we deleted TGFß1 in basal keratinocytes of stratified squamous epithelia. Newborn mice with a homozygous TGFß1 deletion had significant defects in proliferation and differentiation of the epidermis and oral mucosa, and died shortly after birth. Hair follicles were sparse in TGFß1 depleted skin and had delayed development. Additionally, the Wnt pathway transcription factor LEF1 was reduced in hair follicle bulbs and nearly absent from the basal epithelial layer. Hemizygous knockout mice survived to adulthood but were runted and had sparse coats. The skin of these mice had irregular hair follicle morphology and aberrant hair cycle progression, as well as abnormally high melanin expression and delayed melanocyte migration. In contrast to newborn TGFß1 null mice, the epidermis was hyperproliferative, acanthotic and inflamed. Expression of p63, a master regulator of stratified epithelial identity, proliferation and differentiation, was reduced in TGFß1 null newborn epidermis but expanded in the postnatal acanthotic epidermis of TGFß1 hemizygous mice. Thus, TGFß1 is both essential and haploinsufficient with context dependent roles in stratified squamous epithelial development and homeostasis.

TGFß1 regulates HRas-mediated activation of IRE1α through the PERK-RPAP2 axis in keratinocytes.

Transforming Growth Factor ß1 (TGFß1) is a critical regulator of tumor progression in response to HRas. Recently, TGFß1 has been shown to trigger ER stress in many disease models; however, its role in oncogene-induced ER stress is unclear. Oncogenic HRas induces the unfolded protein response (UPR) predominantly via the Inositol-requiring enzyme 1α (IRE1α) pathway to initiate the adaptative responses to ER stress, with importance for both proliferation and senescence. Here, we show a role of the UPR sensor proteins IRE1α and (PKR)-like endoplasmic reticulum kinase (PERK) to mediate the tumor-suppressive roles of TGFß1 in mouse keratinocytes expressing mutant forms of HRas. TGFß1 suppressed IRE1α phosphorylation and activation by HRas both in in vitro and in vivo models while simultaneously activating the PERK pathway. However, the increase in ER stress indicated an uncoupling of ER stress and IRE1α activation by TGFß1. Pharmacological and genetic approaches demonstrated that TGFß1-dependent dephosphorylation of IRE1α was mediated by PERK through RNA Polymerase II Associated Protein 2 (RPAP2), a PERK-dependent IRE1α phosphatase. In addition, TGFß1-mediated growth arrest in oncogenic HRas keratinocytes was partially dependent on PERK-induced IRE1α dephosphorylation and inactivation. Together, these results demonstrate a critical cross-talk between UPR proteins that is important for TGFß1-mediated tumor suppressive responses.