A Dot Product Range on a Circle

The line $ax+by+c=0$ meets the circle $O:x^2+y^2=16$ at two points $M$ and $N$, where the coefficients satisfy $c^2=a^2+b^2$. Let $P$ be an arbitrary point on the circle $O$. Find the range of $\overrightarrow{PM}\cdot\overrightarrow{PN}$.