Despite substantial efforts being made over the years, prostate cancer remains one of the most common cancers to cause death among men [1]. The growth and survival of PCa cells relies on the androgen receptor (AR) signalling hence, androgen deprivation therapy (ADT) is considered as a first line therapy. However, PCa patients acquire resistance to ADT and progress to a lethal form known as castration-resistant prostate cancer (CRPC) [2,3]. Gain-of-function mutations in AR and its coactivators as well as reactivation of AR pathways by constitutively expressed splice variants are known to be primary drivers of the androgen-ablation resistance [4]. Androgen receptor splice variants (AR-Vs) are the truncated form of AR which lacks the ligand binding domain (LBD) at the C-terminal [5]. Among different splice variants, AR-V7, is the most abundant form of AR-Vs [6] which regulates the canonical AR pathways and is responsible for poor survival [[7], [8], [9]]. Given the critical role of AR-V7 in driving CRPC, understanding the mechanisms leading to AR-V7 generation is important to comprehend the disease etiology and develop targeted therapies for treatment.

Nearly more than 90 % human genomes undergo alternative splicing resulting in protein isoforms with distinct structural and functional properties [10]. Studies have reported that the alternative splicing of AR is regulated by many splicing factors such as.

U2AF65 and SRSF1 and their activity can be modulated by splice factor kinases such as Cdc2 like kinase (CLK1) and the SRSF protein kinases (SRPK1) [11]. Any alteration in the RNA splicing of AR results in the generation of AR-V7 [12]. In the light of previously published findings, SR proteins such as SRSF1 are predominantly phosphorylated by CLK1 and SRPK1 and are critical for AR-V7 generation. RNAi screens have been performed to delineate the crosstalk between alternative splicing regulators and cell cycle regulation [13]. Splice factor kinase, CLK1 periodically regulates throughout the cell cycle and results in the alteration in alternative splicing of more than 1300 genes [14].

Previous studies have revealed that AR-V7 regulates the expression of genes involved in cell cycle progression [8]. One of the AR-V7 downstream genes, ubiquitinconjugating enzyme 2C (UBE2C) promotes the degradation of cyclins during cell cycle [15]. Silencing of AR-V7 regulated UBE2C gene led to an increase in S phase which hinders the cancer cell proliferation. Interestingly, here we found that AR-V7 protein levels fluctuate periodically during cell cycle, with minimal-to-no expression during G1 phase. However, there was no effect on AR-V7 mRNA levels. Our results also demonstrate that the knockdown of CLK1 prolongs the S phase with higher expression of AR-V7, p-SRSF1 and CLK1 in the G1/S phase of the cell cycle. Conversely, overexpression of CLK1 results in the accumulation of cells in the G2/M phase with increased AR-V7 and p-SRSF1 expression. Furthermore, using kinase inactive CLK1 mutant, we demonstrate that CLK1 kinase activity is essential for the phosphorylation of SRSF1 which in turn regulates the expression of AR-V7 during different phases of cell cycle. Taken together, our results demonstrate that CLK1- pSRSF1 axis is essential for the cell-cycle dependent regulation of AR-V7 in prostate cancer.