Keratin 19 Promotes Cell Cycle by Regulating GSK3B‐Cyclin D3 Pathway in Breast Cancer Cells

Abstract

Keratin 19 (K19) is a cytoskeletal protein that is utilized as a diagnostic and prognostic marker in select human carcinomas. Despite the positive correlation between higher expression of K19 in tumor and worse patient survival, the role of K19 in breast cancer remains unclear. We previously used K19 knockout (KO) MCF7 breast cancer cells to identify that K19 is required for proper cell cycle progression. Specifically, K19 regulated expression of D‐type cyclins (cyclin D1 and cyclin D3) which promote the G1/S transition. We found that K19 interacts with cyclin D3, and a loss of K19 resulted in decreased protein stability of cyclin D3. To better understand how K19 prevents cyclin D3 degradation, we examined the cyclic adenosine 3′,5′‐monophosphate (cAMP) signaling pathway where cAMP induction causes glycogen synthase kinase GSK3B to promote cyclin D3 degradation. When cells were treated with forskolin to elevate intracellular cAMP level, cyclin D3 turnover was increased in the absence of K19, suggesting that K19 inhibits GSK3B from degrading cyclin D3. Indeed, inhibiting GSK3B activity using LiCl and GSK3B expression via siRNA rescued the cyclin D3 level in K19 KO cells. These data suggest that K19 plays an important role in GSK3B‐mediated cyclin D3 degradation. By using molecular biology tools and biochemical approaches, we are in the process of characterizing specifics of K19‐dependent regulation of cyclin D3 stability including identification of GSK3B and K19 domains responsible for their interaction. We aim to provide evidences for a novel mechanism by which a cytoskeletal protein stabilizes a cell cycle regulator and promotes proper cell cycle progression.Schematic model for the role of K19 in regulation of GSK3B‐mediated cyclin D3 degradation.(a) K19 serve as scaffold for both for GSK3B and cyclin D3, preventing GSK3B to access cyclin D3.(b) In absence of K19, GSK3B has a full access to cyclin D3 for degradation.Figure 1