Cyclin D3 regulates the G1/S transition and is frequently overexpressed in several cancer types including breast cancer, where it promotes tumor progression. Here, we show that a cytoskeletal protein keratin 19 (K19) physically interacts with a serine/threonine kinase GSK3β and prevents GSK3β-dependent degradation of cyclin D3. The absence of K19 allowed active GSK3β to accumulate in the nucleus and degrade cyclin D3. Specifically, the head domain of K19 was required to sustain inhibitory phosphorylation of GSK3β Ser9, prevent nuclear accumulation of GSK3β, and maintain cyclin D3 levels and cell proliferation. K19 was found to interact with GSK3β and K19-GSK3β interaction was mapped out to require Ser10 and Ser35 residues on the head domain of K19. Unlike wildtype K19, S10A and S35A mutants failed to maintain total and nuclear cyclin D3 levels and induce cell proliferation. Finally, we show that the K19-GSK3β-cyclin D3 pathway affected sensitivity of cells towards inhibitors to cyclin dependent kinase 4 and 6 (CDK4/6). Overall, these findings establish a role for K19 in the regulation of GSK3β-cyclin D3 pathway and demonstrate a potential strategy for overcoming resistance to CDK4/6 inhibitors.

Cyclin D3 调节 G1/S 转换，并且经常在包括乳腺癌在内的几种癌症类型中过度表达，从而促进肿瘤进展。在这里，我们展示了细胞骨架蛋白角蛋白 19 (K19) 与丝氨酸/苏氨酸激酶 GSK3β 物理相互作用并阻止 GSK3β 依赖性细胞周期蛋白 D3 的降解。K19 的缺失使活性 GSK3β 在细胞核中积累并降解细胞周期蛋白 D3。具体而言，K19 的头部结构域需要维持 GSK3β Ser9 的抑制性磷酸化，防止 GSK3β 的核积累，并维持细胞周期蛋白 D3 水平和细胞增殖。发现 K19 与 GSK3β 相互作用，并且 K19-GSK3β 相互作用被确定为需要 K19 的头部结构域上的 Ser10 和 Ser35 残基。与野生型 K19 不同，S10A 和 S35A 突变体未能维持总和核细胞周期蛋白 D3 水平并诱导细胞增殖。最后，我们表明 K19-GSK3β-cyclin D3 通路影响细胞对细胞周期蛋白依赖性激酶 4 和 6 (CDK4/6) 抑制剂的敏感性。总体而言，这些发现确立了 K19 在调节 GSK3β-cyclin D3 途径中的作用，并证明了克服对 CDK4/6 抑制剂耐药性的潜在策略。