14‐3‐3σ/Stratifin and p21 limit AKT‐related malignant progression in skin carcinogenesis following MDM2‐associated p53 loss

To study mechanisms driving/inhibiting skin carcinogenesis, stage‐specific expression of 14‐3‐3σ (Stratifin) was analyzed in skin carcinogenesis driven by activated rasHa/fos expression (HK1.ras/fos) and ablation of PTEN‐mediated AKT regulation (K14.creP/Δ5PTENflx/flx). Consistent with 14‐3‐3σ roles in epidermal differentiation, HK1.ras hyperplasia and papillomas displayed elevated 14‐3‐3σ expression in supra‐basal keratinocytes, paralleled by supra‐basal p‐MDM2166 activation and sporadic p‐AKT473 expression. In bi‐genic HK1.fos/Δ5PTENflx/flx hyperplasia, basal‐layer 14‐3‐3σ expression appeared, and alongside p53/p21, was associated with keratinocyte differentiation and keratoacanthoma etiology. Tri‐genic HK1.ras/fos‐Δ5PTENflx/flx hyperplasia/papillomas initially displayed increased basal‐layer 14‐3‐3σ, suggesting attempts to maintain supra‐basal p‐MDM2166 and protect basal‐layer p53. However, HK1.ras/fos‐Δ5PTENflx/flx papillomas exhibited increasing basal‐layer p‐MDM2166 activation that reduced p53, which coincided with malignant conversion. Despite p53 loss, 14‐3‐3σ expression persisted in well‐differentiated squamous cell carcinomas (wdSCCs) and alongside elevated p21, limited malignant progression via inhibiting p‐AKT1473 expression; until 14‐3‐3σ/p21 loss facilitated progression to aggressive SCC exhibiting uniform p‐AKT1473. Analysis of TPA‐promoted HK1.ras‐Δ5PTENflx/flx mouse skin, demonstrated early loss of 14‐3‐3σ/p53/p21 in hyperplasia and papillomas, with increased p‐MDM2166/p‐AKT1473 that resulted in rapid malignant conversion and progression to poorly differentiated SCC. In 2D/3D cultures, membranous 14‐3‐3σ expression observed in normal HaCaT and SP1ras61 papilloma keratinocytes was unexpectedly detected in malignant T52ras61/v‐fos SCC cells cultured in monolayers, but not invasive 3D‐cells. Collectively, these data suggest 14‐3‐3σ/Stratifin exerts suppressive roles in papillomatogenesis via MDM2/p53‐dependent mechanisms; while persistent p53‐independent expression in early wdSCC may involve p21‐mediated AKT1 inhibition to limit malignant progression.